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diff --git a/manual/filesys.texi b/manual/filesys.texi new file mode 100644 index 0000000000..d2afe8623f --- /dev/null +++ b/manual/filesys.texi @@ -0,0 +1,2080 @@ +@node File System Interface, Pipes and FIFOs, Low-Level I/O, Top +@chapter File System Interface + +This chapter describes the GNU C library's functions for manipulating +files. Unlike the input and output functions described in +@ref{I/O on Streams} and @ref{Low-Level I/O}, these +functions are concerned with operating on the files themselves, rather +than on their contents. + +Among the facilities described in this chapter are functions for +examining or modifying directories, functions for renaming and deleting +files, and functions for examining and setting file attributes such as +access permissions and modification times. + +@menu +* Working Directory:: This is used to resolve relative + file names. +* Accessing Directories:: Finding out what files a directory + contains. +* Hard Links:: Adding alternate names to a file. +* Symbolic Links:: A file that ``points to'' a file name. +* Deleting Files:: How to delete a file, and what that means. +* Renaming Files:: Changing a file's name. +* Creating Directories:: A system call just for creating a directory. +* File Attributes:: Attributes of individual files. +* Making Special Files:: How to create special files. +* Temporary Files:: Naming and creating temporary files. +@end menu + +@node Working Directory +@section Working Directory + +@cindex current working directory +@cindex working directory +@cindex change working directory +Each process has associated with it a directory, called its @dfn{current +working directory} or simply @dfn{working directory}, that is used in +the resolution of relative file names (@pxref{File Name Resolution}). + +When you log in and begin a new session, your working directory is +initially set to the home directory associated with your login account +in the system user database. You can find any user's home directory +using the @code{getpwuid} or @code{getpwnam} functions; see @ref{User +Database}. + +Users can change the working directory using shell commands like +@code{cd}. The functions described in this section are the primitives +used by those commands and by other programs for examining and changing +the working directory. +@pindex cd + +Prototypes for these functions are declared in the header file +@file{unistd.h}. +@pindex unistd.h + +@comment unistd.h +@comment POSIX.1 +@deftypefun {char *} getcwd (char *@var{buffer}, size_t @var{size}) +The @code{getcwd} function returns an absolute file name representing +the current working directory, storing it in the character array +@var{buffer} that you provide. The @var{size} argument is how you tell +the system the allocation size of @var{buffer}. + +The GNU library version of this function also permits you to specify a +null pointer for the @var{buffer} argument. Then @code{getcwd} +allocates a buffer automatically, as with @code{malloc} +(@pxref{Unconstrained Allocation}). If the @var{size} is greater than +zero, then the buffer is that large; otherwise, the buffer is as large +as necessary to hold the result. + +The return value is @var{buffer} on success and a null pointer on failure. +The following @code{errno} error conditions are defined for this function: + +@table @code +@item EINVAL +The @var{size} argument is zero and @var{buffer} is not a null pointer. + +@item ERANGE +The @var{size} argument is less than the length of the working directory +name. You need to allocate a bigger array and try again. + +@item EACCES +Permission to read or search a component of the file name was denied. +@end table +@end deftypefun + +Here is an example showing how you could implement the behavior of GNU's +@w{@code{getcwd (NULL, 0)}} using only the standard behavior of +@code{getcwd}: + +@smallexample +char * +gnu_getcwd () +@{ + int size = 100; + char *buffer = (char *) xmalloc (size); + + while (1) + @{ + char *value = getcwd (buffer, size); + if (value != 0) + return buffer; + size *= 2; + free (buffer); + buffer = (char *) xmalloc (size); + @} +@} +@end smallexample + +@noindent +@xref{Malloc Examples}, for information about @code{xmalloc}, which is +not a library function but is a customary name used in most GNU +software. + +@comment unistd.h +@comment BSD +@deftypefun {char *} getwd (char *@var{buffer}) +This is similar to @code{getcwd}, but has no way to specify the size of +the buffer. The GNU library provides @code{getwd} only +for backwards compatibility with BSD. + +The @var{buffer} argument should be a pointer to an array at least +@code{PATH_MAX} bytes long (@pxref{Limits for Files}). In the GNU +system there is no limit to the size of a file name, so this is not +necessarily enough space to contain the directory name. That is why +this function is deprecated. +@end deftypefun + +@comment unistd.h +@comment POSIX.1 +@deftypefun int chdir (const char *@var{filename}) +This function is used to set the process's working directory to +@var{filename}. + +The normal, successful return value from @code{chdir} is @code{0}. A +value of @code{-1} is returned to indicate an error. The @code{errno} +error conditions defined for this function are the usual file name +syntax errors (@pxref{File Name Errors}), plus @code{ENOTDIR} if the +file @var{filename} is not a directory. +@end deftypefun + + +@node Accessing Directories +@section Accessing Directories +@cindex accessing directories +@cindex reading from a directory +@cindex directories, accessing + +The facilities described in this section let you read the contents of a +directory file. This is useful if you want your program to list all the +files in a directory, perhaps as part of a menu. + +@cindex directory stream +The @code{opendir} function opens a @dfn{directory stream} whose +elements are directory entries. You use the @code{readdir} function on +the directory stream to retrieve these entries, represented as +@w{@code{struct dirent}} objects. The name of the file for each entry is +stored in the @code{d_name} member of this structure. There are obvious +parallels here to the stream facilities for ordinary files, described in +@ref{I/O on Streams}. + +@menu +* Directory Entries:: Format of one directory entry. +* Opening a Directory:: How to open a directory stream. +* Reading/Closing Directory:: How to read directory entries from the stream. +* Simple Directory Lister:: A very simple directory listing program. +* Random Access Directory:: Rereading part of the directory + already read with the same stream. +@end menu + +@node Directory Entries +@subsection Format of a Directory Entry + +@pindex dirent.h +This section describes what you find in a single directory entry, as you +might obtain it from a directory stream. All the symbols are declared +in the header file @file{dirent.h}. + +@comment dirent.h +@comment POSIX.1 +@deftp {Data Type} {struct dirent} +This is a structure type used to return information about directory +entries. It contains the following fields: + +@table @code +@item char d_name[] +This is the null-terminated file name component. This is the only +field you can count on in all POSIX systems. + +@item ino_t d_fileno +This is the file serial number. For BSD compatibility, you can also +refer to this member as @code{d_ino}. In the GNU system and most POSIX +systems, for most files this the same as the @code{st_ino} member that +@code{stat} will return for the file. @xref{File Attributes}. + +@item unsigned char d_namlen +This is the length of the file name, not including the terminating null +character. Its type is @code{unsigned char} because that is the integer +type of the appropriate size + +@item unsigned char d_type +This is the type of the file, possibly unknown. The following constants +are defined for its value: + +@table @code +@item DT_UNKNOWN +The type is unknown. On some systems this is the only value returned. + +@item DT_REG +A regular file. + +@item DT_DIR +A directory. + +@item DT_FIFO +A named pipe, or FIFO. @xref{FIFO Special Files}. + +@item DT_SOCK +A local-domain socket. @c !!! @xref{Local Domain}. + +@item DT_CHR +A character device. + +@item DT_BLK +A block device. +@end table + +This member is a BSD extension. Each value except DT_UNKNOWN +corresponds to the file type bits in the @code{st_mode} member of +@code{struct statbuf}. These two macros convert between @code{d_type} +values and @code{st_mode} values: + +@deftypefun int IFTODT (mode_t @var{mode}) +This returns the @code{d_type} value corresponding to @var{mode}. +@end deftypefun + +@deftypefun mode_t DTTOIF (int @var{dirtype}) +This returns the @code{st_mode} value corresponding to @var{dirtype}. +@end deftypefun +@end table + +This structure may contain additional members in the future. + +When a file has multiple names, each name has its own directory entry. +The only way you can tell that the directory entries belong to a +single file is that they have the same value for the @code{d_fileno} +field. + +File attributes such as size, modification times, and the like are part +of the file itself, not any particular directory entry. @xref{File +Attributes}. +@end deftp + +@node Opening a Directory +@subsection Opening a Directory Stream + +@pindex dirent.h +This section describes how to open a directory stream. All the symbols +are declared in the header file @file{dirent.h}. + +@comment dirent.h +@comment POSIX.1 +@deftp {Data Type} DIR +The @code{DIR} data type represents a directory stream. +@end deftp + +You shouldn't ever allocate objects of the @code{struct dirent} or +@code{DIR} data types, since the directory access functions do that for +you. Instead, you refer to these objects using the pointers returned by +the following functions. + +@comment dirent.h +@comment POSIX.1 +@deftypefun {DIR *} opendir (const char *@var{dirname}) +The @code{opendir} function opens and returns a directory stream for +reading the directory whose file name is @var{dirname}. The stream has +type @code{DIR *}. + +If unsuccessful, @code{opendir} returns a null pointer. In addition to +the usual file name errors (@pxref{File Name Errors}), the +following @code{errno} error conditions are defined for this function: + +@table @code +@item EACCES +Read permission is denied for the directory named by @code{dirname}. + +@item EMFILE +The process has too many files open. + +@item ENFILE +The entire system, or perhaps the file system which contains the +directory, cannot support any additional open files at the moment. +(This problem cannot happen on the GNU system.) +@end table + +The @code{DIR} type is typically implemented using a file descriptor, +and the @code{opendir} function in terms of the @code{open} function. +@xref{Low-Level I/O}. Directory streams and the underlying +file descriptors are closed on @code{exec} (@pxref{Executing a File}). +@end deftypefun + +@node Reading/Closing Directory +@subsection Reading and Closing a Directory Stream + +@pindex dirent.h +This section describes how to read directory entries from a directory +stream, and how to close the stream when you are done with it. All the +symbols are declared in the header file @file{dirent.h}. + +@comment dirent.h +@comment POSIX.1 +@deftypefun {struct dirent *} readdir (DIR *@var{dirstream}) +This function reads the next entry from the directory. It normally +returns a pointer to a structure containing information about the file. +This structure is statically allocated and can be rewritten by a +subsequent call. + +@strong{Portability Note:} On some systems, @code{readdir} may not +return entries for @file{.} and @file{..}, even though these are always +valid file names in any directory. @xref{File Name Resolution}. + +If there are no more entries in the directory or an error is detected, +@code{readdir} returns a null pointer. The following @code{errno} error +conditions are defined for this function: + +@table @code +@item EBADF +The @var{dirstream} argument is not valid. +@end table +@end deftypefun + +@comment dirent.h +@comment POSIX.1 +@deftypefun int closedir (DIR *@var{dirstream}) +This function closes the directory stream @var{dirstream}. It returns +@code{0} on success and @code{-1} on failure. + +The following @code{errno} error conditions are defined for this +function: + +@table @code +@item EBADF +The @var{dirstream} argument is not valid. +@end table +@end deftypefun + +@node Simple Directory Lister +@subsection Simple Program to List a Directory + +Here's a simple program that prints the names of the files in +the current working directory: + +@smallexample +@include dir.c.texi +@end smallexample + +The order in which files appear in a directory tends to be fairly +random. A more useful program would sort the entries (perhaps by +alphabetizing them) before printing them; see @ref{Array Sort Function}. + +@c ??? not documented: scandir, alphasort + +@node Random Access Directory +@subsection Random Access in a Directory Stream + +@pindex dirent.h +This section describes how to reread parts of a directory that you have +already read from an open directory stream. All the symbols are +declared in the header file @file{dirent.h}. + +@comment dirent.h +@comment POSIX.1 +@deftypefun void rewinddir (DIR *@var{dirstream}) +The @code{rewinddir} function is used to reinitialize the directory +stream @var{dirstream}, so that if you call @code{readdir} it +returns information about the first entry in the directory again. This +function also notices if files have been added or removed to the +directory since it was opened with @code{opendir}. (Entries for these +files might or might not be returned by @code{readdir} if they were +added or removed since you last called @code{opendir} or +@code{rewinddir}.) +@end deftypefun + +@comment dirent.h +@comment BSD +@deftypefun off_t telldir (DIR *@var{dirstream}) +The @code{telldir} function returns the file position of the directory +stream @var{dirstream}. You can use this value with @code{seekdir} to +restore the directory stream to that position. +@end deftypefun + +@comment dirent.h +@comment BSD +@deftypefun void seekdir (DIR *@var{dirstream}, off_t @var{pos}) +The @code{seekdir} function sets the file position of the directory +stream @var{dirstream} to @var{pos}. The value @var{pos} must be the +result of a previous call to @code{telldir} on this particular stream; +closing and reopening the directory can invalidate values returned by +@code{telldir}. +@end deftypefun + +@node Hard Links +@section Hard Links +@cindex hard link +@cindex link, hard +@cindex multiple names for one file +@cindex file names, multiple + +In POSIX systems, one file can have many names at the same time. All of +the names are equally real, and no one of them is preferred to the +others. + +To add a name to a file, use the @code{link} function. (The new name is +also called a @dfn{hard link} to the file.) Creating a new link to a +file does not copy the contents of the file; it simply makes a new name +by which the file can be known, in addition to the file's existing name +or names. + +One file can have names in several directories, so the the organization +of the file system is not a strict hierarchy or tree. + +In most implementations, it is not possible to have hard links to the +same file in multiple file systems. @code{link} reports an error if you +try to make a hard link to the file from another file system when this +cannot be done. + +The prototype for the @code{link} function is declared in the header +file @file{unistd.h}. +@pindex unistd.h + +@comment unistd.h +@comment POSIX.1 +@deftypefun int link (const char *@var{oldname}, const char *@var{newname}) +The @code{link} function makes a new link to the existing file named by +@var{oldname}, under the new name @var{newname}. + +This function returns a value of @code{0} if it is successful and +@code{-1} on failure. In addition to the usual file name errors +(@pxref{File Name Errors}) for both @var{oldname} and @var{newname}, the +following @code{errno} error conditions are defined for this function: + +@table @code +@item EACCES +You are not allowed to write the directory in which the new link is to +be written. +@ignore +Some implementations also require that the existing file be accessible +by the caller, and use this error to report failure for that reason. +@end ignore + +@item EEXIST +There is already a file named @var{newname}. If you want to replace +this link with a new link, you must remove the old link explicitly first. + +@item EMLINK +There are already too many links to the file named by @var{oldname}. +(The maximum number of links to a file is @w{@code{LINK_MAX}}; see +@ref{Limits for Files}.) + +@item ENOENT +The file named by @var{oldname} doesn't exist. You can't make a link to +a file that doesn't exist. + +@item ENOSPC +The directory or file system that would contain the new link is full +and cannot be extended. + +@item EPERM +In the GNU system and some others, you cannot make links to directories. +Many systems allow only privileged users to do so. This error +is used to report the problem. + +@item EROFS +The directory containing the new link can't be modified because it's on +a read-only file system. + +@item EXDEV +The directory specified in @var{newname} is on a different file system +than the existing file. + +@item EIO +A hardware error occurred while trying to read or write the to filesystem. +@end table +@end deftypefun + +@node Symbolic Links +@section Symbolic Links +@cindex soft link +@cindex link, soft +@cindex symbolic link +@cindex link, symbolic + +The GNU system supports @dfn{soft links} or @dfn{symbolic links}. This +is a kind of ``file'' that is essentially a pointer to another file +name. Unlike hard links, symbolic links can be made to directories or +across file systems with no restrictions. You can also make a symbolic +link to a name which is not the name of any file. (Opening this link +will fail until a file by that name is created.) Likewise, if the +symbolic link points to an existing file which is later deleted, the +symbolic link continues to point to the same file name even though the +name no longer names any file. + +The reason symbolic links work the way they do is that special things +happen when you try to open the link. The @code{open} function realizes +you have specified the name of a link, reads the file name contained in +the link, and opens that file name instead. The @code{stat} function +likewise operates on the file that the symbolic link points to, instead +of on the link itself. + +By contrast, other operations such as deleting or renaming the file +operate on the link itself. The functions @code{readlink} and +@code{lstat} also refrain from following symbolic links, because their +purpose is to obtain information about the link. So does @code{link}, +the function that makes a hard link---it makes a hard link to the +symbolic link, which one rarely wants. + +Prototypes for the functions listed in this section are in +@file{unistd.h}. +@pindex unistd.h + +@comment unistd.h +@comment BSD +@deftypefun int symlink (const char *@var{oldname}, const char *@var{newname}) +The @code{symlink} function makes a symbolic link to @var{oldname} named +@var{newname}. + +The normal return value from @code{symlink} is @code{0}. A return value +of @code{-1} indicates an error. In addition to the usual file name +syntax errors (@pxref{File Name Errors}), the following @code{errno} +error conditions are defined for this function: + +@table @code +@item EEXIST +There is already an existing file named @var{newname}. + +@item EROFS +The file @var{newname} would exist on a read-only file system. + +@item ENOSPC +The directory or file system cannot be extended to make the new link. + +@item EIO +A hardware error occurred while reading or writing data on the disk. + +@ignore +@comment not sure about these +@item ELOOP +There are too many levels of indirection. This can be the result of +circular symbolic links to directories. + +@item EDQUOT +The new link can't be created because the user's disk quota has been +exceeded. +@end ignore +@end table +@end deftypefun + +@comment unistd.h +@comment BSD +@deftypefun int readlink (const char *@var{filename}, char *@var{buffer}, size_t @var{size}) +The @code{readlink} function gets the value of the symbolic link +@var{filename}. The file name that the link points to is copied into +@var{buffer}. This file name string is @emph{not} null-terminated; +@code{readlink} normally returns the number of characters copied. The +@var{size} argument specifies the maximum number of characters to copy, +usually the allocation size of @var{buffer}. + +If the return value equals @var{size}, you cannot tell whether or not +there was room to return the entire name. So make a bigger buffer and +call @code{readlink} again. Here is an example: + +@smallexample +char * +readlink_malloc (char *filename) +@{ + int size = 100; + + while (1) + @{ + char *buffer = (char *) xmalloc (size); + int nchars = readlink (filename, buffer, size); + if (nchars < size) + return buffer; + free (buffer); + size *= 2; + @} +@} +@end smallexample + +@c @group Invalid outside example. +A value of @code{-1} is returned in case of error. In addition to the +usual file name errors (@pxref{File Name Errors}), the following +@code{errno} error conditions are defined for this function: + +@table @code +@item EINVAL +The named file is not a symbolic link. + +@item EIO +A hardware error occurred while reading or writing data on the disk. +@end table +@c @end group +@end deftypefun + +@node Deleting Files +@section Deleting Files +@cindex deleting a file +@cindex removing a file +@cindex unlinking a file + +You can delete a file with the functions @code{unlink} or @code{remove}. + +Deletion actually deletes a file name. If this is the file's only name, +then the file is deleted as well. If the file has other names as well +(@pxref{Hard Links}), it remains accessible under its other names. + +@comment unistd.h +@comment POSIX.1 +@deftypefun int unlink (const char *@var{filename}) +The @code{unlink} function deletes the file name @var{filename}. If +this is a file's sole name, the file itself is also deleted. (Actually, +if any process has the file open when this happens, deletion is +postponed until all processes have closed the file.) + +@pindex unistd.h +The function @code{unlink} is declared in the header file @file{unistd.h}. + +This function returns @code{0} on successful completion, and @code{-1} +on error. In addition to the usual file name errors +(@pxref{File Name Errors}), the following @code{errno} error conditions are +defined for this function: + +@table @code +@item EACCES +Write permission is denied for the directory from which the file is to be +removed, or the directory has the sticky bit set and you do not own the file. + +@item EBUSY +This error indicates that the file is being used by the system in such a +way that it can't be unlinked. For example, you might see this error if +the file name specifies the root directory or a mount point for a file +system. + +@item ENOENT +The file name to be deleted doesn't exist. + +@item EPERM +On some systems, @code{unlink} cannot be used to delete the name of a +directory, or can only be used this way by a privileged user. +To avoid such problems, use @code{rmdir} to delete directories. +(In the GNU system @code{unlink} can never delete the name of a directory.) + +@item EROFS +The directory in which the file name is to be deleted is on a read-only +file system, and can't be modified. +@end table +@end deftypefun + +@comment unistd.h +@comment POSIX.1 +@deftypefun int rmdir (const char *@var{filename}) +@cindex directories, deleting +@cindex deleting a directory +The @code{rmdir} function deletes a directory. The directory must be +empty before it can be removed; in other words, it can only contain +entries for @file{.} and @file{..}. + +In most other respects, @code{rmdir} behaves like @code{unlink}. There +are two additional @code{errno} error conditions defined for +@code{rmdir}: + +@table @code +@item ENOTEMPTY +@itemx EEXIST +The directory to be deleted is not empty. +@end table + +These two error codes are synonymous; some systems use one, and some use +the other. The GNU system always uses @code{ENOTEMPTY}. + +The prototype for this function is declared in the header file +@file{unistd.h}. +@pindex unistd.h +@end deftypefun + +@comment stdio.h +@comment ANSI +@deftypefun int remove (const char *@var{filename}) +This is the ANSI C function to remove a file. It works like +@code{unlink} for files and like @code{rmdir} for directories. +@code{remove} is declared in @file{stdio.h}. +@pindex stdio.h +@end deftypefun + +@node Renaming Files +@section Renaming Files + +The @code{rename} function is used to change a file's name. + +@cindex renaming a file +@comment stdio.h +@comment ANSI +@deftypefun int rename (const char *@var{oldname}, const char *@var{newname}) +The @code{rename} function renames the file name @var{oldname} with +@var{newname}. The file formerly accessible under the name +@var{oldname} is afterward accessible as @var{newname} instead. (If the +file had any other names aside from @var{oldname}, it continues to have +those names.) + +The directory containing the name @var{newname} must be on the same +file system as the file (as indicated by the name @var{oldname}). + +One special case for @code{rename} is when @var{oldname} and +@var{newname} are two names for the same file. The consistent way to +handle this case is to delete @var{oldname}. However, POSIX requires +that in this case @code{rename} do nothing and report success---which is +inconsistent. We don't know what your operating system will do. + +If the @var{oldname} is not a directory, then any existing file named +@var{newname} is removed during the renaming operation. However, if +@var{newname} is the name of a directory, @code{rename} fails in this +case. + +If the @var{oldname} is a directory, then either @var{newname} must not +exist or it must name a directory that is empty. In the latter case, +the existing directory named @var{newname} is deleted first. The name +@var{newname} must not specify a subdirectory of the directory +@code{oldname} which is being renamed. + +One useful feature of @code{rename} is that the meaning of the name +@var{newname} changes ``atomically'' from any previously existing file +by that name to its new meaning (the file that was called +@var{oldname}). There is no instant at which @var{newname} is +nonexistent ``in between'' the old meaning and the new meaning. If +there is a system crash during the operation, it is possible for both +names to still exist; but @var{newname} will always be intact if it +exists at all. + +If @code{rename} fails, it returns @code{-1}. In addition to the usual +file name errors (@pxref{File Name Errors}), the following +@code{errno} error conditions are defined for this function: + +@table @code +@item EACCES +One of the directories containing @var{newname} or @var{oldname} +refuses write permission; or @var{newname} and @var{oldname} are +directories and write permission is refused for one of them. + +@item EBUSY +A directory named by @var{oldname} or @var{newname} is being used by +the system in a way that prevents the renaming from working. This includes +directories that are mount points for filesystems, and directories +that are the current working directories of processes. + +@item ENOTEMPTY +@itemx EEXIST +The directory @var{newname} isn't empty. The GNU system always returns +@code{ENOTEMPTY} for this, but some other systems return @code{EEXIST}. + +@item EINVAL +The @var{oldname} is a directory that contains @var{newname}. + +@item EISDIR +The @var{newname} names a directory, but the @var{oldname} doesn't. + +@item EMLINK +The parent directory of @var{newname} would have too many links. + +@item ENOENT +The file named by @var{oldname} doesn't exist. + +@item ENOSPC +The directory that would contain @var{newname} has no room for another +entry, and there is no space left in the file system to expand it. + +@item EROFS +The operation would involve writing to a directory on a read-only file +system. + +@item EXDEV +The two file names @var{newname} and @var{oldnames} are on different +file systems. +@end table +@end deftypefun + +@node Creating Directories +@section Creating Directories +@cindex creating a directory +@cindex directories, creating + +@pindex mkdir +Directories are created with the @code{mkdir} function. (There is also +a shell command @code{mkdir} which does the same thing.) +@c !!! umask + +@comment sys/stat.h +@comment POSIX.1 +@deftypefun int mkdir (const char *@var{filename}, mode_t @var{mode}) +The @code{mkdir} function creates a new, empty directory whose name is +@var{filename}. + +The argument @var{mode} specifies the file permissions for the new +directory file. @xref{Permission Bits}, for more information about +this. + +A return value of @code{0} indicates successful completion, and +@code{-1} indicates failure. In addition to the usual file name syntax +errors (@pxref{File Name Errors}), the following @code{errno} error +conditions are defined for this function: + +@table @code +@item EACCES +Write permission is denied for the parent directory in which the new +directory is to be added. + +@item EEXIST +A file named @var{filename} already exists. + +@item EMLINK +The parent directory has too many links. + +Well-designed file systems never report this error, because they permit +more links than your disk could possibly hold. However, you must still +take account of the possibility of this error, as it could result from +network access to a file system on another machine. + +@item ENOSPC +The file system doesn't have enough room to create the new directory. + +@item EROFS +The parent directory of the directory being created is on a read-only +file system, and cannot be modified. +@end table + +To use this function, your program should include the header file +@file{sys/stat.h}. +@pindex sys/stat.h +@end deftypefun + +@node File Attributes +@section File Attributes + +@pindex ls +When you issue an @samp{ls -l} shell command on a file, it gives you +information about the size of the file, who owns it, when it was last +modified, and the like. This kind of information is called the +@dfn{file attributes}; it is associated with the file itself and not a +particular one of its names. + +This section contains information about how you can inquire about and +modify these attributes of files. + +@menu +* Attribute Meanings:: The names of the file attributes, + and what their values mean. +* Reading Attributes:: How to read the attributes of a file. +* Testing File Type:: Distinguishing ordinary files, + directories, links... +* File Owner:: How ownership for new files is determined, + and how to change it. +* Permission Bits:: How information about a file's access + mode is stored. +* Access Permission:: How the system decides who can access a file. +* Setting Permissions:: How permissions for new files are assigned, + and how to change them. +* Testing File Access:: How to find out if your process can + access a file. +* File Times:: About the time attributes of a file. +@end menu + +@node Attribute Meanings +@subsection What the File Attribute Values Mean +@cindex status of a file +@cindex attributes of a file +@cindex file attributes + +When you read the attributes of a file, they come back in a structure +called @code{struct stat}. This section describes the names of the +attributes, their data types, and what they mean. For the functions +to read the attributes of a file, see @ref{Reading Attributes}. + +The header file @file{sys/stat.h} declares all the symbols defined +in this section. +@pindex sys/stat.h + +@comment sys/stat.h +@comment POSIX.1 +@deftp {Data Type} {struct stat} +The @code{stat} structure type is used to return information about the +attributes of a file. It contains at least the following members: + +@table @code +@item mode_t st_mode +Specifies the mode of the file. This includes file type information +(@pxref{Testing File Type}) and the file permission bits +(@pxref{Permission Bits}). + +@item ino_t st_ino +The file serial number, which distinguishes this file from all other +files on the same device. + +@item dev_t st_dev +Identifies the device containing the file. The @code{st_ino} and +@code{st_dev}, taken together, uniquely identify the file. The +@code{st_dev} value is not necessarily consistent across reboots or +system crashes, however. + +@item nlink_t st_nlink +The number of hard links to the file. This count keeps track of how +many directories have entries for this file. If the count is ever +decremented to zero, then the file itself is discarded as soon as no +process still holds it open. Symbolic links are not counted in the +total. + +@item uid_t st_uid +The user ID of the file's owner. @xref{File Owner}. + +@item gid_t st_gid +The group ID of the file. @xref{File Owner}. + +@item off_t st_size +This specifies the size of a regular file in bytes. For files that +are really devices and the like, this field isn't usually meaningful. +For symbolic links, this specifies the length of the file name the link +refers to. + +@item time_t st_atime +This is the last access time for the file. @xref{File Times}. + +@item unsigned long int st_atime_usec +This is the fractional part of the last access time for the file. +@xref{File Times}. + +@item time_t st_mtime +This is the time of the last modification to the contents of the file. +@xref{File Times}. + +@item unsigned long int st_mtime_usec +This is the fractional part of the time of last modification to the +contents of the file. @xref{File Times}. + +@item time_t st_ctime +This is the time of the last modification to the attributes of the file. +@xref{File Times}. + +@item unsigned long int st_ctime_usec +This is the fractional part of the time of last modification to the +attributes of the file. @xref{File Times}. + +@c !!! st_rdev +@item unsigned int st_blocks +This is the amount of disk space that the file occupies, measured in +units of 512-byte blocks. + +The number of disk blocks is not strictly proportional to the size of +the file, for two reasons: the file system may use some blocks for +internal record keeping; and the file may be sparse---it may have +``holes'' which contain zeros but do not actually take up space on the +disk. + +You can tell (approximately) whether a file is sparse by comparing this +value with @code{st_size}, like this: + +@smallexample +(st.st_blocks * 512 < st.st_size) +@end smallexample + +This test is not perfect because a file that is just slightly sparse +might not be detected as sparse at all. For practical applications, +this is not a problem. + +@item unsigned int st_blksize +The optimal block size for reading of writing this file, in bytes. You +might use this size for allocating the buffer space for reading of +writing the file. (This is unrelated to @code{st_blocks}.) +@end table +@end deftp + + Some of the file attributes have special data type names which exist +specifically for those attributes. (They are all aliases for well-known +integer types that you know and love.) These typedef names are defined +in the header file @file{sys/types.h} as well as in @file{sys/stat.h}. +Here is a list of them. + +@comment sys/types.h +@comment POSIX.1 +@deftp {Data Type} mode_t +This is an integer data type used to represent file modes. In the +GNU system, this is equivalent to @code{unsigned int}. +@end deftp + +@cindex inode number +@comment sys/types.h +@comment POSIX.1 +@deftp {Data Type} ino_t +This is an arithmetic data type used to represent file serial numbers. +(In Unix jargon, these are sometimes called @dfn{inode numbers}.) +In the GNU system, this type is equivalent to @code{unsigned long int}. +@end deftp + +@comment sys/types.h +@comment POSIX.1 +@deftp {Data Type} dev_t +This is an arithmetic data type used to represent file device numbers. +In the GNU system, this is equivalent to @code{int}. +@end deftp + +@comment sys/types.h +@comment POSIX.1 +@deftp {Data Type} nlink_t +This is an arithmetic data type used to represent file link counts. +In the GNU system, this is equivalent to @code{unsigned short int}. +@end deftp + +@node Reading Attributes +@subsection Reading the Attributes of a File + +To examine the attributes of files, use the functions @code{stat}, +@code{fstat} and @code{lstat}. They return the attribute information in +a @code{struct stat} object. All three functions are declared in the +header file @file{sys/stat.h}. + +@comment sys/stat.h +@comment POSIX.1 +@deftypefun int stat (const char *@var{filename}, struct stat *@var{buf}) +The @code{stat} function returns information about the attributes of the +file named by @w{@var{filename}} in the structure pointed at by @var{buf}. + +If @var{filename} is the name of a symbolic link, the attributes you get +describe the file that the link points to. If the link points to a +nonexistent file name, then @code{stat} fails, reporting a nonexistent +file. + +The return value is @code{0} if the operation is successful, and @code{-1} +on failure. In addition to the usual file name errors +(@pxref{File Name Errors}, the following @code{errno} error conditions +are defined for this function: + +@table @code +@item ENOENT +The file named by @var{filename} doesn't exist. +@end table +@end deftypefun + +@comment sys/stat.h +@comment POSIX.1 +@deftypefun int fstat (int @var{filedes}, struct stat *@var{buf}) +The @code{fstat} function is like @code{stat}, except that it takes an +open file descriptor as an argument instead of a file name. +@xref{Low-Level I/O}. + +Like @code{stat}, @code{fstat} returns @code{0} on success and @code{-1} +on failure. The following @code{errno} error conditions are defined for +@code{fstat}: + +@table @code +@item EBADF +The @var{filedes} argument is not a valid file descriptor. +@end table +@end deftypefun + +@comment sys/stat.h +@comment BSD +@deftypefun int lstat (const char *@var{filename}, struct stat *@var{buf}) +The @code{lstat} function is like @code{stat}, except that it does not +follow symbolic links. If @var{filename} is the name of a symbolic +link, @code{lstat} returns information about the link itself; otherwise, +@code{lstat} works like @code{stat}. @xref{Symbolic Links}. +@end deftypefun + +@node Testing File Type +@subsection Testing the Type of a File + +The @dfn{file mode}, stored in the @code{st_mode} field of the file +attributes, contains two kinds of information: the file type code, and +the access permission bits. This section discusses only the type code, +which you can use to tell whether the file is a directory, whether it is +a socket, and so on. For information about the access permission, +@ref{Permission Bits}. + +There are two predefined ways you can access the file type portion of +the file mode. First of all, for each type of file, there is a +@dfn{predicate macro} which examines a file mode value and returns +true or false---is the file of that type, or not. Secondly, you can +mask out the rest of the file mode to get just a file type code. +You can compare this against various constants for the supported file +types. + +All of the symbols listed in this section are defined in the header file +@file{sys/stat.h}. +@pindex sys/stat.h + +The following predicate macros test the type of a file, given the value +@var{m} which is the @code{st_mode} field returned by @code{stat} on +that file: + +@comment sys/stat.h +@comment POSIX +@deftypefn Macro int S_ISDIR (mode_t @var{m}) +This macro returns nonzero if the file is a directory. +@end deftypefn + +@comment sys/stat.h +@comment POSIX +@deftypefn Macro int S_ISCHR (mode_t @var{m}) +This macro returns nonzero if the file is a character special file (a +device like a terminal). +@end deftypefn + +@comment sys/stat.h +@comment POSIX +@deftypefn Macro int S_ISBLK (mode_t @var{m}) +This macro returns nonzero if the file is a block special file (a device +like a disk). +@end deftypefn + +@comment sys/stat.h +@comment POSIX +@deftypefn Macro int S_ISREG (mode_t @var{m}) +This macro returns nonzero if the file is a regular file. +@end deftypefn + +@comment sys/stat.h +@comment POSIX +@deftypefn Macro int S_ISFIFO (mode_t @var{m}) +This macro returns nonzero if the file is a FIFO special file, or a +pipe. @xref{Pipes and FIFOs}. +@end deftypefn + +@comment sys/stat.h +@comment GNU +@deftypefn Macro int S_ISLNK (mode_t @var{m}) +This macro returns nonzero if the file is a symbolic link. +@xref{Symbolic Links}. +@end deftypefn + +@comment sys/stat.h +@comment GNU +@deftypefn Macro int S_ISSOCK (mode_t @var{m}) +This macro returns nonzero if the file is a socket. @xref{Sockets}. +@end deftypefn + +An alterate non-POSIX method of testing the file type is supported for +compatibility with BSD. The mode can be bitwise ANDed with +@code{S_IFMT} to extract the file type code, and compared to the +appropriate type code constant. For example, + +@smallexample +S_ISCHR (@var{mode}) +@end smallexample + +@noindent +is equivalent to: + +@smallexample +((@var{mode} & S_IFMT) == S_IFCHR) +@end smallexample + +@comment sys/stat.h +@comment BSD +@deftypevr Macro int S_IFMT +This is a bit mask used to extract the file type code portion of a mode +value. +@end deftypevr + +These are the symbolic names for the different file type codes: + +@table @code +@comment sys/stat.h +@comment BSD +@item S_IFDIR +@vindex S_IFDIR +This macro represents the value of the file type code for a directory file. + +@comment sys/stat.h +@comment BSD +@item S_IFCHR +@vindex S_IFCHR +This macro represents the value of the file type code for a +character-oriented device file. + +@comment sys/stat.h +@comment BSD +@item S_IFBLK +@vindex S_IFBLK +This macro represents the value of the file type code for a block-oriented +device file. + +@comment sys/stat.h +@comment BSD +@item S_IFREG +@vindex S_IFREG +This macro represents the value of the file type code for a regular file. + +@comment sys/stat.h +@comment BSD +@item S_IFLNK +@vindex S_IFLNK +This macro represents the value of the file type code for a symbolic link. + +@comment sys/stat.h +@comment BSD +@item S_IFSOCK +@vindex S_IFSOCK +This macro represents the value of the file type code for a socket. + +@comment sys/stat.h +@comment BSD +@item S_IFIFO +@vindex S_IFIFO +This macro represents the value of the file type code for a FIFO or pipe. +@end table + +@node File Owner +@subsection File Owner +@cindex file owner +@cindex owner of a file +@cindex group owner of a file + +Every file has an @dfn{owner} which is one of the registered user names +defined on the system. Each file also has a @dfn{group}, which is one +of the defined groups. The file owner can often be useful for showing +you who edited the file (especially when you edit with GNU Emacs), but +its main purpose is for access control. + +The file owner and group play a role in determining access because the +file has one set of access permission bits for the user that is the +owner, another set that apply to users who belong to the file's group, +and a third set of bits that apply to everyone else. @xref{Access +Permission}, for the details of how access is decided based on this +data. + +When a file is created, its owner is set from the effective user ID of +the process that creates it (@pxref{Process Persona}). The file's group +ID may be set from either effective group ID of the process, or the +group ID of the directory that contains the file, depending on the +system where the file is stored. When you access a remote file system, +it behaves according to its own rule, not according to the system your +program is running on. Thus, your program must be prepared to encounter +either kind of behavior, no matter what kind of system you run it on. + +@pindex chown +@pindex chgrp +You can change the owner and/or group owner of an existing file using +the @code{chown} function. This is the primitive for the @code{chown} +and @code{chgrp} shell commands. + +@pindex unistd.h +The prototype for this function is declared in @file{unistd.h}. + +@comment unistd.h +@comment POSIX.1 +@deftypefun int chown (const char *@var{filename}, uid_t @var{owner}, gid_t @var{group}) +The @code{chown} function changes the owner of the file @var{filename} to +@var{owner}, and its group owner to @var{group}. + +Changing the owner of the file on certain systems clears the set-user-ID +and set-group-ID bits of the file's permissions. (This is because those +bits may not be appropriate for the new owner.) The other file +permission bits are not changed. + +The return value is @code{0} on success and @code{-1} on failure. +In addition to the usual file name errors (@pxref{File Name Errors}), +the following @code{errno} error conditions are defined for this function: + +@table @code +@item EPERM +This process lacks permission to make the requested change. + +Only privileged users or the file's owner can change the file's group. +On most file systems, only privileged users can change the file owner; +some file systems allow you to change the owner if you are currently the +owner. When you access a remote file system, the behavior you encounter +is determined by the system that actually holds the file, not by the +system your program is running on. + +@xref{Options for Files}, for information about the +@code{_POSIX_CHOWN_RESTRICTED} macro. + +@item EROFS +The file is on a read-only file system. +@end table +@end deftypefun + +@comment unistd.h +@comment BSD +@deftypefun int fchown (int @var{filedes}, int @var{owner}, int @var{group}) +This is like @code{chown}, except that it changes the owner of the file +with open file descriptor @var{filedes}. + +The return value from @code{fchown} is @code{0} on success and @code{-1} +on failure. The following @code{errno} error codes are defined for this +function: + +@table @code +@item EBADF +The @var{filedes} argument is not a valid file descriptor. + +@item EINVAL +The @var{filedes} argument corresponds to a pipe or socket, not an ordinary +file. + +@item EPERM +This process lacks permission to make the requested change. For +details, see @code{chmod}, above. + +@item EROFS +The file resides on a read-only file system. +@end table +@end deftypefun + +@node Permission Bits +@subsection The Mode Bits for Access Permission + +The @dfn{file mode}, stored in the @code{st_mode} field of the file +attributes, contains two kinds of information: the file type code, and +the access permission bits. This section discusses only the access +permission bits, which control who can read or write the file. +@xref{Testing File Type}, for information about the file type code. + +All of the symbols listed in this section are defined in the header file +@file{sys/stat.h}. +@pindex sys/stat.h + +@cindex file permission bits +These symbolic constants are defined for the file mode bits that control +access permission for the file: + +@table @code +@comment sys/stat.h +@comment POSIX.1 +@item S_IRUSR +@vindex S_IRUSR +@comment sys/stat.h +@comment BSD +@itemx S_IREAD +@vindex S_IREAD +Read permission bit for the owner of the file. On many systems, this +bit is 0400. @code{S_IREAD} is an obsolete synonym provided for BSD +compatibility. + +@comment sys/stat.h +@comment POSIX.1 +@item S_IWUSR +@vindex S_IWUSR +@comment sys/stat.h +@comment BSD +@itemx S_IWRITE +@vindex S_IWRITE +Write permission bit for the owner of the file. Usually 0200. +@w{@code{S_IWRITE}} is an obsolete synonym provided for BSD compatibility. + +@comment sys/stat.h +@comment POSIX.1 +@item S_IXUSR +@vindex S_IXUSR +@comment sys/stat.h +@comment BSD +@itemx S_IEXEC +@vindex S_IEXEC +Execute (for ordinary files) or search (for directories) permission bit +for the owner of the file. Usually 0100. @code{S_IEXEC} is an obsolete +synonym provided for BSD compatibility. + +@comment sys/stat.h +@comment POSIX.1 +@item S_IRWXU +@vindex S_IRWXU +This is equivalent to @samp{(S_IRUSR | S_IWUSR | S_IXUSR)}. + +@comment sys/stat.h +@comment POSIX.1 +@item S_IRGRP +@vindex S_IRGRP +Read permission bit for the group owner of the file. Usually 040. + +@comment sys/stat.h +@comment POSIX.1 +@item S_IWGRP +@vindex S_IWGRP +Write permission bit for the group owner of the file. Usually 020. + +@comment sys/stat.h +@comment POSIX.1 +@item S_IXGRP +@vindex S_IXGRP +Execute or search permission bit for the group owner of the file. +Usually 010. + +@comment sys/stat.h +@comment POSIX.1 +@item S_IRWXG +@vindex S_IRWXG +This is equivalent to @samp{(S_IRGRP | S_IWGRP | S_IXGRP)}. + +@comment sys/stat.h +@comment POSIX.1 +@item S_IROTH +@vindex S_IROTH +Read permission bit for other users. Usually 04. + +@comment sys/stat.h +@comment POSIX.1 +@item S_IWOTH +@vindex S_IWOTH +Write permission bit for other users. Usually 02. + +@comment sys/stat.h +@comment POSIX.1 +@item S_IXOTH +@vindex S_IXOTH +Execute or search permission bit for other users. Usually 01. + +@comment sys/stat.h +@comment POSIX.1 +@item S_IRWXO +@vindex S_IRWXO +This is equivalent to @samp{(S_IROTH | S_IWOTH | S_IXOTH)}. + +@comment sys/stat.h +@comment POSIX +@item S_ISUID +@vindex S_ISUID +This is the set-user-ID on execute bit, usually 04000. +@xref{How Change Persona}. + +@comment sys/stat.h +@comment POSIX +@item S_ISGID +@vindex S_ISGID +This is the set-group-ID on execute bit, usually 02000. +@xref{How Change Persona}. + +@cindex sticky bit +@comment sys/stat.h +@comment BSD +@item S_ISVTX +@vindex S_ISVTX +This is the @dfn{sticky} bit, usually 01000. + +On a directory, it gives permission to delete a file in the directory +only if you own that file. Ordinarily, a user either can delete all the +files in the directory or cannot delete any of them (based on whether +the user has write permission for the directory). The same restriction +applies---you must both have write permission for the directory and own +the file you want to delete. The one exception is that the owner of the +directory can delete any file in the directory, no matter who owns it +(provided the owner has given himself write permission for the +directory). This is commonly used for the @file{/tmp} directory, where +anyone may create files, but not delete files created by other users. + +Originally the sticky bit on an executable file modified the swapping +policies of the system. Normally, when a program terminated, its pages +in core were immediately freed and reused. If the sticky bit was set on +the executable file, the system kept the pages in core for a while as if +the program were still running. This was advantageous for a program +likely to be run many times in succession. This usage is obsolete in +modern systems. When a program terminates, its pages always remain in +core as long as there is no shortage of memory in the system. When the +program is next run, its pages will still be in core if no shortage +arose since the last run. + +On some modern systems where the sticky bit has no useful meaning for an +executable file, you cannot set the bit at all for a non-directory. +If you try, @code{chmod} fails with @code{EFTYPE}; +@pxref{Setting Permissions}. + +Some systems (particularly SunOS) have yet another use for the sticky +bit. If the sticky bit is set on a file that is @emph{not} executable, +it means the opposite: never cache the pages of this file at all. The +main use of this is for the files on an NFS server machine which are +used as the swap area of diskless client machines. The idea is that the +pages of the file will be cached in the client's memory, so it is a +waste of the server's memory to cache them a second time. In this use +the sticky bit also says that the filesystem may fail to record the +file's modification time onto disk reliably (the idea being that noone +cares for a swap file). +@end table + +The actual bit values of the symbols are listed in the table above +so you can decode file mode values when debugging your programs. +These bit values are correct for most systems, but they are not +guaranteed. + +@strong{Warning:} Writing explicit numbers for file permissions is bad +practice. It is not only nonportable, it also requires everyone who +reads your program to remember what the bits mean. To make your +program clean, use the symbolic names. + +@node Access Permission +@subsection How Your Access to a File is Decided +@cindex permission to access a file +@cindex access permission for a file +@cindex file access permission + +Recall that the operating system normally decides access permission for +a file based on the effective user and group IDs of the process, and its +supplementary group IDs, together with the file's owner, group and +permission bits. These concepts are discussed in detail in +@ref{Process Persona}. + +If the effective user ID of the process matches the owner user ID of the +file, then permissions for read, write, and execute/search are +controlled by the corresponding ``user'' (or ``owner'') bits. Likewise, +if any of the effective group ID or supplementary group IDs of the +process matches the group owner ID of the file, then permissions are +controlled by the ``group'' bits. Otherwise, permissions are controlled +by the ``other'' bits. + +Privileged users, like @samp{root}, can access any file, regardless of +its file permission bits. As a special case, for a file to be +executable even for a privileged user, at least one of its execute bits +must be set. + +@node Setting Permissions +@subsection Assigning File Permissions + +@cindex file creation mask +@cindex umask +The primitive functions for creating files (for example, @code{open} or +@code{mkdir}) take a @var{mode} argument, which specifies the file +permissions for the newly created file. But the specified mode is +modified by the process's @dfn{file creation mask}, or @dfn{umask}, +before it is used. + +The bits that are set in the file creation mask identify permissions +that are always to be disabled for newly created files. For example, if +you set all the ``other'' access bits in the mask, then newly created +files are not accessible at all to processes in the ``other'' +category, even if the @var{mode} argument specified to the creation +function would permit such access. In other words, the file creation +mask is the complement of the ordinary access permissions you want to +grant. + +Programs that create files typically specify a @var{mode} argument that +includes all the permissions that make sense for the particular file. +For an ordinary file, this is typically read and write permission for +all classes of users. These permissions are then restricted as +specified by the individual user's own file creation mask. + +@findex chmod +To change the permission of an existing file given its name, call +@code{chmod}. This function ignores the file creation mask; it uses +exactly the specified permission bits. + +@pindex umask +In normal use, the file creation mask is initialized in the user's login +shell (using the @code{umask} shell command), and inherited by all +subprocesses. Application programs normally don't need to worry about +the file creation mask. It will do automatically what it is supposed to +do. + +When your program should create a file and bypass the umask for its +access permissions, the easiest way to do this is to use @code{fchmod} +after opening the file, rather than changing the umask. + +In fact, changing the umask is usually done only by shells. They use +the @code{umask} function. + +The functions in this section are declared in @file{sys/stat.h}. +@pindex sys/stat.h + +@comment sys/stat.h +@comment POSIX.1 +@deftypefun mode_t umask (mode_t @var{mask}) +The @code{umask} function sets the file creation mask of the current +process to @var{mask}, and returns the previous value of the file +creation mask. + +Here is an example showing how to read the mask with @code{umask} +without changing it permanently: + +@smallexample +mode_t +read_umask (void) +@{ + mask = umask (0); + umask (mask); +@} +@end smallexample + +@noindent +However, it is better to use @code{getumask} if you just want to read +the mask value, because that is reentrant (at least if you use the GNU +operating system). +@end deftypefun + +@comment sys/stat.h +@comment GNU +@deftypefun mode_t getumask (void) +Return the current value of the file creation mask for the current +process. This function is a GNU extension. +@end deftypefun + +@comment sys/stat.h +@comment POSIX.1 +@deftypefun int chmod (const char *@var{filename}, mode_t @var{mode}) +The @code{chmod} function sets the access permission bits for the file +named by @var{filename} to @var{mode}. + +If the @var{filename} names a symbolic link, @code{chmod} changes the +permission of the file pointed to by the link, not those of the link +itself. + +This function returns @code{0} if successful and @code{-1} if not. In +addition to the usual file name errors (@pxref{File Name +Errors}), the following @code{errno} error conditions are defined for +this function: + +@table @code +@item ENOENT +The named file doesn't exist. + +@item EPERM +This process does not have permission to change the access permission of +this file. Only the file's owner (as judged by the effective user ID of +the process) or a privileged user can change them. + +@item EROFS +The file resides on a read-only file system. + +@item EFTYPE +@var{mode} has the @code{S_ISVTX} bit (the ``sticky bit'') set, +and the named file is not a directory. Some systems do not allow setting the +sticky bit on non-directory files, and some do (and only some of those +assign a useful meaning to the bit for non-directory files). + +You only get @code{EFTYPE} on systems where the sticky bit has no useful +meaning for non-directory files, so it is always safe to just clear the +bit in @var{mode} and call @code{chmod} again. @xref{Permission Bits}, +for full details on the sticky bit. +@end table +@end deftypefun + +@comment sys/stat.h +@comment BSD +@deftypefun int fchmod (int @var{filedes}, int @var{mode}) +This is like @code{chmod}, except that it changes the permissions of +the file currently open via descriptor @var{filedes}. + +The return value from @code{fchmod} is @code{0} on success and @code{-1} +on failure. The following @code{errno} error codes are defined for this +function: + +@table @code +@item EBADF +The @var{filedes} argument is not a valid file descriptor. + +@item EINVAL +The @var{filedes} argument corresponds to a pipe or socket, or something +else that doesn't really have access permissions. + +@item EPERM +This process does not have permission to change the access permission of +this file. Only the file's owner (as judged by the effective user ID of +the process) or a privileged user can change them. + +@item EROFS +The file resides on a read-only file system. +@end table +@end deftypefun + +@node Testing File Access +@subsection Testing Permission to Access a File +@cindex testing access permission +@cindex access, testing for +@cindex setuid programs and file access + +When a program runs as a privileged user, this permits it to access +files off-limits to ordinary users---for example, to modify +@file{/etc/passwd}. Programs designed to be run by ordinary users but +access such files use the setuid bit feature so that they always run +with @code{root} as the effective user ID. + +Such a program may also access files specified by the user, files which +conceptually are being accessed explicitly by the user. Since the +program runs as @code{root}, it has permission to access whatever file +the user specifies---but usually the desired behavior is to permit only +those files which the user could ordinarily access. + +The program therefore must explicitly check whether @emph{the user} +would have the necessary access to a file, before it reads or writes the +file. + +To do this, use the function @code{access}, which checks for access +permission based on the process's @emph{real} user ID rather than the +effective user ID. (The setuid feature does not alter the real user ID, +so it reflects the user who actually ran the program.) + +There is another way you could check this access, which is easy to +describe, but very hard to use. This is to examine the file mode bits +and mimic the system's own access computation. This method is +undesirable because many systems have additional access control +features; your program cannot portably mimic them, and you would not +want to try to keep track of the diverse features that different systems +have. Using @code{access} is simple and automatically does whatever is +appropriate for the system you are using. + +@code{access} is @emph{only} only appropriate to use in setuid programs. +A non-setuid program will always use the effective ID rather than the +real ID. + +@pindex unistd.h +The symbols in this section are declared in @file{unistd.h}. + +@comment unistd.h +@comment POSIX.1 +@deftypefun int access (const char *@var{filename}, int @var{how}) +The @code{access} function checks to see whether the file named by +@var{filename} can be accessed in the way specified by the @var{how} +argument. The @var{how} argument either can be the bitwise OR of the +flags @code{R_OK}, @code{W_OK}, @code{X_OK}, or the existence test +@code{F_OK}. + +This function uses the @emph{real} user and group ID's of the calling +process, rather than the @emph{effective} ID's, to check for access +permission. As a result, if you use the function from a @code{setuid} +or @code{setgid} program (@pxref{How Change Persona}), it gives +information relative to the user who actually ran the program. + +The return value is @code{0} if the access is permitted, and @code{-1} +otherwise. (In other words, treated as a predicate function, +@code{access} returns true if the requested access is @emph{denied}.) + +In addition to the usual file name errors (@pxref{File Name +Errors}), the following @code{errno} error conditions are defined for +this function: + +@table @code +@item EACCES +The access specified by @var{how} is denied. + +@item ENOENT +The file doesn't exist. + +@item EROFS +Write permission was requested for a file on a read-only file system. +@end table +@end deftypefun + +These macros are defined in the header file @file{unistd.h} for use +as the @var{how} argument to the @code{access} function. The values +are integer constants. +@pindex unistd.h + +@comment unistd.h +@comment POSIX.1 +@deftypevr Macro int R_OK +Argument that means, test for read permission. +@end deftypevr + +@comment unistd.h +@comment POSIX.1 +@deftypevr Macro int W_OK +Argument that means, test for write permission. +@end deftypevr + +@comment unistd.h +@comment POSIX.1 +@deftypevr Macro int X_OK +Argument that means, test for execute/search permission. +@end deftypevr + +@comment unistd.h +@comment POSIX.1 +@deftypevr Macro int F_OK +Argument that means, test for existence of the file. +@end deftypevr + +@node File Times +@subsection File Times + +@cindex file access time +@cindex file modification time +@cindex file attribute modification time +Each file has three timestamps associated with it: its access time, +its modification time, and its attribute modification time. These +correspond to the @code{st_atime}, @code{st_mtime}, and @code{st_ctime} +members of the @code{stat} structure; see @ref{File Attributes}. + +All of these times are represented in calendar time format, as +@code{time_t} objects. This data type is defined in @file{time.h}. +For more information about representation and manipulation of time +values, see @ref{Calendar Time}. +@pindex time.h + +Reading from a file updates its access time attribute, and writing +updates its modification time. When a file is created, all three +timestamps for that file are set to the current time. In addition, the +attribute change time and modification time fields of the directory that +contains the new entry are updated. + +Adding a new name for a file with the @code{link} function updates the +attribute change time field of the file being linked, and both the +attribute change time and modification time fields of the directory +containing the new name. These same fields are affected if a file name +is deleted with @code{unlink}, @code{remove}, or @code{rmdir}. Renaming +a file with @code{rename} affects only the attribute change time and +modification time fields of the two parent directories involved, and not +the times for the file being renamed. + +Changing attributes of a file (for example, with @code{chmod}) updates +its attribute change time field. + +You can also change some of the timestamps of a file explicitly using +the @code{utime} function---all except the attribute change time. You +need to include the header file @file{utime.h} to use this facility. +@pindex utime.h + +@comment time.h +@comment POSIX.1 +@deftp {Data Type} {struct utimbuf} +The @code{utimbuf} structure is used with the @code{utime} function to +specify new access and modification times for a file. It contains the +following members: + +@table @code +@item time_t actime +This is the access time for the file. + +@item time_t modtime +This is the modification time for the file. +@end table +@end deftp + +@comment time.h +@comment POSIX.1 +@deftypefun int utime (const char *@var{filename}, const struct utimbuf *@var{times}) +This function is used to modify the file times associated with the file +named @var{filename}. + +If @var{times} is a null pointer, then the access and modification times +of the file are set to the current time. Otherwise, they are set to the +values from the @code{actime} and @code{modtime} members (respectively) +of the @code{utimbuf} structure pointed at by @var{times}. + +The attribute modification time for the file is set to the current time +in either case (since changing the timestamps is itself a modification +of the file attributes). + +The @code{utime} function returns @code{0} if successful and @code{-1} +on failure. In addition to the usual file name errors +(@pxref{File Name Errors}), the following @code{errno} error conditions +are defined for this function: + +@table @code +@item EACCES +There is a permission problem in the case where a null pointer was +passed as the @var{times} argument. In order to update the timestamp on +the file, you must either be the owner of the file, have write +permission on the file, or be a privileged user. + +@item ENOENT +The file doesn't exist. + +@item EPERM +If the @var{times} argument is not a null pointer, you must either be +the owner of the file or be a privileged user. This error is used to +report the problem. + +@item EROFS +The file lives on a read-only file system. +@end table +@end deftypefun + +Each of the three time stamps has a corresponding microsecond part, +which extends its resolution. These fields are called +@code{st_atime_usec}, @code{st_mtime_usec}, and @code{st_ctime_usec}; +each has a value between 0 and 999,999, which indicates the time in +microseconds. They correspond to the @code{tv_usec} field of a +@code{timeval} structure; see @ref{High-Resolution Calendar}. + +The @code{utimes} function is like @code{utime}, but also lets you specify +the fractional part of the file times. The prototype for this function is +in the header file @file{sys/time.h}. +@pindex sys/time.h + +@comment sys/time.h +@comment BSD +@deftypefun int utimes (const char *@var{filename}, struct timeval @var{tvp}@t{[2]}) +This function sets the file access and modification times for the file +named by @var{filename}. The new file access time is specified by +@code{@var{tvp}[0]}, and the new modification time by +@code{@var{tvp}[1]}. This function comes from BSD. + +The return values and error conditions are the same as for the @code{utime} +function. +@end deftypefun + +@node Making Special Files +@section Making Special Files +@cindex creating special files +@cindex special files + +The @code{mknod} function is the primitive for making special files, +such as files that correspond to devices. The GNU library includes +this function for compatibility with BSD. + +The prototype for @code{mknod} is declared in @file{sys/stat.h}. +@pindex sys/stat.h + +@comment sys/stat.h +@comment BSD +@deftypefun int mknod (const char *@var{filename}, int @var{mode}, int @var{dev}) +The @code{mknod} function makes a special file with name @var{filename}. +The @var{mode} specifies the mode of the file, and may include the various +special file bits, such as @code{S_IFCHR} (for a character special file) +or @code{S_IFBLK} (for a block special file). @xref{Testing File Type}. + +The @var{dev} argument specifies which device the special file refers to. +Its exact interpretation depends on the kind of special file being created. + +The return value is @code{0} on success and @code{-1} on error. In addition +to the usual file name errors (@pxref{File Name Errors}), the +following @code{errno} error conditions are defined for this function: + +@table @code +@item EPERM +The calling process is not privileged. Only the superuser can create +special files. + +@item ENOSPC +The directory or file system that would contain the new file is full +and cannot be extended. + +@item EROFS +The directory containing the new file can't be modified because it's on +a read-only file system. + +@item EEXIST +There is already a file named @var{filename}. If you want to replace +this file, you must remove the old file explicitly first. +@end table +@end deftypefun + +@node Temporary Files +@section Temporary Files + +If you need to use a temporary file in your program, you can use the +@code{tmpfile} function to open it. Or you can use the @code{tmpnam} +function make a name for a temporary file and then open it in the usual +way with @code{fopen}. + +The @code{tempnam} function is like @code{tmpnam} but lets you choose +what directory temporary files will go in, and something about what +their file names will look like. + +These facilities are declared in the header file @file{stdio.h}. +@pindex stdio.h + +@comment stdio.h +@comment ANSI +@deftypefun {FILE *} tmpfile (void) +This function creates a temporary binary file for update mode, as if by +calling @code{fopen} with mode @code{"wb+"}. The file is deleted +automatically when it is closed or when the program terminates. (On +some other ANSI C systems the file may fail to be deleted if the program +terminates abnormally). +@end deftypefun + +@comment stdio.h +@comment ANSI +@deftypefun {char *} tmpnam (char *@var{result}) +This function constructs and returns a file name that is a valid file +name and that does not name any existing file. If the @var{result} +argument is a null pointer, the return value is a pointer to an internal +static string, which might be modified by subsequent calls. Otherwise, +the @var{result} argument should be a pointer to an array of at least +@code{L_tmpnam} characters, and the result is written into that array. + +It is possible for @code{tmpnam} to fail if you call it too many times. +This is because the fixed length of a temporary file name gives room for +only a finite number of different names. If @code{tmpnam} fails, it +returns a null pointer. +@end deftypefun + +@comment stdio.h +@comment ANSI +@deftypevr Macro int L_tmpnam +The value of this macro is an integer constant expression that represents +the minimum allocation size of a string large enough to hold the +file name generated by the @code{tmpnam} function. +@end deftypevr + +@comment stdio.h +@comment ANSI +@deftypevr Macro int TMP_MAX +The macro @code{TMP_MAX} is a lower bound for how many temporary names +you can create with @code{tmpnam}. You can rely on being able to call +@code{tmpnam} at least this many times before it might fail saying you +have made too many temporary file names. + +With the GNU library, you can create a very large number of temporary +file names---if you actually create the files, you will probably run out +of disk space before you run out of names. Some other systems have a +fixed, small limit on the number of temporary files. The limit is never +less than @code{25}. +@end deftypevr + +@comment stdio.h +@comment SVID +@deftypefun {char *} tempnam (const char *@var{dir}, const char *@var{prefix}) +This function generates a unique temporary filename. If @var{prefix} is +not a null pointer, up to five characters of this string are used as a +prefix for the file name. The return value is a string newly allocated +with @code{malloc}; you should release its storage with @code{free} when +it is no longer needed. + +The directory prefix for the temporary file name is determined by testing +each of the following, in sequence. The directory must exist and be +writable. + +@itemize @bullet +@item +The environment variable @code{TMPDIR}, if it is defined. + +@item +The @var{dir} argument, if it is not a null pointer. + +@item +The value of the @code{P_tmpdir} macro. + +@item +The directory @file{/tmp}. +@end itemize + +This function is defined for SVID compatibility. +@end deftypefun +@cindex TMPDIR environment variable + +@comment stdio.h +@comment SVID +@c !!! are we putting SVID/GNU/POSIX.1/BSD in here or not?? +@deftypevr {SVID Macro} {char *} P_tmpdir +This macro is the name of the default directory for temporary files. +@end deftypevr + +Older Unix systems did not have the functions just described. Instead +they used @code{mktemp} and @code{mkstemp}. Both of these functions +work by modifying a file name template string you pass. The last six +characters of this string must be @samp{XXXXXX}. These six @samp{X}s +are replaced with six characters which make the whole string a unique +file name. Usually the template string is something like +@samp{/tmp/@var{prefix}XXXXXX}, and each program uses a unique @var{prefix}. + +@strong{Note:} Because @code{mktemp} and @code{mkstemp} modify the +template string, you @emph{must not} pass string constants to them. +String constants are normally in read-only storage, so your program +would crash when @code{mktemp} or @code{mkstemp} tried to modify the +string. + +@comment unistd.h +@comment Unix +@deftypefun {char *} mktemp (char *@var{template}) +The @code{mktemp} function generates a unique file name by modifying +@var{template} as described above. If successful, it returns +@var{template} as modified. If @code{mktemp} cannot find a unique file +name, it makes @var{template} an empty string and returns that. If +@var{template} does not end with @samp{XXXXXX}, @code{mktemp} returns a +null pointer. +@end deftypefun + +@comment unistd.h +@comment BSD +@deftypefun int mkstemp (char *@var{template}) +The @code{mkstemp} function generates a unique file name just as +@code{mktemp} does, but it also opens the file for you with @code{open} +(@pxref{Opening and Closing Files}). If successful, it modifies +@var{template} in place and returns a file descriptor open on that file +for reading and writing. If @code{mkstemp} cannot create a +uniquely-named file, it makes @var{template} an empty string and returns +@code{-1}. If @var{template} does not end with @samp{XXXXXX}, +@code{mkstemp} returns @code{-1} and does not modify @var{template}. +@end deftypefun + +Unlike @code{mktemp}, @code{mkstemp} is actually guaranteed to create a +unique file that cannot possibly clash with any other program trying to +create a temporary file. This is because it works by calling +@code{open} with the @code{O_EXCL} flag bit, which says you want to +always create a new file, and get an error if the file already exists. |