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Version: 2008-05-26 (ubuntu - 07/07/09)

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Section: 2 (Appels système)


chmod, fchmod - change permissions of a file


#include <sys/stat.h>

int chmod(const char *path, mode_t mode);
int fchmod(int fd, mode_t mode);

Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

fchmod(): _BSD_SOURCE || _XOPEN_SOURCE >= 500


These system calls change the permissions of a file. They differ only in how the file is specified:
chmod() changes the permissions of the file specified whose pathname is given in path, which is dereferenced if it is a symbolic link.
fchmod() changes the permissions of the file referred to by the open file descriptor fd.

The new file permissions are specified in mode, which is a bit mask created by ORing together zero or more of the following:

S_ISUID (04000)
set-user-ID (set process effective user ID on execve(2))
S_ISGID (02000)
set-group-ID (set process effective group ID on execve(2); mandatory locking, as described in fcntl(2); take a new file's group from parent directory, as described in chown(2) and mkdir(2))
S_ISVTX (01000)
sticky bit (restricted deletion flag, as described in unlink(2))
S_IRUSR (00400)
read by owner
S_IWUSR (00200)
write by owner
S_IXUSR (00100)
execute/search by owner ("search" applies for directories, and means that entries within the directory can be accessed)
S_IRGRP (00040)
read by group
S_IWGRP (00020)
write by group
S_IXGRP (00010)
execute/search by group
S_IROTH (00004)
read by others
S_IWOTH (00002)
write by others
S_IXOTH (00001)
execute/search by others

The effective UID of the calling process must match the owner of the file, or the process must be privileged (Linux: it must have the CAP_FOWNER capability).

If the calling process is not privileged (Linux: does not have the CAP_FSETID capability), and the group of the file does not match the effective group ID of the process or one of its supplementary group IDs, the S_ISGID bit will be turned off, but this will not cause an error to be returned.

As a security measure, depending on the file system, the set-user-ID and set-group-ID execution bits may be turned off if a file is written. (On Linux this occurs if the writing process does not have the CAP_FSETID capability.) On some file systems, only the superuser can set the sticky bit, which may have a special meaning. For the sticky bit, and for set-user-ID and set-group-ID bits on directories, see stat(2).

On NFS file systems, restricting the permissions will immediately influence already open files, because the access control is done on the server, but open files are maintained by the client. Widening the permissions may be delayed for other clients if attribute caching is enabled on them.


On success, zero is returned. On error, -1 is returned, and errno is set appropriately.


Depending on the file system, other errors can be returned. The more general errors for chmod() are listed below:
Search permission is denied on a component of the path prefix. (See also path_resolution(7).)
path points outside your accessible address space.
An I/O error occurred.
Too many symbolic links were encountered in resolving path.
path is too long.
The file does not exist.
Insufficient kernel memory was available.
A component of the path prefix is not a directory.
The effective UID does not match the owner of the file, and the process is not privileged (Linux: it does not have the CAP_FOWNER capability).
The named file resides on a read-only file system.

The general errors for fchmod() are listed below:

The file descriptor fd is not valid.
See above.
See above.
See above.


4.4BSD, SVr4, POSIX.1-2001.


chown(2), execve(2), fchmodat(2), open(2), stat(2), path_resolution(7)


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