chmod() - Unix, Linux System Call
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chmod() - Unix, Linux System Call

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chmod, fchmod - change permissions of a file


#include <sys/types.h>
#include <sys/stat.h> 

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


The mode of the file given by path or referenced by fildes is changed.

Modes are specified by or’ing the following:

S_ISUID 04000 set user ID on execution
S_ISGID 02000 set group ID on execution
S_ISVTX 01000 sticky bit
S_IRUSR 00400 read by owner
S_IWUSR 00200 write by owner
S_IXUSR 00100 execute/search by owner
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:

Error CodeDescription
EACCES Search permission is denied on a component of the path prefix. (See also path_resolution(2).)
EFAULT path points outside your accessible address space.
EIO An I/O error occurred.
ELOOP Too many symbolic links were encountered in resolving path.
ENAMETOOLONG path is too long.
ENOENT The file does not exist.
ENOMEM Insufficient kernel memory was available.
ENOTDIR A component of the path prefix is not a directory.
EPERM 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).
EROFS The named file resides on a read-only file system.
The general errors for fchmod() are listed below:
EBADF The file descriptor fildes is not valid.
EIO See above.
EPERM See above.
EROFS See above.


4.4BSD, SVr4, POSIX.1-2001.


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