chmod - change permissions of a file


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

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


The mode of the file given by path 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 (S_IREAD)| 00400| read by owner |S_IWUSR (S_IWRITE)| 00200| write by owner |S_IXUSR (S_IEXEC)| 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 process must be zero or must match the owner of the file.

If the effective UID of the process is not zero 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.

Depending on the file system, set user ID and set group ID execution bits may be turned off if a file is written. On some file systems, only the super-user 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:

The effective UID does not match the owner of the file, and is not zero.
The named file resides on a read-only file system.
path points outside your accessible address space.
path is too long.
The file does not exist.
Insufficient kernel memory was available.
A component of the path prefix is not a directory.
Search permission is denied on a component of the path prefix.
Too many symbolic links were encountered in resolving path.
An I/O error occurred.


The chmod call conforms to SVr4, SVID, POSIX, X/OPEN, 4.4BSD. SVr4 documents EINTR, ENOLINK and EMULTIHOP returns, but no ENOMEM. POSIX.1 does not document EFAULT, ENOMEM, ELOOP or EIO error conditions, or the macros S_IREAD, S_IWRITE and S_IEXEC.

POSIX and X/OPEN do not document the sticky bit.


open(2), chown(2), execve(2), stat(2)

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