fchmod - change permissions of a file


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

int fchmod(int fildes, mode_t mode);


The mode of the file 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 (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.
Insufficient kernel memory was available.
A component of the path prefix is not a directory.
An I/O error occurred.
The file descriptor fildes is not valid.


The fchmod(2) call conforms to 4.4BSD and SVr4. SVr4 documents additional EINTR and ENOLINK error conditions. POSIX requires the fchmod(2) function if at least one of POSIX_MAPPED_FILES and _POSIX_SHARED_MEMORY_OBJECTS is defined, and documents additional ENOSYS and EINVAL error conditions, but does not document EIO.

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


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

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