fchmodat (2) - Linux Manuals

fchmodat: change permissions of a file


chmod, fchmod, fchmodat - change permissions of a file


#include <sys/stat.h>

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

#include <fcntl.h>           /* Definition of AT_* constants */
#include <sys/stat.h>

int fchmodat(int dirfd, const char *pathname, mode_t mode, int flags);

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


Since glibc 2.24:
    _POSIX_C_SOURCE >= 199309L

Glibc 2.19 to 2.23

Glibc 2.16 to 2.19:

Glibc 2.12 to 2.16:
    _BSD_SOURCE || _XOPEN_SOURCE >= 500 ||
        _POSIX_C_SOURCE >= 200809L

Glibc 2.11 and earlier:


Since glibc 2.10:
_POSIX_C_SOURCE >= 200809L
Before glibc 2.10:


The chmod() and fchmod() system calls change a files mode bits. (The file mode consists of the file permission bits plus the set-user-ID, set-group-ID, and sticky bits.) These system calls differ only in how the file is specified:
chmod() changes the mode of the file specified whose pathname is given in pathname, which is dereferenced if it is a symbolic link.
fchmod() changes the mode of the file referred to by the open file descriptor fd.

The new file mode is 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 filesystem, 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 filesystems, 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 inode(7).

On NFS filesystems, 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.


The fchmodat() system call operates in exactly the same way as chmod(), except for the differences described here.

If the pathname given in pathname is relative, then it is interpreted relative to the directory referred to by the file descriptor dirfd (rather than relative to the current working directory of the calling process, as is done by chmod() for a relative pathname).

If pathname is relative and dirfd is the special value AT_FDCWD, then pathname is interpreted relative to the current working directory of the calling process (like chmod()).

If pathname is absolute, then dirfd is ignored.

flags can either be 0, or include the following flag:

If pathname is a symbolic link, do not dereference it: instead operate on the link itself. This flag is not currently implemented.

See openat(2) for an explanation of the need for fchmodat().


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


Depending on the filesystem, errors other than those listed below 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).)
pathname points outside your accessible address space.
An I/O error occurred.
Too many symbolic links were encountered in resolving pathname.
pathname 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 file is marked immutable or append-only. (See ioctl_iflags(2).)
The named file resides on a read-only filesystem.

The general errors for fchmod() are listed below:

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

The same errors that occur for chmod() can also occur for fchmodat(). The following additional errors can occur for fchmodat():

dirfd is not a valid file descriptor.
Invalid flag specified in flags.
pathname is relative and dirfd is a file descriptor referring to a file other than a directory.
flags specified AT_SYMLINK_NOFOLLOW, which is not supported.


fchmodat() was added to Linux in kernel 2.6.16; library support was added to glibc in version 2.4.


chmod(), fchmod(): 4.4BSD, SVr4, POSIX.1-2001i, POSIX.1-2008.

fchmodat(): POSIX.1-2008.


C library/kernel differences

The GNU C library fchmodat() wrapper function implements the POSIX-specified interface described in this page. This interface differs from the underlying Linux system call, which does not have a flags argument.

Glibc notes

On older kernels where fchmodat() is unavailable, the glibc wrapper function falls back to the use of chmod(). When pathname is a relative pathname, glibc constructs a pathname based on the symbolic link in /proc/self/fd that corresponds to the dirfd argument.


This page is part of release 5.10 of the Linux man-pages project. A description of the project, information about reporting bugs, and the latest version of this page, can be found at https://www.kernel.org/doc/man-pages/.


chmod(1), chown(2), execve(2), open(2), stat(2), inode(7), path_resolution(7), symlink(7)