|CHMOD(2)||Linux Programmer's Manual||CHMOD(2)|
NAME¶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)):
_BSD_SOURCE || _XOPEN_SOURCE >= 500 || _XOPEN_SOURCE && _XOPEN_SOURCE_EXTENDED|| /* Since glibc 2.12: */ _POSIX_C_SOURCE >= 200809Lfchmodat():
- Since glibc 2.10:
- _XOPEN_SOURCE >= 700 || _POSIX_C_SOURCE >= 200809L
- Before glibc 2.10:
DESCRIPTION¶The chmod() and fchmod() 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 pathname, which is dereferenced if it is a symbolic link.
- fchmod() changes the permissions of the file referred to by the open file descriptor fd.
- 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
fchmodat()¶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.
RETURN VALUE¶On success, zero is returned. On error, -1 is returned, and errno is set appropriately.
ERRORS¶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 named file resides on a read-only filesystem.
- The file descriptor fd is not valid.
- See above.
- See above.
- See above.
- 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.
VERSIONS¶fchmodat() was added to Linux in kernel 2.6.16; library support was added to glibc in version 2.4.
CONFORMING TO¶chmod(), fchmod(): 4.4BSD, SVr4, POSIX.1-2001i, POSIX.1-2008. fchmodat(): POSIX.1-2008.
C library/kernel ABI 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.
SEE ALSO¶chown(2), execve(2), open(2), stat(2), path_resolution(7), symlink(7)
COLOPHON¶This page is part of release 3.74 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 http://www.kernel.org/doc/man-pages/.