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NAME |LIBRARY |SYNOPSIS |DESCRIPTION |RETURN VALUE |ERRORS |VERSIONS |STANDARDS |HISTORY |SEE ALSO |COLOPHON | |
chmod(2) System Calls Manualchmod(2)chmod, fchmod, fchmodat - change permissions of a file
Standard C library (libc,-lc)
#include <sys/stat.h>int chmod(const char *path, mode_tmode);int fchmod(intfd, mode_tmode);#include <fcntl.h>/* Definition of AT_* constants */#include <sys/stat.h>int fchmodat(intdirfd, const char *path, mode_tmode, intflags); Feature Test Macro Requirements for glibc (seefeature_test_macros(7)):fchmod(): Since glibc 2.24: _POSIX_C_SOURCE >= 199309L glibc 2.19 to glibc 2.23 _POSIX_C_SOURCE glibc 2.16 to glibc 2.19: _BSD_SOURCE || _POSIX_C_SOURCE glibc 2.12 to glibc 2.16: _BSD_SOURCE || _XOPEN_SOURCE >= 500 || _POSIX_C_SOURCE >= 200809L glibc 2.11 and earlier: _BSD_SOURCE || _XOPEN_SOURCE >= 500fchmodat(): Since glibc 2.10: _POSIX_C_SOURCE >= 200809L Before glibc 2.10: _ATFILE_SOURCE
Thechmod() andfchmod() system calls change a file's 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 inpath, which is dereferenced if it is a symbolic link. •fchmod() changes the mode of the file referred to by the open file descriptorfd. The new file mode is specified inmode, 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 onexecve(2))S_ISGID(02000) set-group-ID (set process effective group ID onexecve(2); mandatory locking, as described infcntl(2); take a new file's group from parent directory, as described inchown(2) andmkdir(2))S_ISVTX(01000) sticky bit (restricted deletion flag, as described inunlink(2))S_IRUSR(00400) read by ownerS_IWUSR(00200) write by ownerS_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 groupS_IWGRP(00020) write by groupS_IXGRP(00010) execute/search by groupS_IROTH(00004) read by othersS_IWOTH(00002) write by othersS_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 theCAP_FOWNERcapability). If the calling process is not privileged (Linux: does not have theCAP_FSETIDcapability), and the group of the file does not match the effective group ID of the process or one of its supplementary group IDs, theS_ISGIDbit 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 theCAP_FSETIDcapability.) 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, seeinode(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.fchmodat() Thefchmodat() system call operates in exactly the same way aschmod(), except for the differences described here. Ifpath is relative, then it is interpreted relative to the directory referred to by the file descriptordirfd (rather than relative to the current working directory of the calling process, as is done bychmod() for a relative pathname). Ifpath is relative anddirfd is the special valueAT_FDCWD, thenpath is interpreted relative to the current working directory of the calling process (likechmod()). Ifpath is absolute, thendirfd is ignored.flags can either be 0, or include the following flags:AT_EMPTY_PATH(since Linux 6.6) Ifpath is an empty string, operate on the file referred to bydirfd (which may have been obtained using theopen(2)O_PATHflag). In this case,dirfd can refer to any type of file, not just a directory. Ifdirfd isAT_FDCWD, the call operates on the current working directory. This flag is Linux-specific; define_GNU_SOURCEto obtain its definition.AT_SYMLINK_NOFOLLOW Ifpath is a symbolic link, do not dereference it: instead operate on the link itself. Seeopenat(2) for an explanation of the need forfchmodat().On success, zero is returned. On error, -1 is returned, anderrno is set to indicate the error.
Depending on the filesystem, errors other than those listed below can be returned. The more general errors forchmod() are listed below:EACCESSearch permission is denied on a component of the path prefix. (See alsopath_resolution(7).)EBADF(fchmod()) The file descriptorfd is not valid.EBADF(fchmodat())path is relative butdirfd is neitherAT_FDCWD nor a valid file descriptor.EFAULTpath points outside your accessible address space.EINVAL(fchmodat()) Invalid flag specified inflags.EIOAn I/O error occurred.ELOOPToo many symbolic links were encountered in resolvingpath.ENAMETOOLONGpath is too long.ENOENTThe file does not exist.ENOMEMInsufficient kernel memory was available.ENOTDIR A component of the path prefix is not a directory.ENOTDIR (fchmodat())path is relative anddirfd is a file descriptor referring to a file other than a directory.ENOTSUP (fchmodat())flags specifiedAT_SYMLINK_NOFOLLOW, which is not supported.EPERMThe effective UID does not match the owner of the file, and the process is not privileged (Linux: it does not have theCAP_FOWNERcapability).EPERMThe file is marked immutable or append-only. (SeeFS_IOC_SETFLAGS(2const).)EROFSThe named file resides on a read-only filesystem.
C library/kernel differences The GNU C libraryfchmodat() wrapper function implements the POSIX-specified interface described in this page. This interface differs from the underlying Linux system call, which doesnot have aflags argument.glibc notes On older kernels wherefchmodat() is unavailable, the glibc wrapper function falls back to the use ofchmod(). Whenpath is a relative pathname, glibc constructs a pathname based on the symbolic link in/proc/self/fd that corresponds to thedirfd argument.
POSIX.1-2008.
chmod()fchmod() 4.4BSD, SVr4, POSIX.1-2001.fchmodat() POSIX.1-2008. Linux 2.6.16, glibc 2.4.AT_SYMLINK_NOFOLLOW glibc 2.32, Linux 6.5.
chmod(1),chown(2),execve(2),open(2),stat(2),inode(7),path_resolution(7),symlink(7)
This page is part of theman-pages (Linux kernel and C library user-space interface documentation) project. Information about the project can be found at ⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report for this manual page, see ⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩. This page was obtained from the tarball man-pages-6.15.tar.gz fetched from ⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on 2025-08-11. If you discover any rendering problems in this HTML version of the page, or you believe there is a better or more up- to-date source for the page, or you have corrections or improvements to the information in this COLOPHON (which isnot part of the original manual page), send a mail to man-pages@man7.orgLinux man-pages 6.15 2025-06-28chmod(2)Pages that refer to this page:chmod(1), access(2), chown(2), execve(2), fcntl_locking(2), F_NOTIFY(2const), mkdir(2), mknod(2), open(2), rename(2), rmdir(2), stat(2), statx(2), syscalls(2), umask(2), unlink(2), euidaccess(3), mode_t(3type), shm_open(3), capabilities(7), inotify(7), landlock(7), shm_overview(7), signal-safety(7), spufs(7), symlink(7), unix(7), logrotate(8), xfs_db(8)
HTML rendering created 2025-09-06 byMichael Kerrisk, author ofThe Linux Programming Interface. For details of in-depthLinux/UNIX system programming training courses that I teach, lookhere. Hosting byjambit GmbH. | |