.\" Copyright (c) 1992 Drew Eckhardt (drew@cs.colorado.edu), March 28, 1992 .\" and Copyright (C) 2006, 2014 Michael Kerrisk .\" .\" %%%LICENSE_START(VERBATIM) .\" Permission is granted to make and distribute verbatim copies of this .\" manual provided the copyright notice and this permission notice are .\" preserved on all copies. .\" .\" Permission is granted to copy and distribute modified versions of this .\" manual under the conditions for verbatim copying, provided that the .\" entire resulting derived work is distributed under the terms of a .\" permission notice identical to this one. .\" .\" Since the Linux kernel and libraries are constantly changing, this .\" manual page may be incorrect or out-of-date. The author(s) assume no .\" responsibility for errors or omissions, or for damages resulting from .\" the use of the information contained herein. The author(s) may not .\" have taken the same level of care in the production of this manual, .\" which is licensed free of charge, as they might when working .\" professionally. .\" .\" Formatted or processed versions of this manual, if unaccompanied by .\" the source, must acknowledge the copyright and authors of this work. .\" %%%LICENSE_END .\" .\" Modified by Michael Haardt .\" Modified 1993-07-21 by Rik Faith .\" Modified 1997-01-12 by Michael Haardt .\" : NFS details .\" Modified 2004-06-23 by Michael Kerrisk .\" .TH CHMOD 2 2014-08-19 "Linux" "Linux Programmer's Manual" .SH NAME chmod, fchmod, fchmodat \- change permissions of a file .SH SYNOPSIS .nf .B #include .sp .BI "int chmod(const char *" pathname ", mode_t " mode ); .br .BI "int fchmod(int " fd ", mode_t " mode ); .sp .BR "#include " " /* Definition of AT_* constants */" .B #include .sp .BI "int fchmodat(int " dirfd ", const char *" pathname ", mode_t " \ mode ", int " flags ); .fi .sp .in -4n Feature Test Macro Requirements for glibc (see .BR feature_test_macros (7)): .in .sp .ad l .PD 0 .BR fchmod (): .RS 4 _BSD_SOURCE || _XOPEN_SOURCE\ >=\ 500 || _XOPEN_SOURCE\ &&\ _XOPEN_SOURCE_EXTENDED .br || /* Since glibc 2.12: */ _POSIX_C_SOURCE\ >=\ 200809L .RE .PD .sp .BR fchmodat (): .PD 0 .ad l .RS 4 .TP 4 Since glibc 2.10: _XOPEN_SOURCE\ >=\ 700 || _POSIX_C_SOURCE\ >=\ 200809L .TP Before glibc 2.10: _ATFILE_SOURCE .RE .ad .PD .ad .SH DESCRIPTION The .BR chmod () and .BR fchmod () system calls change the permissions of a file. They differ only in how the file is specified: .IP * 2 .BR chmod () changes the permissions of the file specified whose pathname is given in .IR pathname , which is dereferenced if it is a symbolic link. .IP * .BR fchmod () changes the permissions of the file referred to by the open file descriptor .IR fd . .PP The new file permissions are specified in .IR mode , which is a bit mask created by ORing together zero or more of the following: .TP 18 .BR S_ISUID " (04000)" set-user-ID (set process effective user ID on .BR execve (2)) .TP .BR S_ISGID " (02000)" set-group-ID (set process effective group ID on .BR execve (2); mandatory locking, as described in .BR fcntl (2); take a new file's group from parent directory, as described in .BR chown (2) and .BR mkdir (2)) .TP .BR S_ISVTX " (01000)" sticky bit (restricted deletion flag, as described in .BR unlink (2)) .TP .BR S_IRUSR " (00400)" read by owner .TP .BR S_IWUSR " (00200)" write by owner .TP .BR S_IXUSR " (00100)" execute/search by owner ("search" applies for directories, and means that entries within the directory can be accessed) .TP .BR S_IRGRP " (00040)" read by group .TP .BR S_IWGRP " (00020)" write by group .TP .BR S_IXGRP " (00010)" execute/search by group .TP .BR S_IROTH " (00004)" read by others .TP .BR S_IWOTH " (00002)" write by others .TP .BR S_IXOTH " (00001)" execute/search by others .PP 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 .B CAP_FOWNER capability). If the calling process is not privileged (Linux: does not have the .B 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 .B 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 .B 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 .BR stat (2). 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. .\" .\" .SS fchmodat() The .BR fchmodat () system call operates in exactly the same way as .BR chmod (), except for the differences described here. If the pathname given in .I pathname is relative, then it is interpreted relative to the directory referred to by the file descriptor .I dirfd (rather than relative to the current working directory of the calling process, as is done by .BR chmod () for a relative pathname). If .I pathname is relative and .I dirfd is the special value .BR AT_FDCWD , then .I pathname is interpreted relative to the current working directory of the calling process (like .BR chmod ()). If .I pathname is absolute, then .I dirfd is ignored. .I flags can either be 0, or include the following flag: .TP .B AT_SYMLINK_NOFOLLOW If .I pathname is a symbolic link, do not dereference it: instead operate on the link itself. This flag is not currently implemented. .PP See .BR openat (2) for an explanation of the need for .BR fchmodat (). .SH RETURN VALUE On success, zero is returned. On error, \-1 is returned, and .I errno is set appropriately. .SH ERRORS Depending on the filesystem, errors other than those listed below can be returned. The more general errors for .BR chmod () are listed below: .TP .B EACCES Search permission is denied on a component of the path prefix. (See also .BR path_resolution (7).) .TP .B EFAULT .I pathname points outside your accessible address space. .TP .B EIO An I/O error occurred. .TP .B ELOOP Too many symbolic links were encountered in resolving .IR pathname . .TP .B ENAMETOOLONG .I pathname is too long. .TP .B ENOENT The file does not exist. .TP .B ENOMEM Insufficient kernel memory was available. .TP .B ENOTDIR A component of the path prefix is not a directory. .TP .B EPERM The effective UID does not match the owner of the file, and the process is not privileged (Linux: it does not have the .B CAP_FOWNER capability). .TP .B EROFS The named file resides on a read-only filesystem. .PP The general errors for .BR fchmod () are listed below: .TP .B EBADF The file descriptor .I fd is not valid. .TP .B EIO See above. .TP .B EPERM See above. .TP .B EROFS See above. .PP The same errors that occur for .BR chmod () can also occur for .BR fchmodat (). The following additional errors can occur for .BR fchmodat (): .TP .B EBADF .I dirfd is not a valid file descriptor. .TP .B EINVAL Invalid flag specified in .IR flags . .TP .B ENOTDIR .I pathname is relative and .I dirfd is a file descriptor referring to a file other than a directory. .TP .B ENOTSUP .I flags specified .BR AT_SYMLINK_NOFOLLOW , which is not supported. .SH VERSIONS .BR fchmodat () was added to Linux in kernel 2.6.16; library support was added to glibc in version 2.4. .SH CONFORMING TO .BR chmod (), .BR fchmod (): 4.4BSD, SVr4, POSIX.1-2001i, POSIX.1-2008. .BR fchmodat (): POSIX.1-2008. .SH NOTES .SS C library/kernel ABI differences The GNU C library .BR fchmodat () wrapper function implements the POSIX-specified interface described in this page. This interface differs from the underlying Linux system call, which does .I not have a .I flags argument. .SS Glibc notes On older kernels where .BR fchmodat () is unavailable, the glibc wrapper function falls back to the use of .BR chmod (). When .I pathname is a relative pathname, glibc constructs a pathname based on the symbolic link in .IR /proc/self/fd that corresponds to the .IR dirfd argument. .SH SEE ALSO .BR chown (2), .BR execve (2), .BR open (2), .BR stat (2), .BR path_resolution (7), .BR symlink (7) .SH COLOPHON This page is part of release 3.74 of the Linux .I 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/.