| .\" 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| .\" manual page may be incorrect or out-of-date.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 <michael@moria.de> |
| .\" Modified 1993-07-21 by Rik Faith <faith@cs.unc.edu> |
| .\" Modified 1997-01-12 by Michael Haardt |
| .\" <michael@cantor.informatik.rwth-aachen.de>: NFS details |
| .\" Modified 2004-06-23 by Michael Kerrisk <mtk.manpages@gmail.com> |
| .\" |
| .TH CHMOD 2 2021-03-22 "Linux" "Linux Programmer's Manual" |
| .SH NAME |
| chmod, fchmod, fchmodat \- change permissions of a file |
| .SH SYNOPSIS |
| .nf |
| .B #include <sys/stat.h> |
| .PP |
| .BI "int chmod(const char *" pathname ", mode_t " mode ); |
| .BI "int fchmod(int " fd ", mode_t " mode ); |
| .PP |
| .BR "#include <fcntl.h>" " /* Definition of AT_* constants */" |
| .B #include <sys/stat.h> |
| .PP |
| .BI "int fchmodat(int " dirfd ", const char *" pathname ", mode_t " \ |
| mode ", int " flags ); |
| .fi |
| .PP |
| .RS -4 |
| Feature Test Macro Requirements for glibc (see |
| .BR feature_test_macros (7)): |
| .RE |
| .PP |
| .nf |
| .BR fchmod (): |
| Since glibc 2.24: |
| _POSIX_C_SOURCE >= 199309L |
| .\" || (_XOPEN_SOURCE && _XOPEN_SOURCE_EXTENDED) |
| Glibc 2.19 to 2.23 |
| _POSIX_C_SOURCE |
| Glibc 2.16 to 2.19: |
| _BSD_SOURCE || _POSIX_C_SOURCE |
| Glibc 2.12 to 2.16: |
| _BSD_SOURCE || _XOPEN_SOURCE >= 500 |
| || _POSIX_C_SOURCE >= 200809L |
| Glibc 2.11 and earlier: |
| _BSD_SOURCE || _XOPEN_SOURCE >= 500 |
| .\" || (_XOPEN_SOURCE && _XOPEN_SOURCE_EXTENDED) |
| .fi |
| .PP |
| .BR fchmodat (): |
| .nf |
| Since glibc 2.10: |
| _POSIX_C_SOURCE >= 200809L |
| Before glibc 2.10: |
| _ATFILE_SOURCE |
| .fi |
| .SH DESCRIPTION |
| The |
| .BR chmod () |
| and |
| .BR 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: |
| .IP * 2 |
| .BR chmod () |
| changes the mode 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 mode of the file referred to by the open file descriptor |
| .IR fd . |
| .PP |
| The new file mode is 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). |
| .PP |
| 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. |
| .PP |
| 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 inode (7). |
| .PP |
| 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. |
| .PP |
| 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). |
| .PP |
| 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 ()). |
| .PP |
| If |
| .I pathname |
| is absolute, then |
| .I dirfd |
| is ignored. |
| .PP |
| .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 to indicate the error. |
| .SH ERRORS |
| Depending on the filesystem, |
| errors other than those listed below can be returned. |
| .PP |
| 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 EPERM |
| The file is marked immutable or append-only. |
| (See |
| .BR ioctl_iflags (2).) |
| .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. |
| .PP |
| .BR fchmodat (): |
| POSIX.1-2008. |
| .SH NOTES |
| .SS C library/kernel 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 chmod (1), |
| .BR chown (2), |
| .BR execve (2), |
| .BR open (2), |
| .BR stat (2), |
| .BR inode (7), |
| .BR path_resolution (7), |
| .BR symlink (7) |
