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PROLOG
This manual page is part of the POSIX Programmer's Manual. The Linux implementation of this interface
may differ (consult the corresponding Linux manual page for details of Linux behavior), or the interface
may not be implemented on Linux.
NAME
chmod, fchmodat — change mode of a file
SYNOPSIS
#include <sys/stat.h>
int chmod(const char *path, mode_t mode);
#include <fcntl.h>
int fchmodat(int fd, const char *path, mode_t mode, int flag);
DESCRIPTION
The chmod() function shall change S_ISUID, S_ISGID, S_ISVTX, and the file permission bits of the file
named by the pathname pointed to by the path argument to the corresponding bits in the mode argument. The
application shall ensure that the effective user ID of the process matches the owner of the file or the
process has appropriate privileges in order to do this.
S_ISUID, S_ISGID, S_ISVTX, and the file permission bits are described in <sys/stat.h>.
If the calling process does not have appropriate privileges, and if the group ID of the file does not
match the effective group ID or one of the supplementary group IDs and if the file is a regular file, bit
S_ISGID (set-group-ID on execution) in the file's mode shall be cleared upon successful return from
chmod().
Additional implementation-defined restrictions may cause the S_ISUID and S_ISGID bits in mode to be
ignored.
Upon successful completion, chmod() shall mark for update the last file status change timestamp of the
file.
The fchmodat() function shall be equivalent to the chmod() function except in the case where path
specifies a relative path. In this case the file to be changed is determined relative to the directory
associated with the file descriptor fd instead of the current working directory. If the access mode of
the open file description associated with the file descriptor is not O_SEARCH, the function shall check
whether directory searches are permitted using the current permissions of the directory underlying the
file descriptor. If the access mode is O_SEARCH, the function shall not perform the check.
Values for flag are constructed by a bitwise-inclusive OR of flags from the following list, defined in
<fcntl.h>:
AT_SYMLINK_NOFOLLOW
If path names a symbolic link, then the mode of the symbolic link is changed.
If fchmodat() is passed the special value AT_FDCWD in the fd parameter, the current working directory
shall be used. If also flag is zero, the behavior shall be identical to a call to chmod().
RETURN VALUE
Upon successful completion, these functions shall return 0. Otherwise, these functions shall return -1
and set errno to indicate the error. If -1 is returned, no change to the file mode occurs.
ERRORS
These functions shall fail if:
EACCES Search permission is denied on a component of the path prefix.
ELOOP A loop exists in symbolic links encountered during resolution of the path argument.
ENAMETOOLONG
The length of a component of a pathname is longer than {NAME_MAX}.
ENOENT A component of path does not name an existing file or path is an empty string.
ENOTDIR
A component of the path prefix names an existing file that is neither a directory nor a symbolic
link to a directory, or the path argument contains at least one non-<slash> character and ends
with one or more trailing <slash> characters and the last pathname component names an existing
file that is neither a directory nor a symbolic link to a directory.
EPERM The effective user ID does not match the owner of the file and the process does not have
appropriate privileges.
EROFS The named file resides on a read-only file system.
The fchmodat() function shall fail if:
EACCES The access mode of the open file description associated with fd is not O_SEARCH and the
permissions of the directory underlying fd do not permit directory searches.
EBADF The path argument does not specify an absolute path and the fd argument is neither AT_FDCWD nor a
valid file descriptor open for reading or searching.
ENOTDIR
The path argument is not an absolute path and fd is a file descriptor associated with a non-
directory file.
These functions may fail if:
EINTR A signal was caught during execution of the function.
EINVAL The value of the mode argument is invalid.
ELOOP More than {SYMLOOP_MAX} symbolic links were encountered during resolution of the path argument.
ENAMETOOLONG
The length of a pathname exceeds {PATH_MAX}, or pathname resolution of a symbolic link produced an
intermediate result with a length that exceeds {PATH_MAX}.
The fchmodat() function may fail if:
EINVAL The value of the flag argument is invalid.
EOPNOTSUPP
The AT_SYMLINK_NOFOLLOW bit is set in the flag argument, path names a symbolic link, and the
system does not support changing the mode of a symbolic link.
The following sections are informative.
EXAMPLES
Setting Read Permissions for User, Group, and Others
The following example sets read permissions for the owner, group, and others.
#include <sys/stat.h>
const char *path;
...
chmod(path, S_IRUSR|S_IRGRP|S_IROTH);
Setting Read, Write, and Execute Permissions for the Owner Only
The following example sets read, write, and execute permissions for the owner, and no permissions for
group and others.
#include <sys/stat.h>
const char *path;
...
chmod(path, S_IRWXU);
Setting Different Permissions for Owner, Group, and Other
The following example sets owner permissions for CHANGEFILE to read, write, and execute, group
permissions to read and execute, and other permissions to read.
#include <sys/stat.h>
#define CHANGEFILE "/etc/myfile"
...
chmod(CHANGEFILE, S_IRWXU|S_IRGRP|S_IXGRP|S_IROTH);
Setting and Checking File Permissions
The following example sets the file permission bits for a file named /home/cnd/mod1, then calls the
stat() function to verify the permissions.
#include <sys/types.h>
#include <sys/stat.h>
int status;
struct stat buffer
...
chmod("/home/cnd/mod1", S_IRWXU|S_IRWXG|S_IROTH|S_IWOTH);
status = stat("/home/cnd/mod1", &buffer);
APPLICATION USAGE
In order to ensure that the S_ISUID and S_ISGID bits are set, an application requiring this should use
stat() after a successful chmod() to verify this.
Any file descriptors currently open by any process on the file could possibly become invalid if the mode
of the file is changed to a value which would deny access to that process. One situation where this could
occur is on a stateless file system. This behavior will not occur in a conforming environment.
RATIONALE
This volume of POSIX.1‐2017 specifies that the S_ISGID bit is cleared by chmod() on a regular file under
certain conditions. This is specified on the assumption that regular files may be executed, and the
system should prevent users from making executable setgid() files perform with privileges that the caller
does not have. On implementations that support execution of other file types, the S_ISGID bit should be
cleared for those file types under the same circumstances.
Implementations that use the S_ISUID bit to indicate some other function (for example, mandatory record
locking) on non-executable files need not clear this bit on writing. They should clear the bit for
executable files and any other cases where the bit grants special powers to processes that change the
file contents. Similar comments apply to the S_ISGID bit.
The purpose of the fchmodat() function is to enable changing the mode of files in directories other than
the current working directory without exposure to race conditions. Any part of the path of a file could
be changed in parallel to a call to chmod(), resulting in unspecified behavior. By opening a file
descriptor for the target directory and using the fchmodat() function it can be guaranteed that the
changed file is located relative to the desired directory. Some implementations might allow changing the
mode of symbolic links. This is not supported by the interfaces in the POSIX specification. Systems with
such support provide an interface named lchmod(). To support such implementations fchmodat() has a flag
parameter.
FUTURE DIRECTIONS
None.
SEE ALSO
access(), chown(), exec, fstatat(), fstatvfs(), mkdir(), mkfifo(), mknod(), open()
The Base Definitions volume of POSIX.1‐2017, <fcntl.h>, <sys_stat.h>, <sys_types.h>
COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1-2017, Standard
for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 7, 2018 Edition, Copyright (C) 2018 by the Institute of Electrical and Electronics
Engineers, Inc and The Open Group. In the event of any discrepancy between this version and the original
IEEE and The Open Group Standard, the original IEEE and The Open Group Standard is the referee document.
The original Standard can be obtained online at http://www.opengroup.org/unix/online.html .
Any typographical or formatting errors that appear in this page are most likely to have been introduced
during the conversion of the source files to man page format. To report such errors, see
https://www.kernel.org/doc/man-pages/reporting_bugs.html .