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PROLOG | NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | EXAMPLES | APPLICATION USAGE | RATIONALE | FUTURE DIRECTIONS | SEE ALSO | COPYRIGHT |
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CHMOD(3P) POSIX Programmer's Manual CHMOD(3P)
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.
chmod, fchmodat — change mode of a file
#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);
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().
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.
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.
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);
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.
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.
None.
access(3p), chown(3p), exec(1p), fstatat(3p), fstatvfs(3p),
mkdir(3p), mkfifo(3p), mknod(3p), open(3p)
The Base Definitions volume of POSIX.1‐2017, fcntl.h(0p),
sys_stat.h(0p), sys_types.h(0p)
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 .
IEEE/The Open Group 2017 CHMOD(3P)
Pages that refer to this page: sys_stat.h(0p), chmod(1p), access(3p), chown(3p), exec(3p), fchmod(3p), fchmodat(3p), fstatat(3p), fstatvfs(3p), lockf(3p), mkdir(3p), mkfifo(3p), mknod(3p), open(3p), posix_spawn(3p), write(3p)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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