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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 relative to directory file
descriptor
#include <sys/stat.h>
int chmod(const char *path, mode_t mode);
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 file descriptor was opened without O_SEARCH, the
function shall check whether directory searches are permitted using
the current permissions of the directory underlying the file
descriptor. If the file descriptor was opened with 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 fd was not opened with 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.1‐2008 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.1‐2008, 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, 2013 Edition, Standard for Information
Technology -- Portable Operating System Interface (POSIX), The Open
Group Base Specifications Issue 7, Copyright (C) 2013 by the
Institute of Electrical and Electronics Engineers, Inc and The Open
Group. (This is POSIX.1-2008 with the 2013 Technical Corrigendum 1
applied.) 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.unix.org/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 2013 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)