| chmod(2) | System Calls Manual | chmod(2) |
chmod, fchmod, fchmodat - change permissions of a file
Standard C library (libc, -lc)
#include <sys/stat.h>
int chmod(const char *pathname, mode_t mode); int fchmod(int fd, mode_t mode);
#include <fcntl.h> /* Definition of AT_* constants */ #include <sys/stat.h>
int fchmodat(int dirfd, const char *pathname, mode_t mode, int flags);
fchmod():
Since glibc 2.24:
_POSIX_C_SOURCE >= 199309L
glibc 2.19 to glibc 2.23
_POSIX_C_SOURCE
glibc 2.16 to glibc 2.19:
_BSD_SOURCE || _POSIX_C_SOURCE
glibc 2.12 to glibc 2.16:
_BSD_SOURCE || _XOPEN_SOURCE >= 500
|| _POSIX_C_SOURCE >= 200809L
glibc 2.11 and earlier:
_BSD_SOURCE || _XOPEN_SOURCE >= 500
fchmodat():
Since glibc 2.10:
_POSIX_C_SOURCE >= 200809L
Before glibc 2.10:
_ATFILE_SOURCE
The chmod() and fchmod() system calls change a file's 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:
The new file mode is specified in mode, which is a bit mask created by ORing together zero or more of the following:
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 CAP_FOWNER capability).
If the calling process is not privileged (Linux: does not have the 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 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 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 inode(7).
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.
The fchmodat() system call operates in exactly the same way as chmod(), except for the differences described here.
If the pathname given in pathname is relative, then it is interpreted relative to the directory referred to by the file descriptor dirfd (rather than relative to the current working directory of the calling process, as is done by chmod() for a relative pathname).
If pathname is relative and dirfd is the special value AT_FDCWD, then pathname is interpreted relative to the current working directory of the calling process (like chmod()).
If pathname is absolute, then dirfd is ignored.
flags can either be 0, or include the following flag:
See openat(2) for an explanation of the need for fchmodat().
On success, zero is returned. On error, -1 is returned, and errno is set to indicate the error.
Depending on the filesystem, errors other than those listed below can be returned.
The more general errors for chmod() are listed below:
The GNU C library fchmodat() wrapper function implements the POSIX-specified interface described in this page. This interface differs from the underlying Linux system call, which does not have a flags argument.
On older kernels where fchmodat() is unavailable, the glibc wrapper function falls back to the use of chmod(). When pathname is a relative pathname, glibc constructs a pathname based on the symbolic link in /proc/self/fd that corresponds to the dirfd argument.
POSIX.1-2008.
chmod(1), chown(2), execve(2), open(2), stat(2), inode(7), path_resolution(7), symlink(7)
| 2023-03-30 | Linux man-pages 6.05.01 |