CHMOD(2) | System Calls Manual | CHMOD(2)

# NAME

`chmod`, `fchmodat`, `fchmod` — change mode of file

# SYNOPSIS

`#include <sys/stat.h>`

int
`chmod`(const char *path, mode_t mode);

int
`fchmod`(int fd, mode_t mode);

`#include <sys/stat.h>`
`#include <fcntl.h>`

int
`fchmodat`(int fd, const char *path, mode_t mode, int flag);

# DESCRIPTION

The `chmod`() function sets the file permission bits of the file specified by the pathname path to mode. `chmod`() verifies that the process owner (user) either owns the specified file or is the superuser.

The mode argument is the bitwise OR of zero or more of the permission bit masks from the following list:

    #define S_IRWXU 0000700    /* RWX mask for owner */
    #define S_IRUSR 0000400    /* R for owner */
    #define S_IWUSR 0000200    /* W for owner */
    #define S_IXUSR 0000100    /* X for owner */

    #define S_IRWXG 0000070    /* RWX mask for group */
    #define S_IRGRP 0000040    /* R for group */
    #define S_IWGRP 0000020    /* W for group */
    #define S_IXGRP 0000010    /* X for group */

    #define S_IRWXO 0000007    /* RWX mask for other */
    #define S_IROTH 0000004    /* R for other */
    #define S_IWOTH 0000002    /* W for other */
    #define S_IXOTH 0000001    /* X for other */

    #define S_ISUID 0004000    /* set user id on execution */
    #define S_ISGID 0002000    /* set group id on execution */
    #define S_ISVTX 0001000    /* save swapped text even after use */

Mode `ISVTX` (the sticky bit) on a file has no effect, although the superuser can still set it. If it is set on a directory, an unprivileged user may not delete or rename files of other users in that directory. The sticky bit may be set by any user on a directory which the user owns or has appropriate permissions. For more details of the properties of the sticky bit, see sticky(8).

Writing or changing the owner of a file turns off the set-user-ID and set-group-ID bits unless the user is the superuser. This makes the system somewhat more secure by protecting set-user-ID (set-group-ID) files from remaining set-user-ID (set-group-ID) if they are modified, at the expense of a degree of compatibility.

The `fchmodat`() function is equivalent to `chmod`() 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 `fchmodat`() is passed the special value `AT_FDCWD` (defined in `<fcntl.h>`) in the fd parameter, the current working directory is used. If flag is also zero, the behavior is identical to a call to `chmod`().

The flag argument is the bitwise OR of zero or more of the following values:

`AT_SYMLINK_NOFOLLOW`
  If path names a symbolic link, then the mode of the symbolic link is changed.

The `fchmod`() function is equivalent to `chmod`() except that the file whose permissions are changed is specified by the file descriptor fd.

# RETURN VALUES

Upon successful completion, the value 0 is returned; otherwise the value -1 is returned and the global variable errno is set to indicate the error.

# ERRORS

The `chmod`() and `fchmodat`() functions will fail and the file mode will be unchanged if:

[`ENOTDIR`]
  A component of the path prefix is not a directory.
[`ENAMETOOLONG`]
  A component of a pathname exceeded `NAME_MAX` characters, or an entire pathname (including the terminating NUL) exceeded `PATH_MAX` bytes.
[`ENOENT`]
  The named file does not exist.
[`EACCES`]
  Search permission is denied for a component of the path prefix.
[`EINVAL`]
  mode contains bits other than the file type and those described above.
[`ELOOP`]
  Too many symbolic links were encountered in translating the pathname.
[`EPERM`]
  The effective user ID does not match the owner of the file and the effective user ID is not the superuser.
[`EROFS`]
  The named file resides on a read-only file system.
[`EFAULT`]
  path points outside the process's allocated address space.
[`EIO`]
  An I/O error occurred while reading from or writing to the file system.
[`EFTYPE`]
  An unprivileged user attempted to set a file's mode to `ISVTX`.

Additionally, the `fchmodat`() function will fail if:

[`EINVAL`]
  The value of the flag argument was neither zero nor `AT_SYMLINK_NOFOLLOW`.
[`EBADF`]
  The path argument specifies a relative path and the fd argument is neither `AT_FDCWD` nor a valid file descriptor.
[`ENOTDIR`]
  The path argument specifies a relative path and the fd argument is a valid file descriptor but it does not reference a directory.
[`EOPNOTSUPP`]
  The flag argument specifies `AT_SYMLINK_NOFOLLOW` on a symbolic link and the file system does not support that operation.
[`EACCES`]
  The path argument specifies a relative path but search permission is denied for the directory which the fd file descriptor references.

`fchmod`() will fail and the file mode will be unchanged if:

[`EBADF`]
  The descriptor is not valid.
[`EINVAL`]
  fd refers to a socket, not to a file.
[`EINVAL`]
  mode contains bits other than the file type and those described above.
[`EPERM`]
  The effective user ID does not match the owner of the file and the effective user ID is not the superuser.
[`EROFS`]
  The file resides on a read-only file system.
[`EIO`]
  An I/O error occurred while reading from or writing to the file system.

# SEE ALSO

chmod(1), chown(2), open(2), stat(2), sticky(8)

# STANDARDS

The `chmod`(), `fchmod`(), and `fchmodat`() functions are expected to conform to IEEE Std 1003.1-2008 (“POSIX.1”).

# HISTORY

The `chmod`() system call first appeared in Version 1 AT&T UNIX; `fchmod`() in 4.1cBSD; and `fchmodat`() has been available since OpenBSD 5.0.

May 20, 2025 | Debian