CHMOD(2) System Calls Manual CHMOD(2)

chmod, fchmod, fchmodat — change mode of file

#include <sys/types.h>
#include <sys/stat.h>

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

int
fchmod(int fildes, mode_t mode);

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

The function () sets the file permission bits of the file specified by the pathname path to mode. () sets the permission bits of the specified file descriptor fildes. chmod() verifies that the process owner (user) either owns the file specified by path (or fildes), or is the super-user.

The () 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. The values for the flag are constructed by a bitwise-inclusive OR of flags from the following list, defined in <fcntl.h>:

If path names a symbolic link, then the mode of the symbolic link is changed.
If path names a symbolic link, then the mode of the symbolic link is changed and if if the path has any other symbolic links, an error is returned.

If () is passed the special value AT_FDCWD in the fd parameter, the current working directory is used. If also flag is zero, the behavior is identical to a call to chmod(). A mode is created from permission bit masks defined in ⟨sys/stat.h⟩:

#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 */

The ISVTX (the ) indicates to the system which executable files are shareable (the default) and the system maintains the program text of the files in the swap area. The sticky bit may only be set by the super user on shareable executable files.

If mode ISVTX (the `sticky bit') 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(7).

Writing or changing the owner of a file turns off the set-user-id and set-group-id bits unless the user is the super-user. 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.

Upon successful completion, a value of 0 is returned. Otherwise, a value of -1 is returned and errno is set to indicate the error.

The chmod() system call will fail and the file mode will be unchanged if:

[]
Search permission is denied for a component of the path prefix.
[]
Path points outside the process's allocated address space.
[]
Its execution was interrupted by a signal.
[]
An I/O error occurred while reading from or writing to the file system.
[]
Too many symbolic links were encountered in translating the pathname. This is taken to be indicative of a looping symbolic link.
[]
If AT_SYMLINK_NOFOLLOW_ANY was passed and a symbolic link was encountered in translating the pathname.
[]
A component of a pathname exceeded {NAME_MAX} characters, or an entire path name exceeded {PATH_MAX} characters.
[]
The named file does not exist.
[]
A component of the path prefix is not a directory.
[]
The effective user ID does not match the owner of the file and the effective user ID is not the super-user.
[]
The named file resides on a read-only file system.

fchmod() will fail if:

[]
fildes is not a valid file descriptor.
[]
fildes refers to a socket, not to a file.
[]
mode is not a valid file mode.
[]
Its execution was interrupted by a signal.
[]
An I/O error occurred while reading from or writing to the file system.
[]
The effective user ID does not match the owner of the file and the effective user ID is not the super-user.
[]
The file resides on a read-only file system.

In addition to the chmod() errors, fchmodat() fails if:

[]
The path argument does not specify an absolute path and the fd argument is neither AT_FDCWD nor a valid file descriptor open for searching.
[]
The value of the flag argument is not valid.
[]
The path argument is not an absolute path and fd is neither AT_FDCWD nor a file descriptor associated with a directory.

#include <sys/types.h> #include <sys/stat.h>

The include file <sys/types.h> is necessary.

chmod(1), chown(2), open(2), stat(2), compat(5), sticky(7)

The chmod() function is expected to conform to IEEE Std 1003.1-1988 (“POSIX.1”). The fchmodat() function is expected to conform to POSIX.1-2008 .

The fchmod() function call appeared in 4.2BSD. The fchmodat() system call appeared in OS X 10.10

June 4, 1993 BSD 4