chmod(2) (man pages section 2: System Calls)

man pages section 2: System Calls

chmod(2)

Name

chmod, fchmod– change access permission mode of file

Synopsis

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

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

int fchmod(int fildes, mode_t mode);

Description

The chmod() and fchmod() functions set the access permission portion of the mode of the file whose name is given by path or referenced by the open file descriptor fildes to the bit pattern contained in mode. Access permission bits are interpreted as follows:

`S_ISUID`	04000	Set user ID on execution.
`S_ISGID`	020#0	Set group ID on execution if # is `7`, `5`, `3`, or `1`. Enable mandatory file/record locking if # is `6`, `4`, `2`, or `0`.
`S_ISVTX`	01000	Sticky bit.
`S_IRWXU`	00700	Read, write, execute by owner.
`S_IRUSR`	00400	Read by owner.
`S_IWUSR`	00200	Write by owner.
`S_IXUSR`	00100	Execute (search if a directory) by owner.
`S_IRWXG`	00070	Read, write, execute by group.
`S_IRGRP`	00040	Read by group.
`S_IWGRP`	00020	Write by group.
`S_IXGRP`	00010	Execute by group.
`S_IRWXO`	00007	Read, write, execute (search) by others.
`S_IROTH`	00004	Read by others.
`S_IWOTH`	00002	Write by others.
`S_IXOTH`	00001	Execute by others.

Modes are constructed by the bitwise OR operation of the access permission bits.

The effective user ID of the process must match the owner of the file or the process must have the appropriate privilege to change the mode of a file.

If the process is not a privileged process and the file is not a directory, mode bit 01000 (save text image on execution) is cleared.

If neither the process is privileged nor the file's group is a member of the process's supplementary group list, and the effective group ID of the process does not match the group ID of the file, mode bit 02000 (set group ID on execution) is cleared.

If a directory is writable and has S_ISVTX (the sticky bit) set, files within that directory can be removed or renamed only if one or more of the following is true (see unlink(2) and rename(2)):

the user owns the file
the user owns the directory
the file is writable by the user
the user is a privileged user

If a regular file is not executable and has S_ISVTX set, the file is assumed to be a swap file. In this case, the system's page cache will not be used to hold the file's data. If the S_ISVTX bit is set on any other file, the results are unspecified.

If a directory has the set group ID bit set, a given file created within that directory will have the same group ID as the directory. Otherwise, the newly created file's group ID will be set to the effective group ID of the creating process.

If the mode bit 02000 (set group ID on execution) is set and the mode bit 00010 (execute or search by group) is not set, mandatory file/record locking will exist on a regular file, possibly affecting future calls to open(2), creat(2), read(2), and write(2) on this file.

If fildes references a shared memory object, fchmod() need only affect the S_IRUSR, S_IRGRP, S_IROTH, S_IWUSR, S_IWGRP, S_IWOTH, S_IXUSR, S_IXGRP, and S_IXOTH file permission bits.

If fildes refers to a socket, fchmod() does not fail but no action is taken.

If fildes refers to a stream that is attached to an object in the file system name space with fattach(3C), the fchmod() call performs no action and returns successfully.

Upon successful completion, chmod() and fchmod() mark for update the st_ctime field of the file.

Return Values

Upon successful completion, 0 is returned. Otherwise, -1 is returned, the file mode is unchanged, and errno is set to indicate the error.

Errors

The chmod() and fchmod() functions will fail if:

EIO

An I/O error occurred while reading from or writing to the file system.

EPERM

The effective user ID does not match the owner of the file and the process does not have appropriate privilege.

The {PRIV_FILE_OWNER} privilege overrides constraints on ownership when changing permissions on a file.

The {PRIV_FILE_SETID} privilege overrides constraints on ownership when adding the setuid or setgid bits to an executable file or a directory. When adding the setuid bit to a root owned executable, additional restrictions apply. See privileges(5).

The chmod() function will fail if:

EACCES: Search permission is denied on a component of the path prefix of path. The privilege {FILE_DAC_SEARCH} overrides file permissions restrictions in that case.
EFAULT: The path argument points to an illegal address.
ELOOP: A loop exists in symbolic links encountered during the resolution of the path argument.
ENAMETOOLONG: The length of the path argument exceeds PATH_MAX, or the length of a path component exceeds NAME_MAX while _POSIX_NO_TRUNC is in effect.
ENOENT: Either a component of the path prefix or the file referred to by path does not exist or is a null pathname.
ENOLINK: The fildes argument points to a remote machine and the link to that machine is no longer active.
ENOTDIR: A component of the prefix of path is not a directory.
EROFS: The file referred to by path resides on a read-only file system.

The fchmod() function will fail if:

EBADF: The fildes argument is not an open file descriptor
ENOLINK: The path argument points to a remote machine and the link to that machine is no longer active.
EROFS: The file referred to by fildes resides on a read-only file system.

The chmod() and fchmod() functions may fail if:

EINTR: A signal was caught during execution of the function.
EINVAL: The value of the mode argument is invalid.

The chmod() function may fail if:

ELOOP: More than {SYMLOOP_MAX} symbolic links were encountered during the resolution of the path argument.
ENAMETOOLONG: As a result of encountering a symbolic link in resolution of thepath argument, the length of the substituted pathname strings exceeds {PATH_MAX}.

The fchmod() function may fail if:

EINVAL: The fildes argument refers to a pipe and the system disallows execution of this function on a pipe.

Examples

Example 1 Set 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);

Example 2 Set 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);

Example 3 Set 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);

Example 4 Set and Checking File Permissions

The following example sets the file permission bits for a file named /home/cnd/mod1, then calls the stat(2) 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;);

Usage

If chmod() or fchmod() is used to change the file group owner permissions on a file with non-trivial ACL entries, only the ACL mask is set to the new permissions and the group owner permission bits in the file's mode field (defined in mknod(2)) are unchanged. A non-trivial ACL entry is one whose meaning cannot be represented in the file's mode field alone. The new ACL mask permissions might change the effective permissions for additional users and groups that have ACL entries on the file.

Attributes

See attributes(5) for descriptions of the following attributes:

ATTRIBUTE TYPE	ATTRIBUTE VALUE
Interface Stability	Standard
MT-Level	Async-Signal-Safe