NAME

chmod, fchmod, fchmodat - 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);

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

DESCRIPTION

The chmod(), fchmod(), and fchmodat() 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:

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)):

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.

The fchmodat() function behaves similarly to chmod(), except when path is a relative path, it is resolved relative to the directory specified by filedes. If filedes has the value AT_FDCWD, then path will be resolved relative to the current working directory. The argument flag should be zero, but may include the value AT_SYMLINK_NOFOLLOW, which indicates that if path refers to a symbolic link, then permissions should be changed on the symbolic link itself. However, changing permissions of symbolic links is not supported on illumos, and will result in an error.

Upon successful completion, chmod(), fchmod(), fchmodat() 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(), fchmod(), and fchmodat() functions will fail if:

EIO

EPERM

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

EACCES

EFAULT

ELOOP

ENAMETOOLONG

ENOENT

ENOLINK

ENOTDIR

EROFS

The fchmod() function will fail if:

EBADF

ENOLINK

EROFS

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

EINTR

EINVAL

The fchmodat() will fail if:

EBADF

EINVAL

ENOTDIR

EOPNOTSUPP

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

ELOOP

ENAMETOOLONG

The fchmod() function may fail if:

EINVAL

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(7) for descriptions of the following attributes:

SEE ALSO

chmod(1), chown(2), creat(2), fcntl(2), mknod(2), open(2), read(2), rename(2), stat(2), write(2), fattach(3C), mkfifo(3C), stat.h(3HEAD), attributes(7), privileges(7), standards(7)

Programming Interfaces Guide