chmod(2) — Linux manual page


chmod(2)                   System Calls Manual                  chmod(2)

NAME         top

       chmod, fchmod, fchmodat - change permissions of a file

LIBRARY         top

       Standard C library (libc, -lc)

SYNOPSIS         top

       #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);

   Feature Test Macro Requirements for glibc (see

           Since glibc 2.24:
               _POSIX_C_SOURCE >= 199309L
           glibc 2.19 to glibc 2.23
           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

           Since glibc 2.10:
               _POSIX_C_SOURCE >= 200809L
           Before glibc 2.10:

DESCRIPTION         top

       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:

       •  chmod() changes the mode of the file specified whose pathname
          is given in pathname, which is dereferenced if it is a
          symbolic link.

       •  fchmod() changes the mode of the file referred to by the open
          file descriptor fd.

       The new file mode is specified in mode, which is a bit mask
       created by ORing together zero or more of the following:

       S_ISUID  (04000)
              set-user-ID (set process effective user ID on execve(2))

       S_ISGID  (02000)
              set-group-ID (set process effective group ID on execve(2);
              mandatory locking, as described in fcntl(2); take a new
              file's group from parent directory, as described in
              chown(2) and mkdir(2))

       S_ISVTX  (01000)
              sticky bit (restricted deletion flag, as described in

       S_IRUSR  (00400)
              read by owner

       S_IWUSR  (00200)
              write by owner

       S_IXUSR  (00100)
              execute/search by owner ("search" applies for directories,
              and means that entries within the directory can be

       S_IRGRP  (00040)
              read by group

       S_IWGRP  (00020)
              write by group

       S_IXGRP  (00010)
              execute/search by group

       S_IROTH  (00004)
              read by others

       S_IWOTH  (00002)
              write by others

       S_IXOTH  (00001)
              execute/search by others

       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:

              If pathname is a symbolic link, do not dereference it:
              instead operate on the link itself.  This flag is not
              currently implemented.

       See openat(2) for an explanation of the need for fchmodat().

RETURN VALUE         top

       On success, zero is returned.  On error, -1 is returned, and
       errno is set to indicate the error.

ERRORS         top

       Depending on the filesystem, errors other than those listed below
       can be returned.

       The more general errors for chmod() are listed below:

       EACCES Search permission is denied on a component of the path
              prefix.  (See also path_resolution(7).)

       EBADF  (fchmod()) The file descriptor fd is not valid.

       EBADF  (fchmodat()) pathname is relative but dirfd is neither
              AT_FDCWD nor a valid file descriptor.

       EFAULT pathname points outside your accessible address space.

       EINVAL (fchmodat()) Invalid flag specified in flags.

       EIO    An I/O error occurred.

       ELOOP  Too many symbolic links were encountered in resolving

              pathname is too long.

       ENOENT The file does not exist.

       ENOMEM Insufficient kernel memory was available.

              A component of the path prefix is not a directory.

              (fchmodat()) pathname is relative and dirfd is a file
              descriptor referring to a file other than a directory.

              (fchmodat()) flags specified AT_SYMLINK_NOFOLLOW, which is
              not supported.

       EPERM  The effective UID does not match the owner of the file,
              and the process is not privileged (Linux: it does not have
              the CAP_FOWNER capability).

       EPERM  The file is marked immutable or append-only.  (See

       EROFS  The named file resides on a read-only filesystem.

VERSIONS         top

   C library/kernel differences
       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.

   glibc notes
       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

STANDARDS         top


HISTORY         top

              4.4BSD, SVr4, POSIX.1-2001.

              POSIX.1-2008.  Linux 2.6.16, glibc 2.4.

SEE ALSO         top

       chmod(1), chown(2), execve(2), open(2), stat(2), inode(7),
       path_resolution(7), symlink(7)

Linux man-pages (unreleased)     (date)                         chmod(2)

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