CHMOD(2)		   Linux Programmer's Manual		      CHMOD(2)



NAME
       chmod, fchmod, fchmodat - change permissions of a file

SYNOPSIS
       #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 feature_test_macros(7)):

       fchmod():
	   /* Since glibc 2.16: */ _POSIX_C_SOURCE
	       || /* Glibc versions <= 2.19: */ _BSD_SOURCE
	       || /* Glibc versions <= 2.15: */ _XOPEN_SOURCE >= 500
	       || /* Glibc 2.12 to 2.15: */ _POSIX_C_SOURCE >= 200809L

       fchmodat():
	   Since glibc 2.10:
	       _POSIX_C_SOURCE >= 200809L
	   Before glibc 2.10:
	       _ATFILE_SOURCE

DESCRIPTION
       The  chmod()  and fchmod() system calls change a files 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	direc-
			 tory, as described in chown(2) and mkdir(2))

       S_ISVTX	(01000)	 sticky bit (restricted deletion flag, as described in
			 unlink(2))

       S_IRUSR	(00400)	 read by owner

       S_IWUSR	(00200)	 write by owner

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

       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
       stat(2).

       On NFS filesystems, restricting the permissions will immediately influ-
       ence  already  open  files,  because  the access control is done on the
       server, but open files are maintained by the client.  Widening the per-
       missions	 may  be  delayed  for	other  clients if attribute caching is
       enabled on them.

   fchmodat()
       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:

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

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

RETURN VALUE
       On success, zero is returned.  On error, -1 is returned, and  errno  is
       set appropriately.

ERRORS
       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).)

       EFAULT pathname points outside your accessible address space.

       EIO    An I/O error occurred.

       ELOOP  Too many symbolic links were encountered in resolving pathname.

       ENAMETOOLONG
	      pathname is too long.

       ENOENT The file does not exist.

       ENOMEM Insufficient kernel memory was available.

       ENOTDIR
	      A component of the path prefix is not a directory.

       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
	      ioctl_iflags(2).)

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

       The general errors for fchmod() are listed below:

       EBADF  The file descriptor fd is not valid.

       EIO    See above.

       EPERM  See above.

       EROFS  See above.

       The same errors that occur for chmod() can also occur  for  fchmodat().
       The following additional errors can occur for fchmodat():

       EBADF  dirfd is not a valid file descriptor.

       EINVAL Invalid flag specified in flags.

       ENOTDIR
	      pathname is relative and dirfd is a file descriptor referring to
	      a file other than a directory.

       ENOTSUP
	      flags specified AT_SYMLINK_NOFOLLOW, which is not supported.

VERSIONS
       fchmodat() was added to Linux in kernel	2.6.16;	 library  support  was
       added to glibc in version 2.4.

CONFORMING TO
       chmod(), fchmod(): 4.4BSD, SVr4, POSIX.1-2001i, POSIX.1-2008.

       fchmodat(): POSIX.1-2008.

NOTES
   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 argument.

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

COLOPHON
       This page is part of release 4.10 of the Linux  man-pages  project.   A
       description  of	the project, information about reporting bugs, and the
       latest	 version    of	  this	  page,	   can	   be	  found	    at
       https://www.kernel.org/doc/man-pages/.



Linux				  2016-12-12			      CHMOD(2)