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



NAME
       chmod, fchmod - change permissions of a file

SYNOPSIS
       #include <sys/stat.h>

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

   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

       fchmod(): _BSD_SOURCE || _XOPEN_SOURCE >= 500

DESCRIPTION
       These  system  calls change the permissions of a file.  They differ only in how the
       file is specified:

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

       * fchmod()  changes  the  permissions  of  the  file  referred  to by the open file
         descriptor fd.

       The new file permissions are 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); manda-
                         tory 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 unlink(2))

       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 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 capa-
       bility), 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 file system, 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  file  sys-
       tems,  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 file systems, 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.

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

ERRORS
       Depending  on  the  file  system,  other  errors 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 path points outside your accessible address space.

       EIO    An I/O error occurred.

       ELOOP  Too many symbolic links were encountered in resolving path.

       ENAMETOOLONG
              path 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).

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

       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.

CONFORMING TO
       4.4BSD, SVr4, POSIX.1-2001.

SEE ALSO
       chown(2), execve(2), fchmodat(2), open(2), stat(2), path_resolution(7)

COLOPHON
       This page is part of release 3.22 of the Linux man-pages project.  A description of
       the  project, and information about reporting bugs, can be found at http://www.ker-
       nel.org/doc/man-pages/.



Linux                             2008-05-26                          CHMOD(2)

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