Ubuntu Manpage: chmod, fchmod, fchmodat - change permissions of a file

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():
           _BSD_SOURCE || _XOPEN_SOURCE >= 500 || _XOPEN_SOURCE && _XOPEN_SOURCE_EXTENDED
           || /* Since glibc 2.12: */ _POSIX_C_SOURCE >= 200809L

       fchmodat():
           Since glibc 2.10:
               _XOPEN_SOURCE >= 700 || _POSIX_C_SOURCE >= 200809L
           Before glibc 2.10:
               _ATFILE_SOURCE

DESCRIPTION

       The chmod() and fchmod() 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  pathname,  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);  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 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 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 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.

   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 implemented.

       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).

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.

COLOPHON

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