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File: libc.info,  Node: File Owner,  Next: Permission Bits,  Prev: Testing File Type,  Up: File Attributes
14.9.4 File Owner
-----------------
Every file has an "owner" which is one of the registered user names
defined on the system.  Each file also has a "group" which is one of the
defined groups.  The file owner can often be useful for showing you who
edited the file (especially when you edit with GNU Emacs), but its main
purpose is for access control.
   The file owner and group play a role in determining access because
the file has one set of access permission bits for the owner, another
set that applies to users who belong to the file's group, and a third
set of bits that applies to everyone else.  *Note Access Permission::,
for the details of how access is decided based on this data.
   When a file is created, its owner is set to the effective user ID of
the process that creates it (*note Process Persona::).  The file's group
ID may be set to either the effective group ID of the process, or the
group ID of the directory that contains the file, depending on the
system where the file is stored.  When you access a remote file system,
it behaves according to its own rules, not according to the system your
program is running on.  Thus, your program must be prepared to encounter
either kind of behavior no matter what kind of system you run it on.
   You can change the owner and/or group owner of an existing file using
the 'chown' function.  This is the primitive for the 'chown' and 'chgrp'
shell commands.
   The prototype for this function is declared in 'unistd.h'.
 -- Function: int chown (const char *FILENAME, uid_t OWNER, gid_t GROUP)
     Preliminary: | MT-Safe | AS-Safe | AC-Safe | *Note POSIX Safety
     Concepts::.
     The 'chown' function changes the owner of the file FILENAME to
     OWNER, and its group owner to GROUP.
     Changing the owner of the file on certain systems clears the
     set-user-ID and set-group-ID permission bits.  (This is because
     those bits may not be appropriate for the new owner.)  Other file
     permission bits are not changed.
     The return value is '0' on success and '-1' on failure.  In
     addition to the usual file name errors (*note File Name Errors::),
     the following 'errno' error conditions are defined for this
     function:
     'EPERM'
          This process lacks permission to make the requested change.
          Only privileged users or the file's owner can change the
          file's group.  On most file systems, only privileged users can
          change the file owner; some file systems allow you to change
          the owner if you are currently the owner.  When you access a
          remote file system, the behavior you encounter is determined
          by the system that actually holds the file, not by the system
          your program is running on.
          *Note Options for Files::, for information about the
          '_POSIX_CHOWN_RESTRICTED' macro.
     'EROFS'
          The file is on a read-only file system.
 -- Function: int fchown (int FILEDES, uid_t OWNER, gid_t GROUP)
     Preliminary: | MT-Safe | AS-Safe | AC-Safe | *Note POSIX Safety
     Concepts::.
     This is like 'chown', except that it changes the owner of the open
     file with descriptor FILEDES.
     The return value from 'fchown' is '0' on success and '-1' on
     failure.  The following 'errno' error codes are defined for this
     function:
     'EBADF'
          The FILEDES argument is not a valid file descriptor.
     'EINVAL'
          The FILEDES argument corresponds to a pipe or socket, not an
          ordinary file.
     'EPERM'
          This process lacks permission to make the requested change.
          For details see 'chmod' above.
     'EROFS'
          The file resides on a read-only file system.