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File: libc.info,  Node: Setting Permissions,  Next: Testing File Access,  Prev: Access Permission,  Up: File Attributes

14.9.7 Assigning File Permissions
---------------------------------

The primitive functions for creating files (for example, 'open' or
'mkdir') take a MODE argument, which specifies the file permissions to
give the newly created file.  This mode is modified by the process's
"file creation mask", or "umask", before it is used.

   The bits that are set in the file creation mask identify permissions
that are always to be disabled for newly created files.  For example, if
you set all the "other" access bits in the mask, then newly created
files are not accessible at all to processes in the "other" category,
even if the MODE argument passed to the create function would permit
such access.  In other words, the file creation mask is the complement
of the ordinary access permissions you want to grant.

   Programs that create files typically specify a MODE argument that
includes all the permissions that make sense for the particular file.
For an ordinary file, this is typically read and write permission for
all classes of users.  These permissions are then restricted as
specified by the individual user's own file creation mask.

   To change the permission of an existing file given its name, call
'chmod'.  This function uses the specified permission bits and ignores
the file creation mask.

   In normal use, the file creation mask is initialized by the user's
login shell (using the 'umask' shell command), and inherited by all
subprocesses.  Application programs normally don't need to worry about
the file creation mask.  It will automatically do what it is supposed to
do.

   When your program needs to create a file and bypass the umask for its
access permissions, the easiest way to do this is to use 'fchmod' after
opening the file, rather than changing the umask.  In fact, changing the
umask is usually done only by shells.  They use the 'umask' function.

   The functions in this section are declared in 'sys/stat.h'.

 -- Function: mode_t umask (mode_t MASK)
     Preliminary: | MT-Safe | AS-Safe | AC-Safe | *Note POSIX Safety
     Concepts::.

     The 'umask' function sets the file creation mask of the current
     process to MASK, and returns the previous value of the file
     creation mask.

     Here is an example showing how to read the mask with 'umask'
     without changing it permanently:

          mode_t
          read_umask (void)
          {
            mode_t mask = umask (0);
            umask (mask);
            return mask;
          }

     However, on GNU/Hurd systems it is better to use 'getumask' if you
     just want to read the mask value, because it is reentrant.

 -- Function: mode_t getumask (void)
     Preliminary: | MT-Safe | AS-Safe | AC-Safe | *Note POSIX Safety
     Concepts::.

     Return the current value of the file creation mask for the current
     process.  This function is a GNU extension and is only available on
     GNU/Hurd systems.

 -- Function: int chmod (const char *FILENAME, mode_t MODE)
     Preliminary: | MT-Safe | AS-Safe | AC-Safe | *Note POSIX Safety
     Concepts::.

     The 'chmod' function sets the access permission bits for the file
     named by FILENAME to MODE.

     If FILENAME is a symbolic link, 'chmod' changes the permissions of
     the file pointed to by the link, not those of the link itself.

     This function returns '0' if successful and '-1' if not.  In
     addition to the usual file name errors (*note File Name Errors::),
     the following 'errno' error conditions are defined for this
     function:

     'ENOENT'
          The named file doesn't exist.

     'EPERM'
          This process does not have permission to change the access
          permissions of this file.  Only the file's owner (as judged by
          the effective user ID of the process) or a privileged user can
          change them.

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

     'EFTYPE'
          MODE has the 'S_ISVTX' bit (the "sticky bit") set, and the
          named file is not a directory.  Some systems do not allow
          setting the sticky bit on non-directory files, and some do
          (and only some of those assign a useful meaning to the bit for
          non-directory files).

          You only get 'EFTYPE' on systems where the sticky bit has no
          useful meaning for non-directory files, so it is always safe
          to just clear the bit in MODE and call 'chmod' again.  *Note
          Permission Bits::, for full details on the sticky bit.

 -- Function: int fchmod (int FILEDES, mode_t MODE)
     Preliminary: | MT-Safe | AS-Safe | AC-Safe | *Note POSIX Safety
     Concepts::.

     This is like 'chmod', except that it changes the permissions of the
     currently open file given by FILEDES.

     The return value from 'fchmod' 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, or
          something else that doesn't really have access permissions.

     'EPERM'
          This process does not have permission to change the access
          permissions of this file.  Only the file's owner (as judged by
          the effective user ID of the process) or a privileged user can
          change them.

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


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