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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.
The
umaskfunction 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
umaskwithout changing it permanently:mode_t read_umask (void) { mode_t mask = umask (0); umask (mask); return mask; }However, it is better to use
getumaskif you just want to read the mask value, because it is reentrant (at least if you use the GNU operating system).
Return the current value of the file creation mask for the current process. This function is a GNU extension.
The
chmodfunction sets the access permission bits for the file named by filename to mode.If filename is a symbolic link,
chmodchanges the permissions of the file pointed to by the link, not those of the link itself.This function returns
0if successful and-1if not. In addition to the usual file name errors (see File Name Errors), the followingerrnoerror 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_ISVTXbit (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
EFTYPEon 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 callchmodagain. See Permission Bits, for full details on the sticky bit.
This is like
chmod, except that it changes the permissions of the currently open file given by filedes.The return value from
fchmodis0on success and-1on failure. The followingerrnoerror 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.