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     The umask function sets the file-permission mask of the current
     process to the mode specified by <pmode>.
 
     The file-permission mask is used to modify the permission setting
     of new files created by creat, open, or sopen. If a bit in the
     mask is 1, the corresponding bit in the file's requested
     permission value is set to 0 (disallowed). If a bit in the mask is
     0, the corresponding bit is left unchanged. The permission setting
     for a new file is not set until the file is closed for the first
     time.
 
     The argument <pmode> is a constant expression containing one or
     both of the manifest constants S_IWRITE and S_IREAD (defined in
     SYS\STAT.H). When both constants are given, they are combined
     with the bitwise-OR operator (|).
 
     For example, if the write bit is set in the mask, any new files
     are read-only.
 
     Note that under DOS and OS/2, all files are readable──it is not
     possible to give write-only permission. Therefore, setting the
     read bit with umask has no effect on the file's modes.
 
     Return Value
 
     The umask function returns the previous value of <pmode>. There is
     no error return.
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