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UMASK(2) System Calls Manual UMASK(2)

umask
set file creation mode mask

#include <sys/types.h>
#include <sys/stat.h>
mode_t
umask(mode_t numask);

The umask() routine sets the process's file mode creation mask to numask and returns the previous value of the mask. Only the read, write, and execute file permission bits of numask are honored, all others are ignored.
The file mode creation mask is used by the bind(2), mkdir(2), mkdirat(2), mkfifo(2), mkfifoat(2), mknod(2), mknodat(2), open(2) and openat(2) system calls to turn off corresponding bits requested in the file mode (see chmod(2)). This clearing allows each user to restrict the default access to his files.
The default mask value is S_IWGRP|S_IWOTH (022, write access for the owner only). Child processes inherit the mask of the calling process.

The previous value of the file mode mask is returned by the call.

The umask() function is always successful.

chmod(2), mkdir(2), mkfifo(2), mknod(2), open(2)

The umask() function conforms to IEEE Std 1003.1-2008 (“POSIX.1”).

The umask() system call first appeared in Version 7 AT&T UNIX.
October 12, 2015 OpenBSD-6.0