DESCRIPTION
umask() sets the calling process's file mode creation mask (umask) to
mask & 0777 (i.e., only the file permission bits of mask are used), and
returns the previous value of the mask.
The umask is used by open(2), mkdir(2), and other system calls that
create files to modify the permissions placed on newly created files or
directories. Specifically, permissions in the umask are turned off
from the mode argument to open(2) and mkdir(2).
The constants that should be used to specify mask are described under
stat(2).
The typical default value for the process umask is S_IWGRP | S_IWOTH
(octal 022). In the usual case where the mode argument to open(2) is
specified as:
S_IRUSR | S_IWUSR | S_IRGRP | S_IWGRP | S_IROTH | S_IWOTH
(octal 0666) when creating a new file, the permissions on the resulting
file will be:
S_IRUSR | S_IWUSR | S_IRGRP | S_IROTH
(because 0666 & ~022 = 0644; i.e., rw-r--r--).
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