System call, and CLI front-end.
man 2 umask
man umask
Shows/sets permissions that will be removed by default when a new file is created by the process.
Each process has umask information associated to it. On Linux 4.0, it is present on task_struct->fs->umask
umask is inherited at fork time.
It does not seem to be possible to get the umask for a given process except the current one without debugging system calls: http://www.hostatic.ro/2011/04/11/getting-the-umask-of-a-running-process/
0002
touch a
ls -l a
#rw-rw-r--
umask
#002
#ok the other w was removed
#the x are not set by touch by default
umask 0
touch b
#rw-rw-rw-
umask 777
touch c
#---------