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).
Alternatively, if the parent directory has a default ACL (see acl(5)), the umask is ignored, the default ACL is inherited, the permission bits are set based on the inherited ACL, and permission bits absent in the mode argument are turned off. For example, the following default ACL is equivalent to a umask of 022:
u::rwx,g::r-x,o::r-x
Combining the effect of this default ACL with a mode argument of 0666 (rw-rw-rw-), the resulting file permissions would be 0644 (rw-r--r--).
The constants that should be used to specify mask are described in inode(7).
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--).
It is impossible to use umask() to fetch a process's umask without at the same time changing it. A second call to umask() would then be needed to restore the umask. The nonatomicity of these two steps provides the potential for races in multithreaded programs.
Since Linux 4.7, the umask of any process can be viewed via the Umask field of /proc/[pid]/status. Inspecting this field in /proc/self/status allows a process to retrieve its umask without at the same time changing it.
The umask setting also affects the permissions assigned to POSIX IPC objects (mq_open(3), sem_open(3), shm_open(3)), FIFOs (mkfifo(3)), and UNIX domain sockets (unix(7)) created by the process. The umask does not affect the permissions assigned to System V IPC objects created by the process (using msgget(2), semget(2), shmget(2)).