umask(2)

Purpose

umask() sets the calling process’s file mode creation mask to mask & 0777 and returns the previous value. The umask is used by open(2), mkdir(2), and other file-creating calls to turn permission bits off from the mode argument: a file created with mode 0666 under a umask of 022 gets 0644. If the parent directory has a default ACL, the umask is ignored and the inherited ACL plus the mode argument determine the permissions instead.

Synopsis

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

Arguments and behavior

mask is a file permission bit mask (S_IRUSR and friends, inode(7)); only the low 0777 bits are used. The typical default is S_IWGRP | S_IWOTH (022). The call always succeeds and cannot fail.

Return value and errors

Returns the previous umask. No error conditions exist.

Flags and constants

Permission bit constants for mask come from <sys/stat.h> (S_IRUSR 0400, S_IWUSR 0200, S_IXUSR 0100, and the group/other equivalents). There is no flag argument.

Security and notes

• The read-modify-restore idiom (umask(0) then restore) is not atomic and races in multithreaded programs; the man page calls this out explicitly.
• Since Linux 4.7 the umask of any process is visible as the Umask field of /proc/[pid]/status, so a process can read its own umask from /proc/self/status without changing it.
• Children created by fork(2) inherit the umask; execve(2) leaves it unchanged.
• The umask affects POSIX IPC objects (mq_open(3), sem_open(3), shm_open(3)), FIFOs, and UNIX domain sockets; it does not affect System V IPC objects (msgget(2), semget(2), shmget(2)).
• It cannot be set by another process, and there is no way to fetch it without setting it (other than the /proc field above).

References

• man2/umask.2 in the cached man-pages tree (dated 2021-03-22).
• Syscalls ledger row SC-umask; kernel tables in source-cache/kernel.
• Siblings: open (mode & umask application), stat.
• Domain hub for status and sibling pages.