umask (3) - glibc

Split from the chapter glibc: File System Interface into the per-function layout; see the method page and the glibc ledger for coverage status. Kernel side: umask(2); the unreadable-mask problem: getumask.md.

Declaration and gates

#include <sys/stat.h>

mode_t umask(mode_t mask);

Declared ungated at /usr/include/x86_64-linux-gnu/sys/stat.h line 380, directly above the Hurd-only getumask (lines 384-386).

Semantics

Sets the calling process’s file mode creation mask to mask & 0777 and returns the previous mask; it always succeeds (umask.2). Every creator that takes a mode argument – open, creat, mkdir, mkfifo, mknod, POSIX IPC opens (mq_open, sem_open, shm_open), UNIX-domain sockets – computes the new node’s permissions as mode & ~umask, so the mask is the complement of the permissions you intend to grant (manual Assigning File Permissions). A default ACL on the parent directory overrides the umask entirely (umask.2). The shell initializes it at login; fork inherits it and execve leaves it unchanged.

Gotchas and portability

• Reading the mask requires writing it: the manual’s canonical read_umask does umask(0) then restores – nonatomic, so in a multithreaded program another thread can create a file through the wide-open window (umask.2 NOTES). On Hurd use getumask; since Linux 4.7 a process can read its own mask without changing it from /proc/self/status’s Umask field.
• Programs that must bypass the inherited mask should fchmod the opened file instead of mutating the process-wide mask – the manual reserves umask changes “usually only for shells” (Assigning File Permissions).
• The mask never affects SysV IPC objects (msgget/semget/ shmget) – only POSIX IPC, FIFOs, files and sockets (umask.2 NOTES).
• musl 1.2.5: bare SYS_umask wrapper, identical semantics (src/stat/umask.c). Grouped sibling page with the chmod/chown family: chmod-umask-owners.md.

Errors

None: “This system call always succeeds and the previous value of the mask is returned” (umask.2 RETURN VALUE).

References

• glibc manual: filesys.texi, node Assigning File Permissions
• man2: umask.2
• headers: /usr/include/x86_64-linux-gnu/sys/stat.h lines 380-386
• symbols: nm -D --with-symbol-versions libc.so.6