diff --git a/lib/std/c.zig b/lib/std/c.zig
index 57d5beae56b325547b5605af2528c025da88d51a..1334b2f2c12c5eea7df28068a6b0aef09ac9a815 100644
--- a/lib/std/c.zig
+++ b/lib/std/c.zig
@@ -173,6 +173,7 @@ pub extern "c" fn readlink(noalias path: [*:0]const u8, noalias buf: [*]u8, bufs
 pub extern "c" fn readlinkat(dirfd: c.fd_t, noalias path: [*:0]const u8, noalias buf: [*]u8, bufsize: usize) isize;
 pub extern "c" fn fchmod(fd: c.fd_t, mode: c.mode_t) c_int;
 pub extern "c" fn fchown(fd: c.fd_t, owner: c.uid_t, group: c.gid_t) c_int;
+pub extern "c" fn umask(mode: c.mode_t) c.mode_t;
 
 pub extern "c" fn rmdir(path: [*:0]const u8) c_int;
 pub extern "c" fn getenv(name: [*:0]const u8) ?[*:0]u8;