diff --git a/sys/include/posix.h b/sys/include/posix.h
index 4d440cd..c2b2a47 100644
--- a/sys/include/posix.h
+++ b/sys/include/posix.h
@@ -141,6 +141,7 @@ extern void *POSIX_calloc(int *errnop, size_t nelem, size_t size);
 extern void POSIX_free(void *ptr);
 extern unsigned int POSIX_sleep(unsigned int seconds);
 extern int POSIX_pipe(int *errnop, int fildes[2]);
+extern int POSIX_umask(int *errnop, int mask);
 
 extern int POSIX_getuid(int *errnop);
 extern int POSIX_geteuid(int *errnop);
diff --git a/sys/src/lib/posix/processes.c b/sys/src/lib/posix/processes.c
index e9be8f2..ab46eb8 100644
--- a/sys/src/lib/posix/processes.c
+++ b/sys/src/lib/posix/processes.c
@@ -277,6 +277,12 @@ OnIgnoredSignalInterrupt:
 	return pid;
 }
 
+int
+POSIX_umask(int *errnop, int mask)
+{
+	return 0;
+}
+
 int
 POSIX_waitpid(int *errnop, int reqpid, int *status, int options)
 {