Add a "fork" of musl as //fusl.

fusl/src/signal/sigaction.c

+#include <signal.h>
+#include <errno.h>
+#include <string.h>
+#include "syscall.h"
+#include "pthread_impl.h"
+#include "libc.h"
+#include "ksigaction.h"
+
+static int unmask_done;
+static unsigned long handler_set[_NSIG/(8*sizeof(long))];
+
+void __get_handler_set(sigset_t *set)
+{
+        memcpy(set, handler_set, sizeof handler_set);
+}
+
+int __libc_sigaction(int sig, const struct sigaction *restrict sa, struct sigaction *restrict old)
+{
+        struct k_sigaction ksa, ksa_old;
+        if (sa) {
+                if ((uintptr_t)sa->sa_handler > 1UL) {
+                        a_or_l(handler_set+(sig-1)/(8*sizeof(long)),
+                                1UL<<(sig-1)%(8*sizeof(long)));
+
+                        /* If pthread_create has not yet been called,
+                         * implementation-internal signals might not
+                         * yet have been unblocked. They must be
+                         * unblocked before any signal handler is
+                         * installed, so that an application cannot
+                         * receive an illegal sigset_t (with them
+                         * blocked) as part of the ucontext_t passed
+                         * to the signal handler. */
+                        if (!libc.threaded && !unmask_done) {
+                                __syscall(SYS_rt_sigprocmask, SIG_UNBLOCK,
+                                        SIGPT_SET, 0, _NSIG/8);
+                                unmask_done = 1;
+                        }
+                }
+                ksa.handler = sa->sa_handler;
+                ksa.flags = sa->sa_flags | SA_RESTORER;
+                ksa.restorer = (sa->sa_flags & SA_SIGINFO) ? __restore_rt : __restore;
+                memcpy(&ksa.mask, &sa->sa_mask, sizeof ksa.mask);
+        }
+        if (syscall(SYS_rt_sigaction, sig, sa?&ksa:0, old?&ksa_old:0, sizeof ksa.mask))
+                return -1;
+        if (old) {
+                old->sa_handler = ksa_old.handler;
+                old->sa_flags = ksa_old.flags;
+                memcpy(&old->sa_mask, &ksa_old.mask, sizeof ksa_old.mask);
+        }
+        return 0;
+}
+
+int __sigaction(int sig, const struct sigaction *restrict sa, struct sigaction *restrict old)
+{
+        if (sig-32U < 3 || sig-1U >= _NSIG-1) {
+                errno = EINVAL;
+                return -1;
+        }
+        return __libc_sigaction(sig, sa, old);
+}
+
+weak_alias(__sigaction, sigaction);