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

fusl/src/thread/pthread_atfork.c

+#include <pthread.h>
+#include "libc.h"
+
+static struct atfork_funcs {
+        void (*prepare)(void);
+        void (*parent)(void);
+        void (*child)(void);
+        struct atfork_funcs *prev, *next;
+} *funcs;
+
+static volatile int lock[2];
+
+void __fork_handler(int who)
+{
+        struct atfork_funcs *p;
+        if (!funcs) return;
+        if (who < 0) {
+                LOCK(lock);
+                for (p=funcs; p; p = p->next) {
+                        if (p->prepare) p->prepare();
+                        funcs = p;
+                }
+        } else {
+                for (p=funcs; p; p = p->prev) {
+                        if (!who && p->parent) p->parent();
+                        else if (who && p->child) p->child();
+                        funcs = p;
+                }
+                UNLOCK(lock);
+        }
+}
+
+int pthread_atfork(void (*prepare)(void), void (*parent)(void), void (*child)(void))
+{
+        struct atfork_funcs *new = malloc(sizeof *new);
+        if (!new) return -1;
+
+        LOCK(lock);
+        new->next = funcs;
+        new->prev = 0;
+        new->prepare = prepare;
+        new->parent = parent;
+        new->child = child;
+        if (funcs) funcs->prev = new;
+        funcs = new;
+        UNLOCK(lock);
+        return 0;
+}