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

fusl/src/internal/libc.h

+#ifndef LIBC_H
+#define LIBC_H
+
+#include <stdlib.h>
+#include <stdio.h>
+#include <limits.h>
+
+struct __locale_map;
+
+struct __locale_struct {
+        const struct __locale_map *volatile cat[6];
+};
+
+struct tls_module {
+        struct tls_module *next;
+        void *image;
+        size_t len, size, align, offset;
+};
+
+struct __libc {
+        int can_do_threads;
+        int threaded;
+        int secure;
+        volatile int threads_minus_1;
+        size_t *auxv;
+        struct tls_module *tls_head;
+        size_t tls_size, tls_align, tls_cnt;
+        size_t page_size;
+        struct __locale_struct global_locale;
+};
+
+#ifndef PAGE_SIZE
+#define PAGE_SIZE libc.page_size
+#endif
+
+#ifdef __PIC__
+#define ATTR_LIBC_VISIBILITY __attribute__((visibility("hidden")))
+#else
+#define ATTR_LIBC_VISIBILITY
+#endif
+
+extern struct __libc __libc ATTR_LIBC_VISIBILITY;
+#define libc __libc
+
+extern size_t __hwcap ATTR_LIBC_VISIBILITY;
+extern size_t __sysinfo ATTR_LIBC_VISIBILITY;
+extern char *__progname, *__progname_full;
+
+/* Designed to avoid any overhead in non-threaded processes */
+void __lock(volatile int *) ATTR_LIBC_VISIBILITY;
+void __unlock(volatile int *) ATTR_LIBC_VISIBILITY;
+int __lockfile(FILE *) ATTR_LIBC_VISIBILITY;
+void __unlockfile(FILE *) ATTR_LIBC_VISIBILITY;
+#define LOCK(x) __lock(x)
+#define UNLOCK(x) __unlock(x)
+
+void __synccall(void (*)(void *), void *);
+int __setxid(int, int, int, int);
+
+extern char **__environ;
+
+#undef weak_alias
+#define weak_alias(old, new) \
+        extern __typeof(old) new __attribute__((weak, alias(#old)))
+
+#undef LFS64_2
+#define LFS64_2(x, y) weak_alias(x, y)
+
+#undef LFS64
+#define LFS64(x) LFS64_2(x, x##64)
+
+#endif