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

fusl/src/stdio/fwrite.c

+#include "stdio_impl.h"
+#include <string.h>
+
+size_t __fwritex(const unsigned char *restrict s, size_t l, FILE *restrict f)
+{
+        size_t i=0;
+
+        if (!f->wend && __towrite(f)) return 0;
+
+        if (l > f->wend - f->wpos) return f->write(f, s, l);
+
+        if (f->lbf >= 0) {
+                /* Match /^(.*\n|)/ */
+                for (i=l; i && s[i-1] != '\n'; i--);
+                if (i) {
+                        if (f->write(f, s, i) < i)
+                                return i;
+                        s += i;
+                        l -= i;
+                }
+        }
+
+        memcpy(f->wpos, s, l);
+        f->wpos += l;
+        return l+i;
+}
+
+size_t fwrite(const void *restrict src, size_t size, size_t nmemb, FILE *restrict f)
+{
+        size_t k, l = size*nmemb;
+        FLOCK(f);
+        k = __fwritex(src, l, f);
+        FUNLOCK(f);
+        return k==l ? nmemb : k/size;
+}
+
+weak_alias(fwrite, fwrite_unlocked);