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

fusl/src/stdio/ftell.c

Index: fusl/src/stdio/ftell.c
diff --git a/fusl/src/stdio/ftell.c b/fusl/src/stdio/ftell.c
new file mode 100644
index 0000000000000000000000000000000000000000..bb62897a4c7cc710295fd62aae8bb727b116a4d5
--- /dev/null
+++ b/fusl/src/stdio/ftell.c
@@ -0,0 +1,37 @@
+#include "stdio_impl.h"
+#include <limits.h>
+#include <errno.h>
+
+off_t __ftello_unlocked(FILE *f)
+{
+	off_t pos = f->seek(f, 0,
+		(f->flags & F_APP) && f->wpos > f->wbase
+		? SEEK_END : SEEK_CUR);
+	if (pos < 0) return pos;
+
+	/* Adjust for data in buffer. */
+	return pos - (f->rend - f->rpos) + (f->wpos - f->wbase);
+}
+
+off_t __ftello(FILE *f)
+{
+	off_t pos;
+	FLOCK(f);
+	pos = __ftello_unlocked(f);
+	FUNLOCK(f);
+	return pos;
+}
+
+long ftell(FILE *f)
+{
+	off_t pos = __ftello(f);
+	if (pos > LONG_MAX) {
+		errno = EOVERFLOW;
+		return -1;
+	}
+	return pos;
+}
+
+weak_alias(__ftello, ftello);
+
+LFS64(ftello);