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

fusl/src/stdio/fseek.c

+#include "stdio_impl.h"
+
+int __fseeko_unlocked(FILE *f, off_t off, int whence)
+{
+        /* Adjust relative offset for unread data in buffer, if any. */
+        if (whence == SEEK_CUR) off -= f->rend - f->rpos;
+
+        /* Flush write buffer, and report error on failure. */
+        if (f->wpos > f->wbase) {
+                f->write(f, 0, 0);
+                if (!f->wpos) return -1;
+        }
+
+        /* Leave writing mode */
+        f->wpos = f->wbase = f->wend = 0;
+
+        /* Perform the underlying seek. */
+        if (f->seek(f, off, whence) < 0) return -1;
+
+        /* If seek succeeded, file is seekable and we discard read buffer. */
+        f->rpos = f->rend = 0;
+        f->flags &= ~F_EOF;
+        
+        return 0;
+}
+
+int __fseeko(FILE *f, off_t off, int whence)
+{
+        int result;
+        FLOCK(f);
+        result = __fseeko_unlocked(f, off, whence);
+        FUNLOCK(f);
+        return result;
+}
+
+int fseek(FILE *f, long off, int whence)
+{
+        return __fseeko(f, off, whence);
+}
+
+weak_alias(__fseeko, fseeko);
+
+LFS64(fseeko);