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

fusl/src/stdio/__stdio_write.c

+#include "stdio_impl.h"
+#include <sys/uio.h>
+
+size_t __stdio_write(FILE *f, const unsigned char *buf, size_t len)
+{
+        struct iovec iovs[2] = {
+                { .iov_base = f->wbase, .iov_len = f->wpos-f->wbase },
+                { .iov_base = (void *)buf, .iov_len = len }
+        };
+        struct iovec *iov = iovs;
+        size_t rem = iov[0].iov_len + iov[1].iov_len;
+        int iovcnt = 2;
+        ssize_t cnt;
+        for (;;) {
+                cnt = syscall(SYS_writev, f->fd, iov, iovcnt);
+                if (cnt == rem) {
+                        f->wend = f->buf + f->buf_size;
+                        f->wpos = f->wbase = f->buf;
+                        return len;
+                }
+                if (cnt < 0) {
+                        f->wpos = f->wbase = f->wend = 0;
+                        f->flags |= F_ERR;
+                        return iovcnt == 2 ? 0 : len-iov[0].iov_len;
+                }
+                rem -= cnt;
+                if (cnt > iov[0].iov_len) {
+                        cnt -= iov[0].iov_len;
+                        iov++; iovcnt--;
+                }
+                iov[0].iov_base = (char *)iov[0].iov_base + cnt;
+                iov[0].iov_len -= cnt;
+        }
+}