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

fusl/src/network/sendmmsg.c

+#define _GNU_SOURCE
+#include <sys/socket.h>
+#include <limits.h>
+#include <errno.h>
+#include "syscall.h"
+
+int sendmmsg(int fd, struct mmsghdr *msgvec, unsigned int vlen, unsigned int flags)
+{
+#if LONG_MAX > INT_MAX
+        /* Can't use the syscall directly because the kernel has the wrong
+         * idea for the types of msg_iovlen, msg_controllen, and cmsg_len,
+         * and the cmsg blocks cannot be modified in-place. */
+        int i;
+        if (vlen > IOV_MAX) vlen = IOV_MAX; /* This matches the kernel. */
+        if (!vlen) return 0;
+        for (i=0; i<vlen; i++) {
+                /* As an unfortunate inconsistency, the sendmmsg API uses
+                 * unsigned int for the resulting msg_len, despite sendmsg
+                 * returning ssize_t. However Linux limits the total bytes
+                 * sent by sendmsg to INT_MAX, so the assignment is safe. */
+                ssize_t r = sendmsg(fd, &msgvec[i].msg_hdr, flags);
+                if (r < 0) goto error;
+                msgvec[i].msg_len = r;
+        }
+error:
+        return i ? i : -1;
+#else
+        return syscall_cp(SYS_sendmmsg, fd, msgvec, vlen, flags);
+#endif
+}