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

fusl/src/network/sendmmsg.c

Index: fusl/src/network/sendmmsg.c
diff --git a/fusl/src/network/sendmmsg.c b/fusl/src/network/sendmmsg.c
new file mode 100644
index 0000000000000000000000000000000000000000..eeae1d0a5822503f5c572a46e3d3696f6c95b1b9
--- /dev/null
+++ b/fusl/src/network/sendmmsg.c
@@ -0,0 +1,30 @@
+#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
+}