socket-util: Support sendmmsg() regardless of platform.

This will have its first user in an upcoming commit.

Signed-off-by: Ben Pfaff <blp@ovn.org>

lib/socket-util.h

@@ -87,6 +87,33 @@ int make_unix_socket(int style, bool nonblock,
                      const char *bind_path, const char *connect_path);
 int get_unix_name_len(const struct sockaddr_un *sun, socklen_t sun_len);
 
+/* Universal sendmmsg support.
+ *
+ * Some platforms, such as new enough Linux and FreeBSD, support sendmmsg, but
+ * other platforms (or older ones) do not.  We add the following infrastructure
+ * to allow all code to use sendmmsg, regardless of platform support:
+ *
+ *   - For platforms that lack sendmmsg entirely, we emulate it.
+ *
+ *   - Some platforms have sendmmsg() in the C library but not in the kernel.
+ *     For example, this is true if a Linux system has a newer glibc with an
+ *     old kernel.  To compensate, even if sendmmsg() appears to be available,
+ *     we still wrap it with a handler that uses our emulation if sendmmsg()
+ *     returns ENOSYS.
+ */
+#ifndef HAVE_STRUCT_MMSGHDR_MSG_LEN
+struct mmsghdr {
+    struct msghdr msg_hdr;
+    unsigned int msg_len;
+};
+#endif
+#ifndef HAVE_SENDMMSG
+int sendmmsg(int, struct mmsghdr *, unsigned int, unsigned int);
+#else
+#define sendmmsg wrap_sendmmsg
+int wrap_sendmmsg(int, struct mmsghdr *, unsigned int, unsigned int);
+#endif
+
 /* Helpers for calling ioctl() on an AF_INET socket. */
 struct ifreq;
 int af_inet_ioctl(unsigned long int command, const void *arg);