Add shim for sendmmsg when sendmmsg does not exist

CMakeLists.txt

@@ -245,6 +245,18 @@ else()
             message(STATUS "SO_ATTACH_REUSEPORT_CBPF is missing. Server receive performance will be reduced")
         endif()
 
+        check_c_source_compiles(
+            "
+            #define _GNU_SOURCE
+            #include <sys/socket.h>
+            #include <stddef.h>
+            int main() { return sendmmsg(0, NULL, 0, 0); }
+            "
+            HAS_SENDMMSG)
+        if(NOT HAS_SENDMMSG)
+            message(STATUS "sendmmsg is missing. Send performance will be reduced")
+        endif()
+
         # Error if flags are missing in CI
         if(QUIC_CI AND NOT QUIC_SKIP_CI_CHECKS)
             if (NOT HAS_UDP_SEGMENT)
@@ -276,6 +288,9 @@ else()
 
     set(QUIC_COMMON_FLAGS "")
     set(QUIC_COMMON_DEFINES _GNU_SOURCE)
+    if (HAS_SENDMMSG)
+        list(APPEND QUIC_COMMON_DEFINES HAS_SENDMMSG)
+    endif()
     set(QUIC_WARNING_FLAGS -Werror -Wall -Wextra -Wformat=2 -Wno-type-limits
         -Wno-unknown-pragmas -Wno-multichar -Wno-missing-field-initializers
         CACHE INTERNAL "")

src/platform/datapath_epoll.c

@@ -28,6 +28,31 @@
 CXPLAT_STATIC_ASSERT((SIZEOF_STRUCT_MEMBER(QUIC_BUFFER, Length) <= sizeof(size_t)), "(sizeof(QUIC_BUFFER.Length) == sizeof(size_t) must be TRUE.");
 CXPLAT_STATIC_ASSERT((SIZEOF_STRUCT_MEMBER(QUIC_BUFFER, Buffer) == sizeof(void*)), "(sizeof(QUIC_BUFFER.Buffer) == sizeof(void*) must be TRUE.");
 
+#ifdef HAS_SENDMMSG
+#define CXPLAT_SENDMMSG sendmmsg
+#else
+static
+int
+cxplat_sendmmsg_shim(
+    int fd,
+    struct mmsghdr* Messages,
+    unsigned int MessageLen,
+    int Flags
+    )
+{
+    unsigned int SuccessCount = 0;
+    while (SuccessCount < MessageLen) {
+        int Result = sendmsg(fd, &Messages[SuccessCount].msg_hdr, Flags);
+        if (Result < 0) {
+            return SuccessCount == 0 ? Result : (int)SuccessCount;
+        }
+        Messages[SuccessCount].msg_len = Result;
+        SuccessCount++;
+    }
+    return SuccessCount;
+}
+#define CXPLAT_SENDMMSG cxplat_sendmmsg_shim
+#endif
 
 //
 // The maximum single buffer size for sending coalesced payloads.
@@ -2573,7 +2598,7 @@ CxPlatSocketSendInternal(
     while (SendData->SentMessagesCount < TotalMessagesCount) {
 
         int SuccessfullySentMessages =
-            sendmmsg(
+            CXPLAT_SENDMMSG(
                 SocketContext->SocketFd,
                 Mhdrs + SendData->SentMessagesCount,
                 (unsigned int)(TotalMessagesCount - SendData->SentMessagesCount),