sendmmsg总是返回1

我正在尝试使用sendmmsg。我的程序工作正常，但是sendmmsg总是一次发送1个数据包（返回1）。我已经通过gdb检查了vlen设置为正确的值（128），正如我说的那样，它确实可以发送数据包。在插座上启用sendmmsg，我有什么特别的事情要做吗？如果这有所不同，它是数据报（UDP）套接字。另外，这在Ubuntu 16和RHEL 7.2。

更新：另外，如果它很重要，我正在以非阻滞模式运行，所以我并不感到惊讶，它返回少于128，只是它总是返回1。

更新：添加MCVE：

#include <sys/socket.h>
#include <netinet/ip.h>
#include <arpa/inet.h>
#include <unistd.h>
#include <cstring>
#include <string>
#include <vector>
#include <stdexcept>
#include <iostream>
const int SND_BUFFER_SIZE = 212992;
const int PORT = 3333;
const int NUM_MESSAGES = 128;
const int BYTES_PER_MESSAGE = 1024;
int numSendCalls = 0;
int numPacketsSent = 0;
void SetSndBufferSize(int socketFd) {
    int sndBufferSize = SND_BUFFER_SIZE;
    socklen_t sndBufferSizeLen = sizeof(int);
    // Set the value
    if(setsockopt(socketFd, SOL_SOCKET, SO_SNDBUF, (char*) &sndBufferSize, sndBufferSizeLen) != 0) {
        throw std::runtime_error("Could not set the send buffer size (errno=" + std::string(strerror(errno)) + ")");
    }
    // Confirm the value was set as desired
    if(getsockopt(socketFd, SOL_SOCKET, SO_SNDBUF, (char*) &sndBufferSize, &sndBufferSizeLen) != 0) {
        throw std::runtime_error("Could not fetch the send buffer size after setting it (errno=" + std::string(strerror(errno)) + ")");
    }
    if(sndBufferSize != (SND_BUFFER_SIZE * 2)) {
        throw std::runtime_error("The underlying OS could not set the send buffer size to what we wanted (" + std::to_string(SND_BUFFER_SIZE) + ") and instead it was (" + std::to_string(sndBufferSize / 2) + ")");
    }
}
struct sockaddr_in MakeAddress(std::string destinationAddress, uint16_t destinationPort) {
    struct sockaddr_in result;
    result.sin_family = AF_INET;
    result.sin_port = htons(destinationPort);
    if(inet_aton(destinationAddress.c_str(), &result.sin_addr) == 0) {
        throw std::runtime_error("Invalid IPv4 address " + destinationAddress);
    }
    return result;
}
void Connect(int socketFd) {
    auto addr = MakeAddress("127.0.0.1", 3333);
    if(connect(socketFd, (struct sockaddr*)&addr, sizeof(addr)) != 0) {
        throw std::runtime_error("Failed to connect to address (" + std::string(strerror(errno)) + ")");
    }
}
std::vector<std::string> MakeDataBuffers() {
    std::vector<std::string> result;
    for(int i = 0; i < NUM_MESSAGES; i++) {
        result.emplace_back(BYTES_PER_MESSAGE, 'W');
    }
    return result;
}
std::vector<struct iovec> MakeIovecs(std::vector<std::string> & dataBuffers) {
    std::vector<struct iovec> result;
    for(int i = 0; i < NUM_MESSAGES; i++) {
        result.emplace_back();
        result[i].iov_base = (void*) dataBuffers[i].data();
        result[i].iov_len = BYTES_PER_MESSAGE;
    }
    return result;
}
std::vector<struct mmsghdr> MakeHeaders(std::vector<struct iovec> & iovecs) {
    std::vector<struct mmsghdr> result;
    for(int i = 0; i < NUM_MESSAGES; i++) {
        result.emplace_back();
        result[i].msg_hdr.msg_name = NULL;
        result[i].msg_hdr.msg_namelen = 0;
        result[i].msg_hdr.msg_iov = (struct iovec*) &iovecs[i];
        result[i].msg_hdr.msg_iovlen = 1;
        result[i].msg_hdr.msg_control = NULL;
        result[i].msg_hdr.msg_controllen = 0;
        result[i].msg_hdr.msg_flags = 0;
        result[i].msg_len = 0;
    }
    return result;
}
void Send(int socketFd, std::vector<struct mmsghdr> & msgvec) {
    struct mmsghdr * msgvecdata = (struct mmsghdr*) msgvec.data();
    unsigned int size = msgvec.size();
    int numUpdated = sendmmsg(socketFd, msgvecdata, size, 0);
    numSendCalls++;
    if(numUpdated < 0) {
        throw std::runtime_error("Error sending packets (" + std::string(strerror(errno)) + ")");
    }
    numPacketsSent += numUpdated;
}
void Reset(std::vector<struct mmsghdr> & msgvec) {
    for(auto it = msgvec.begin(); it < msgvec.end(); it++) {
        it->msg_len = 0;
    }
}
int main() {
    auto socketFd = socket(AF_INET, SOCK_DGRAM | SOCK_NONBLOCK, 0);
    SetSndBufferSize(socketFd);
    Connect(socketFd);
    auto dataBuffers = MakeDataBuffers();
    auto iovecs = MakeIovecs(dataBuffers);
    auto headers = MakeHeaders(iovecs);
    for(int i = 0; i < 1000; i++) {
        Send(socketFd, headers);
        Reset(headers);
    }
    std::cout << "Sent: " << std::to_string(numPacketsSent) << " packets across " << std::to_string(numSendCalls) + " calls" << std::endl;
    close(socketFd);
    return 0;
}