巧妙的获取本机ip地址

get_local_addr.c

#include <stdio.h>
#include <unistd.h>
#include <string.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <fcntl.h>

char* get_local_addr(char* buf, size_t len);

int main(int argc, char** argv)
{
    char strLocalAddr[16] = "0.0.0.0\0";
    get_local_addr(strLocalAddr, sizeof(strLocalAddr));
    printf("local address: %s\n", strLocalAddr);
    return 0;
}

char* get_local_addr(char* buf, size_t len)
{
    if(16 > len)
        return buf;

    memset(buf, '\0', len);

    int32_t iClientSockfd = socket(AF_INET, SOCK_DGRAM, 0);
    if(iClientSockfd < 0 )
    {
        return buf;
    }

    struct sockaddr_in stINETAddr;
    stINETAddr.sin_addr.s_addr = inet_addr("192.168.0.1");
    stINETAddr.sin_family = AF_INET;
    stINETAddr.sin_port = htons(18888);

    int32_t iCurrentFlag = fcntl(iClientSockfd, F_GETFL, 0);
    fcntl(iClientSockfd, F_SETFL, iCurrentFlag | O_NONBLOCK);

    if( 0 != connect(iClientSockfd, (struct sockaddr *) &stINETAddr, sizeof(stINETAddr)) )
    {
        close(iClientSockfd);
        return buf;
    }

    struct sockaddr_in stINETAddrLocal;
    socklen_t iAddrLenLocal = sizeof(stINETAddrLocal);
    getsockname(iClientSockfd, (struct sockaddr *)&stINETAddrLocal, &iAddrLenLocal);

    close(iClientSockfd);
    snprintf(buf, len, "%s", inet_ntoa(stINETAddrLocal.sin_addr));
    return buf;
}

第33行：
对于双网卡（通常是一块内网一块外网）设备，设置socket服务端ip为外网地址时（比如8.8.8.8）则将获得本机外网网卡ip，设置socket服务端ip为内网地址时（比如192.168.0.1）则将获得本机内网网卡ip。