get route source address

get_src_addr.cc

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

std::string get_src_addr_popen(const std::string& addr) {
  // assemble command string
  char command[64];
  snprintf(command, 64, "ip route get %s", addr.c_str());

  // execute command
  FILE* fp = popen(command, "r");
  if (fp == NULL) {
    return "";  // error
  }

  // get stdout of command
  char buffer[64];
  if (fgets(buffer, 64, fp) == NULL) {
    return "";  // error
  }
  buffer[strlen(buffer) - 1] = '\0';  // chomp '\n'

  // get source address
  char* pos = strstr(buffer, "src ");
  if (pos == NULL) {
    return "";  // error
  }

  pclose(fp);  // close file pointer
  return std::string(pos + 4);
}

std::string get_src_addr_socket(const std::string& addr) {
  // create address structure
  struct sockaddr_in sin;
  socklen_t len = sizeof(sin);

  // set address configurations
  sin.sin_family = AF_INET;
  inet_aton(addr.c_str(), &sin.sin_addr);
  sin.sin_port = htons(0);

  // create socket
  int sockfd = socket(AF_INET, SOCK_DGRAM, 0);
  if (sockfd == -1) {
    return "";  // error
  }

  // connect socket
  int rc_conn = connect(sockfd, (struct sockaddr*)&sin, sizeof(sin));
  if (rc_conn == -1) {
    return "";  // error
  }

  // get local address
  int rc_get = getsockname(sockfd, (struct sockaddr*)&sin, &len);
  if (rc_get == -1) {
    return "";  // error
  }

  return std::string(inet_ntoa(sin.sin_addr));
}

#include <iostream>

int main(int argc, char** argv) {
  std::cout << get_src_addr_popen(argv[1]) << std::endl;
  std::cout << get_src_addr_socket(argv[1]) << std::endl;
  return 0;
}