网络编程 UDP 发送端 bind 作用

upd 发送端调用bind函数的效果：把socket特定到一个指定的端口，如果不调用bind，内核会随机分配一个端口。

upd 发送端调用bind函数的目的：假如有2个发送端，接收端需要识别是从哪个发送端过来的，就可以分别在发送端调用bind函数，这样一来，接收端就能够知道是哪个发送端过来的数据了。

运行方法：先运行【1，先接收再发送】，再运行【2，先发送再接收】

1，先接收再发送：

#include <stdio.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <string.h>
#include <netdb.h>//getaddrinfo
int main(){
  int sock;
  sockaddr_in addr;
  sockaddr_in senderinfo;
  socklen_t addrlen;
  char buf[2048];
  char senderstr[16];
  int n;
  sock = socket(AF_INET, SOCK_DGRAM, 0);
  addr.sin_family = AF_INET;
  addr.sin_port = htons(12345);
  addr.sin_addr.s_addr = INADDR_ANY;
  bind(sock, (sockaddr*)&addr, sizeof(addr));
  while(1){
    memset(buf, 0, sizeof(buf));
    addrlen = sizeof(senderinfo);
    n = recvfrom(sock, buf, sizeof(buf) - 1, 0,
         (sockaddr*)&senderinfo, &addrlen);
    inet_ntop(AF_INET, &senderinfo.sin_addr, senderstr, sizeof(senderstr));
    printf("recvfrom: %s, port=%d\n", senderstr, ntohs(senderinfo.sin_port));
    
    sendto(sock, buf, n, 0, (sockaddr*)&senderinfo, addrlen);
    printf("send data to: %s, port=%d\n", senderstr, ntohs(senderinfo.sin_port));
    
  }
}

github源代码

执行方式：

g++ -g udp_recv_send.cpp -std=c++11 -o se
./re

2，先发送再接收：

#include <stdio.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <string.h>
#include <netdb.h>//getaddrinfo
void print_port_num(int sock){
  sockaddr_in s;
  socklen_t sz = sizeof(s);
  getsockname(sock, (sockaddr*)&s, &sz);
  printf("%d\n", ntohs(s.sin_port));
}
int main(int argc, char* argv[]){
  int sock;
  sockaddr_in addr;
  sockaddr_in senderinfo;
  socklen_t senderinfolen;
  int n;
  char buf[2048];
  sockaddr_in myname;
  
  if(argc != 2){
    return 1;
  }
  sock = socket(AF_INET, SOCK_DGRAM, 0);
  addr.sin_family = AF_INET;
  addr.sin_port = htons(12345);
  inet_pton(AF_INET, argv[1], &addr.sin_addr.s_addr);
  
  myname.sin_family = AF_INET;
  myname.sin_addr.s_addr = INADDR_ANY;
  myname.sin_port = htons(34567);
  bind(sock, (sockaddr*)&myname, sizeof(myname));
  
  n = sendto(sock, "HELLO", 5, 0, (sockaddr*)&addr, sizeof(addr));
  if(n < 1){
    perror("sendto");
    return 1;
  }
  print_port_num(sock);
  memset(buf, 0, sizeof(buf));
  senderinfolen = sizeof(senderinfo);
  recvfrom(sock, buf, sizeof(buf), 0,
       (sockaddr*)&senderinfo, &senderinfolen);
  printf("%s\n", buf);
  close(sock);
  return 0;
}