c/c++ 网络编程 UDP 发送端 bind 作用

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));
    
  }
}

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;
}

./a.out 127.0.0.1

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