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;

}

github源代码

./a.out 127.0.0.1

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