echosrv.c

#include <sys/types.h>          /* See NOTES */
#include <sys/socket.h>
#include <unistd.h>
#include <netinet/in.h>
#include <arpa/inet.h>

#include <stdlib.h>
#include <stdio.h>
#include <errno.h>
#include <string.h>

#define ERR_EIXT(m) \
        do \
        {  \
            perror(m); \
            exit(EXIT_FAILURE); \
        } while(0)

int main(int argc, char const *argv[])
{
    int listenfd;
    if((listenfd = socket(PF_INET, SOCK_STREAM, IPPROTO_TCP)) < 0 )
        ERR_EIXT("socket failed");
    // listenfd = socket(PF_INET, SOCK_STREAM, 0);
    struct  sockaddr_in servaddr;
    memset(&servaddr, 0, sizeof(servaddr));
    servaddr.sin_family = AF_INET;
    servaddr.sin_port = htons(5188);
    servaddr.sin_addr.s_addr = htonl(INADDR_ANY);   //INADDR_ANY表示本机的任意地址, 网络字节序
    // servaddr.sin_addr.s_addr = inet_addr("127.0.0.1"); //显示指定本机的IP地址
    // inet_aton("127.0.0.1", &servaddr.sin_addr)
    if(bind(listenfd, (struct sockaddr*)&servaddr, sizeof(servaddr)) < 0)
        ERR_EIXT("bind failed");
    if(listen(listenfd, SOMAXCONN) < 0)
        ERR_EIXT("listen failed");  // SOMAXCONN是队列允许的最大值

    struct sockaddr_in peeraddr;
    socklen_t peerlen = sizeof(peeraddr);  //peerlen一定要有初始值
    int conn;
    if((conn = accept(listenfd, (struct sockaddr*)&peeraddr, &peerlen)) < 0)
        ERR_EIXT("accept failed");

    char recvbuf[1024];
    while(1) {
        memset(recvbuf, 0, sizeof(recvbuf));
        int ret = read(conn, recvbuf, sizeof(recvbuf)); //conn是已连接套接字, 是主动套接字
        fputs(recvbuf, stdout);
        write(conn, recvbuf, ret);
    }

    return 0;
}