多进程并发服务器

使用多进程并发服务器时要考虑以下几点：

1. 父进程最大文件描述个数（父进程中需要close关闭accept返回的新文件描述符）
2. 系统内创建进程个数（与内存大小相关）
3. 进程创建过多是否降低整体服务性能（进程调度）

server.c
#include "wrap.h"
#include <arpa/inet.h>
#include <netinet/in.h>
#include <signal.h>
#include <stdio.h>
#include <string.h>
#include <sys/types.h>
#include <sys/wait.h>

#define MAXLINE 80
#define SERV_PORT 800

void do_sigchild(int num) {
    while (waitpid(0, NULL, WNOHANG) > 0){};
}
int main(int argc, char *argv[]) {
    struct sockaddr_in servaddr, cliaddr;
    socklen_t cliaddr_len;
    int listenfd, connfd;
    char buf[MAXLINE];
    char str[INET_ADDRSTRLEN];
    int i, n;
    pid_t pid;

    struct sigaction newact;
    newact.sa_handler = do_sigchild;
    sigemptyset(&newact.sa_mask);
    newact.sa_flags = 0;
    sigaction(SIGCHLD, &newact, NULL);

    listenfd = Socket(AF_INET, SOCK_STREAM, 0);

    bzero(&servaddr, sizeof(servaddr));
    servaddr.sin_family = AF_INET;
    servaddr.sin_addr.s_addr = htonl(INADDR_ANY);
    servaddr.sin_port = htons(SERV_PORT);

    Bind(listenfd, (struct sockaddr *) &servaddr, sizeof(servaddr));

    Listen(listenfd, 20);

    printf("Accepting connections ...\n");
    while (1) {
        cliaddr_len = sizeof(cliaddr);
        connfd = Accept(listenfd, (struct sockaddr *) &cliaddr, &cliaddr_len);

        pid = fork();
        if (pid == 0) {
            Close(listenfd);
            while (1) {
                n = Read(connfd, buf, MAXLINE);
                if (n == 0) {
                    printf("the other side has been closed.\n");
                    break;
                }
                printf("received from %s at PORT %d\n",
                       inet_ntop(AF_INET, &cliaddr.sin_addr, str, sizeof(str)),
                       ntohs(cliaddr.sin_port));
                for (i = 0; i < n; i++)
                    buf[i] = toupper(buf[i]);
                Write(connfd, buf, n);
            }
            Close(connfd);
            return 0;
        } else if (pid > 0) {
            Close(connfd);
        } else
            perr_exit("fork");
    }
    Close(listenfd);
    return 0;
}

client.c
#include "wrap.h"
#include <netinet/in.h>
#include <stdio.h>
#include <string.h>
#include <unistd.h>

#define MAXLINE 80
#define SERV_PORT 6666

int main(int argc, char *argv[]) {
    struct sockaddr_in servaddr;
    char buf[MAXLINE];
    int sockfd, n;

    sockfd = Socket(AF_INET, SOCK_STREAM, 0);

    bzero(&servaddr, sizeof(servaddr));
    servaddr.sin_family = AF_INET;
    inet_pton(AF_INET, "127.0.0.1", &servaddr.sin_addr);
    servaddr.sin_port = htons(SERV_PORT);

    Connect(sockfd, (struct sockaddr *) &servaddr, sizeof(servaddr));

    while (fgets(buf, MAXLINE, stdin) != NULL) {
        Write(sockfd, buf, strlen(buf));
        n = Read(sockfd, buf, MAXLINE);
        if (n == 0) {
            printf("the other side has been closed.\n");
            break;
        } else
            Write(STDOUT_FILENO, buf, n);
    }
    Close(sockfd);
    return 0;
}