/* Listing 1.3 * * Client/Server Example Using socketpair(2) * and fork(2) : */#include <stdio.h>#include <stdlib.h>#include <unistd.h>#include <errno.h>#include <string.h>#include <time.h>#include <sys/types.h>#include <sys/socket.h>#include <sys/wait.h>/* * As of RedHat-6.0, these are still not defined : */#ifndef SHUT_WR#define SHUT_RD     0#define SHUT_WR     1#define SHUT_RDWR   2#endif/* * Main program : */intmain(int argc,char **argv) {    int z;          /* Status return code */    int s[2];       /* Pair of sockets */    char *msgp;     /* A message pointer */    int mlen;       /* Message length */    char buf[80];   /* Work buffer */    pid_t chpid;    /* Child PID */    /*     * Create a pair of local sockets :     */    z = socketpair(AF_LOCAL,SOCK_STREAM,0,s);    if ( z == -1 ) {        fprintf(stderr,"%s: socketpair(2)\n",            strerror(errno));        exit(1);    }    /*     * Now fork() into two processes :     */    if ( (chpid = fork()) == (pid_t)-1 ) {        /*         * Failed to fork into two processes :         */        fprintf(stderr,"%s: fork(2)\n",            strerror(errno));        exit(1);    } else if ( chpid == 0 ) {        /*         * This is the child process (client) :         */        char rxbuf[80]; /* Receive buffer */        printf("Parent PID is %ld\n",(long)getppid());        close(s[0]);    /* Server uses s[1] */        s[0] = -1;      /* Forget this unit */        /*         * Form the message and its length :         */        msgp = "%A %d-%b-%Y %l:%M %p";        mlen = strlen(msgp);        printf("Child sending request '%s'\n",msgp);        fflush(stdout);        /*         * Write a request to the server :         */        z = write(s[1],msgp,mlen);        if ( z < 0 ) {            fprintf(stderr,"%s: write(2)\n",                strerror(errno));            exit(1);        }        /*         * Now indicate that we will not be writing         * anything further to our socket, by shutting         * down the write side of the socket :         */        if ( shutdown(s[1],SHUT_WR) == -1 ) {            fprintf(stderr,"%s: shutdown(2)\n",                strerror(errno));            exit(1);        }        /*         * Receive the reply from the server :         */        z = read(s[1],rxbuf,sizeof rxbuf);        if ( z < 0 ) {            fprintf(stderr,"%s: read(2)\n",                strerror(errno));            exit(1);        }        /*         * Put a null byte at the end of what we         * received from the server:         */        rxbuf[z] = 0;        /*         * Report the results :         */        printf("Server returned '%s'\n",rxbuf);        fflush(stdout);        close(s[1]);    /* Close our end now */    } else {        /*         * This is the parent process (server) :         */        int status; /* Child termination status */        char txbuf[80];         /* Reply buffer */        time_t td;       /* Current date & time */        printf("Child PID is %ld\n",(long)chpid);        fflush(stdout);        close(s[1]);    /* Client uses s[1] */        s[1] = -1;      /* Forget this descriptor */        /*         * Wait for a request from the client :         */        z = read(s[0],buf,sizeof buf);        if ( z < 0 ) {            fprintf(stderr,"%s: read(2)\n",                strerror(errno));            exit(1);        }        /*         * Put a null byte at the end of the         * message we received from the client:         */        buf[z] = 0;        /*         * Now perform the server function on the         * received message :         */        time(&td);          /* Get current time */        strftime(txbuf,sizeof txbuf,  /* Buffer */            buf,                /* Input format */            localtime(&td));      /* Input time */        /*         * Send back the response to client :         */        z = write(s[0],txbuf,strlen(txbuf));        if ( z < 0 ) {            fprintf(stderr,"%s: write(2)\n",                strerror(errno));            exit(1);        }        /*         * Close our end of the socket :         */        close(s[0]);        /*         * Wait for the child process to exit..         * See text.         */        waitpid(chpid,&status,0);    }    return 0;}