Subject: Bug in socket fork code?
From: [email-masked] ("Ajay M. Desai")

30 Dec 1996 15:41:10 -0800
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The following code compiles but hangs. If the server fork is removed
it works just fine.

-- 
|-----------------------------------------------------|
| Ajay M. Desai                                       |
| Software Engineering Consultant      MCI Metro      |
| Home: mailto:[email-masked]         Vienna, VA     |
| Work: mailto:[email-masked]      703 918-0845   |
| MCI Mail ID: 214-0881                               |
|-----------------------------------------------------|

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/*
 * Connects to port 1234 on the local host.
 */
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <netdb.h>
#include <stdio.h>
#include <string.h>
#include <unistd.h>

#define NSTRS     3     /* no. of strings */

/*
 * Strings we send to the client.
 */
char *strs[NSTRS] = {
    "This is the first string from the server.\n",
    "This is the second string from the server.\n",
    "This is the third string from the server.\n"
};

extern int errno;

int main()
{
    char c;
    FILE *fp;
    int fromlen;
    char hostname[64];
    struct hostent *hp;
    register int i, s, ns;
    struct sockaddr_in sin, fsin;

    /*
     * Before we can do anything, we need
     * to know our hostname.
     */
    gethostname(hostname, sizeof(hostname));

    /*
     * Now we look up our host to get
     * its network number.
     */
    if ((hp = gethostbyname(hostname)) == NULL) {
        fprintf(stderr, "%s: host unknown.\n", hostname);
        exit(1);
    }

    /*
     * Get a socket to work with.  This socket will
     * be in the Internet domain, and will be a
     * stream socket.
     */
    if ((s = socket(AF_INET, SOCK_STREAM, 0)) < 0) {
        perror("server: socket");
        exit(1);
    }

    /*
     * Create the address that we will be binding to.
     * We use port 1234 but put it into network
     * byte order.  Also, we use bcopy (see 
     * Chapter 14) to copy the network number.
     */
    sin.sin_family = AF_INET;
    sin.sin_port = htons(1234);
    bcopy(hp->h_addr, &sin.sin_addr, hp->h_length);

    /*
     * Try to bind the address to the socket.
     */
    if (bind(s, (struct sock_addr*)&sin, sizeof(sin)) < 0) {
        perror("server: bind");
        exit(1);
    }

    /*
     * Listen on the socket.
     */
    if (listen(s, 5) < 0) {
        perror("server: listen");
        exit(1);
    }

	while(1){
		/*
		 * Accept connections.  When we accept one, ns
		 * will be connected to the client.  fsin will
		 * contain the address of the client.
		 */
		if ((ns = accept(s, (struct sock_addr*)&fsin, &fromlen)) < 0) {
			perror("server: accept");
			exit(1);
		}

		if (fork() == 0){
			/*
			 * We'll use stdio for reading the socket.
			 */
			fp = fdopen(ns, "r");

			/*
			 * First we send some strings to the client.
			 */
			for (i = 0; i < NSTRS; i++)
				send(ns, strs[i], strlen(strs[i]), 0);

			/*
			 * Then we read some strings from the client
			 * and print them out.
			 */
			for (i = 0; i < NSTRS; i++) {
				while ((c = fgetc(fp)) != EOF) {
					putchar(c);

					if (c == '\n')
						break;
				}
			}

			/*
			 * We can simply use close() to terminate the
			 * connection, since we're done with both sides.
			 */
			close(ns);
			exit(0);
		}
		else{
			close(ns); // parent
		}
	}

    return(0);
}

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/*
 * Connects to the local host at port 1234.
 */
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <netdb.h>
#include <stdio.h>
#include <unistd.h>

#define NSTRS     3     /* no. of strings  */

/*
 * Strings we send to the server.
 */
char *strs[NSTRS] = {
    "This is the first string from the client.\n",
    "This is the second string from the client.\n",
    "This is the third string from the client.\n"
};

extern int errno;

main()
{
    char c;
    FILE *fp;
    char hostname[64];
    register int i, s;
    struct hostent *hp;
    struct sockaddr_in sin;

    /*
     * Before we can do anything, we need to know
     * our hostname.
     */
    gethostname(hostname, sizeof(hostname));

    /*
     * Next, we need to look up the network
     * address of our host.
     */
    if ((hp = gethostbyname(hostname)) == NULL) {
        fprintf(stderr, "%s: unknown host.\n", hostname);
        exit(1);
    }

    /*
     * Get a socket to work with.  This socket will
     * be in the Internet domain, and will be a
     * stream socket.
     */
    if ((s = socket(AF_INET, SOCK_STREAM, 0)) < 0) {
        perror("client: socket");
        exit(1);
    }

    /*
     * Create the address we will be connecting to.
     * We use port 1234 but put it into network
     * byte order.  Also, we use bcopy (see Chapter
     * 14) to copy the network number.
     */
    sin.sin_family = AF_INET;
    sin.sin_port = htons(1234);
    bcopy(hp->h_addr, &sin.sin_addr, hp->h_length);

    /*
     * Try to connect to the address.  For this to
     * succeed, the server must already have bound
     * this address, and must have issued a listen()
     * request.
     */
    if (connect(s, (struct sock_addr*)&sin, sizeof(sin)) < 0) {
        perror("client: connect");
        exit(1);
    }

    /*
     * We'll use stdio for reading
     * the socket.
     */
    fp = fdopen(s, "r");

    /*
     * First we read some strings from the server
     * and print them out.
     */
    for (i = 0; i < NSTRS; i++) {
        while ((c = fgetc(fp)) != EOF) {
            putchar(c);

            if (c == '\n')
                break;
        }
    }

    /*
     * Now we send some strings to the server.
     */
    for (i = 0; i < NSTRS; i++)
        send(s, strs[i], strlen(strs[i]), 0);

    /*
     * We can simply use close() to terminate the
     * connection, since we're done with both sides.
     */
    close(s);

    exit(0);
}

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all: server.exe client.exe

server.exe: server.c
	gcc -g -o $@ $<

client.exe: client.c
	gcc -g -o $@ $<

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