C AF_UNIX中的连接拒绝套接字编程

Question

Hi i'm trying to make a simple program that forks and then should act like client/server. Here is my code:

int main (){
int sfd,fdc;
struct sockaddr_un sa;
strncpy(sa.sun_path,SOCKNAME,UNIX_PATH_MAX);
sa.sun_family = AF_UNIX;
char buf[N+1];

if (fork() != 0){
    sfd = socket(AF_UNIX,SOCK_STREAM,0);
    bind (sfd, (struct sockaddr *) &sa ,sizeof(sa));
    listen(sfd, SOMAXCONN);
    fdc= accept(sfd,NULL,0);
    read(fdc,buf,N);
    printf("Server got: %s\n",buf);
    write(fdc,"bye!",5);
    close(fdc);
    close(sfd);
    exit(EXIT_SUCCESS);
}
else {
    sfd = socket(AF_UNIX,SOCK_STREAM,0);

    while(connect(sfd, (struct sockaddr *)&sa,sizeof(sa)) == -1){
        if (errno == ENOENT){
            printf("Aspetto 1 sec\n");
            sleep(1);
        }
        else {perror(NULL); exit(EXIT_FAILURE) ;}   
    }
    write(sfd,"Hallo!",7);
    read(sfd,buf,N);
    printf("Client got: %s\n",buf);
    close(sfd);
    exit(EXIT_SUCCESS);
    }

I don't understand why this fail when i try to connect the client to server. I got this error: Connection Refused. I can't really find where the problem is, thanks for your help.

Answer 1

First, regarding simple program that forks and then should act like client/server : Having a client and server in the same logical block is not a good idea. although it can be written to be syntactically correct (ie it will compile and build), I do not see how it will ever be practical.

There are many tutorial examples, using Posix C to create separate server and client apps. Here is one . Here is another , and a third .

One general suggestion, when using a family of functions that have prerequisites (such as a handle for a socket, or for a previous successful call of another function), you should always use the return value. For example, in the following lines of your example code:

if (fork() != 0){
    sfd = socket(AF_UNIX,SOCK_STREAM,0);
    bind (sfd, (struct sockaddr *) &sa ,sizeof(sa));
    listen(sfd, SOMAXCONN);
    fdc= accept(sfd,NULL,0); 

the only line you choose to verify is the result of fork (that is good). But you should also check the results of each of the others, using the results as a prerequisite of continuing to the next step.

At the very least something like this:

if (fork() != 0)
{
    sfd = socket(AF_UNIX,SOCK_STREAM,0);
    if(sfd < 0)
    {
        //notify user
        return -1;
    }
    ret = bind (sfd, (struct sockaddr *) &sa ,sizeof(sa));
    if(ret < 0)
    {
        //check value of errno, and notify user
        return -1;
    }
    ret = listen(sfd, SOMAXCONN);
    if(ret < 0)
    {
        //check value of errno, and notify user
        return -1;
    }
    new_s= accept(sfd,NULL,0); 
    if(new_s < 0)
    {
        //check value of errno, and notify user
        return -1;
    }
    // continue...