socket() returns 0 in C client server application

问题

I'm working on an application that contains several server sockets that each run in a unique thread.
An external utility (script) is called by one of the threads. This script calls a utility (client) that sends a message to one of the server sockets.

Initially, I was using system() to execute this external script, but we couldn't use that because we had to make sure the server sockets were closed in the child that was forked to execute the external script.
I now call fork() and execvp() myself. I fork() and then in the child I close all the server sockets and then call execvp() to execute the script.

Now, all of that works fine. The problem is that at times the script reports errors to the server app. The script sends these errors by calling another application (client) which opens a TCP socket and sends the appropriate data. My issue is that the client app gets a value of 0 returned by the socket() system call.

NOTE: This ONLY occurs when the script/client app is called using my forkExec() function. If the script/client app is called manually the socket() call performs appropriately and things work fine.

Based on that information I suspect it's something in my fork() execvp() code below... Any ideas?

void forkExec()
{    
    int stat;

    stat = fork();
    if (stat < 0)
    {
        printf("Error forking child: %s", strerror(errno));
    }
    else if (stat == 0)
    {
        char *progArgs[3];

        /*
         * First, close the file descriptors that the child 
         * shouldn't keep open
         */
        close(ServerFd);
        close(XMLSocket);
        close(ClientFd);
        close(EventSocket);
        close(monitorSocket);

        /* build the arguments for script */
        progArgs[0] = calloc(1, strlen("/path_to_script")+1);
        strcpy(progArgs[0], "/path_to_script");
        progArgs[1] = calloc(1, strlen(arg)+1);
        strcpy(progArgs[1], arg);
        progArgs[2] = NULL; /* Array of args must be NULL terminated for execvp() */

        /* launch the script */
        stat = execvp(progArgs[0], progArgs);
        if (stat != 0)
        {
            printf("Error executing script: '%s' '%s' : %s", progArgs[0], progArgs[1], strerror(errno));
        }
        free(progArgs[0]);
        free(progArgs[1]);
        exit(0);
    }

    return;
}

Client app code:

static int connectToServer(void)
{
int socketFD = 0;
int status;
struct sockaddr_in address;
struct hostent* hostAddr = gethostbyname("localhost");

socketFD = socket(PF_INET, SOCK_STREAM, 0);

The above call returns 0.

if (socketFD < 0)
{
    fprintf(stderr, "%s-%d: Failed to create socket: %s", 
                                __func__, __LINE__, strerror(errno));
    return (-1);
}

memset(&address, 0, sizeof(struct sockaddr));
address.sin_family = AF_INET;
memcpy(&(address.sin_addr.s_addr), hostAddr->h_addr, hostAddr->h_length);
address.sin_port = htons(POLLING_SERVER_PORT);

status = connect(socketFD, (struct sockaddr *)&address, sizeof(address));
if (status < 0)
{
    if (errno != ECONNREFUSED)
    {
        fprintf(stderr, "%s-%d: Failed to connect to server socket: %s",
                   __func__, __LINE__, strerror(errno));
    }
    else
    {
        fprintf(stderr, "%s-%d: Server not yet available...%s",
                   __func__, __LINE__, strerror(errno));
        close(socketFD);
        socketFD = 0;
    }
}

return socketFD;
}

FYI
OS: Linux
Arch: ARM32
Kernel: 2.6.26

回答1:

socket() returns -1 on error.

A return of 0 means socket() succeeded and gave you file descriptor 0. I suspect that one of the file descriptors that you close has file descriptor 0 and once it's closed the next call to a function that allocated a file descriptor will return fd 0 as it's available.

回答2:

A socket with value 0 is fine, it means stdin was closed which will make fd 0 available for reuse - such as by a socket.

chances are one of the filedescriptors you close in the forkExec() child path(XMLSocket/ServerFd) etc.) was fd 0 . That'll start the child with fd 0 closed, which won't happen when you run the app from a command line, as fd 0 will be already open as the stdin of the shell.

If you want your socket to not be 0,1 or 2 (stdin/out/err) call the following in your forkExec() function after all the close() calls

void reserve_tty()
{
  int fd;

  for(fd=0; fd < 3; fd++)
    int nfd;
    nfd = open("/dev/null", O_RDWR);

    if(nfd<0) /* We're screwed. */
    continue;

    if(nfd==fd)
    continue;

    dup2(nfd, fd);
    if(nfd > 2)
     close(nfd);

}

Check for socket returning -1 which means an error occured.

回答3:

Don't forget a call to

waitpid()

End of "obvious question mode". I'm assuming a bit here but you're not doing anything with the pid returned by the fork() call. (-:

回答4:

As it is mentioned in another comment, you really should not close 0,1 or 2 (stdin/out/err), you can put a check to make sure you do not close those and so it will not be assigned as new fd`s when you request for a new socket

来源：https://stackoverflow.com/questions/2135159/socket-returns-0-in-c-client-server-application

标签

Linux

exec

arm

sockets

fork