Program goes into a deadlock when invoking fork()

Question

I've implemented a pipe that's based on a shared memory , and I have a problem when I try to invoke fork with a main program .

The following main :

# include "my_shm_piper.h"
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/wait.h>
#include <unistd.h>
#include <fcntl.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <sys/shm.h>
#include <semaphore.h>
#include <sys/mman.h>
int main()
{
    int spd[2], pid, rb;
    char buff[4096];

    fork();  // that fork is okay , but if we put it after initPipe() , there's a deadlock

    initPipe();

    if (my_pipe(spd) < 0)
    {
        perror("my_pipe");
        exit(1);
    }

    if (fork()) 
    {
        rb = my_read(spd[0], buff, sizeof(buff));
        if (rb > 0)
            write(1, buff, rb);
    }

    else
    {
        my_write(spd[1], "hello world!\n", sizeof("hello world!\n"));
    }

    my_close(spd[0]);
    my_close(spd[1]);
    removePipe();
    return 0;
}

Is using on an Anonymous-pipe that's implemented using shared memory library.

When I put the 1st command of fork() , as above , then my program is working as expected , all the hello-world-s are presented .

But when I put the fork after initPipe() , there's deadlock , and the program hangs :

int main()

{
    int spd[2], pid, rb;
    char buff[4096];

    initPipe();
    fork();   // now the fork() is after the initialization ,and we have a deadlock

    if (my_pipe(spd) < 0)
    {
        perror("my_pipe");
        exit(1);
    }

        // from here the same as above 
}

I think that the initialization stage for the fork() is happening only once , and not twice , as in the first main() .

I guess that there's something wrong with the writing/reading stage , but I can't seem to find the exact source .

I'd appreciate for your help with the matter

Thanks

EDIT:

the struct in the H. file :

struct PipeShm
{
    int init;
    int flag;
    sem_t *mutex;
    char * ptr1;
    char * ptr2;
    int status1;
    int status2;
    int semaphoreFlag;
};

this is initPipe:

int initPipe()
{
    if (!myPipe.init)
    {
        myPipe.mutex = mmap (NULL, sizeof *myPipe.mutex, PROT_READ | PROT_WRITE,MAP_SHARED | MAP_ANONYMOUS, -1, 0);
        if (!sem_init (myPipe.mutex, 1, 1))
        {
            myPipe.init = TRUE;
        }
        else
            perror ("initPipe");
    }
    return 1;   // always successful
}

this is my_pipe():

int my_pipe(int spd[2])
{
    spd[0] = shmget(2009, SHMSIZE, 0);      // for reading
    spd[1] = shmget(2009, SHMSIZE, 0666 | IPC_CREAT);  // for writing

     if (spd[0] == -1 || spd[1] == -1)
     {
             perror("shmget");
             exit(EXIT_FAILURE);
             return -1;
     }

     return 1;

}

This is the reading :

ssize_t my_read(int spd, void *buf, size_t count)
{

    char array[4096];
    memset (array, '\0', 4096);
    ssize_t returnVal = 0;

    sem_wait (myPipe.mutex);

    int sval;
    sem_getvalue (myPipe.mutex, &sval);

    printf ("my_read - wait %d\n", sval);
    if (sem_wait (myPipe.mutex))
    perror ("sem_wait");

    printf ("my_read - proceed\n");

    if (myPipe.flag == FALSE)
    {
        myPipe.ptr1 = shmat (spd, NULL, 0); // attaching the segment
        if (myPipe.ptr1 == (void *) -1)
            error_out ("shmat");

        strncpy (array, myPipe.ptr1, count);
        array[count] = '\0';

        returnVal = strlen (array);
        buf = (void *) array;

        printf ("Output:%s", array);

    }

    else if (myPipe.flag == TRUE)
    {
        const size_t region_size = sysconf (_SC_PAGE_SIZE);
        myPipe.ptr1 = mmap (0, region_size, PROT_READ | PROT_WRITE, MAP_SHARED, spd, 0);
        if (myPipe.ptr1 == (void *) -1)
            error_out ("mmap");

        strncpy (array, myPipe.ptr1, count);
        array[count] = '\0';

        returnVal = strlen (array);
        buf = (void *) array;

        printf ("Output:%s", array);

    }

    return returnVal;

}

and this is the writing :

ssize_t my_write(int spd, const void *buf, size_t count)
{

       ssize_t returnVal = 0;
       sleep(1); // debug to ensure that read goes first for testing.
        if (myPipe.flag == FALSE)
        {
            myPipe.ptr2 = shmat (spd, NULL, 0); // attaching the segment
            if (myPipe.ptr2 == (void *) -1)
                error_out ("shmat");

            char *d = (char *) buf;

            returnVal = snprintf (myPipe.ptr2, count, "%s", d);
        }
        else
        {
            const size_t region_size = sysconf (_SC_PAGE_SIZE);

            // Map the region into memory.
            myPipe.ptr2 = mmap (0, region_size, PROT_READ | PROT_WRITE, MAP_SHARED, spd, 0);
            if (myPipe.ptr2 == MAP_FAILED)
                error_out ("mmap");
            char *d = (char *) buf;

            returnVal = snprintf (myPipe.ptr2, count, "%s", d);
        }

        sem_post (myPipe.mutex);
        return returnVal;

}

The processes are hanging like that - in the second main (this is the output on the console):

my_read - wait 0
my_read - proceed
Output:hello world!
// here it just gets stuck

And in the first main , the output in console is:

my_read  - wait 0
my_read  - wait 0
my_read - proceed
my_read - proceed
Output:hello world!
Output:hello world!
// here the program is done , the end 

Answer 1

Given the 'working' code, you have two pipes each has one reader and one write Given the 'notworking' code, you have one pipe with 2 readers and 2 writers and try to create the same unnamed pipe twice.

when 2 readers and 2 writers, The second creation of the same pipe will return -1 and errno set to EEXISTS.

Answer 2

Ah. Tricky one, but I was bored enough to figure it out.

Basically, your waits and posts are not balanced. If you look at your read() function, you do two waits:

sem_wait (myPipe.mutex);
// some more code
if (sem_wait (myPipe.mutex))
  perror ("sem_wait");

But your write function only does one post.

But why does it work if the fork happens before initPipe()? Because you're initializing the semaphore to 1:

if (!sem_init (myPipe.mutex, 1, 1))

Since you're forking before the init, you have two different semaphores with value 1; each one will receive a post and two waits, so you're fine.

In the other case, you have a single semaphore with value 1, which will receive two posts and four waits. 1 + 2 < 4, so one of your readers will hang on sem_wait.

BTW, that's a pretty complicated way of writing pipe(2).