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POProgram goes into a deadlock when invoking fork()
 

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  1. POProgram goes into a deadlock when invoking fork()
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    <p>I've implemented a pipe that's based on a shared memory , and I have a problem when I try to invoke <code>fork</code> with a <code>main</code> program . </p> <p>The following main : </p> <pre><code># include "my_shm_piper.h" #include &lt;stdio.h&gt; #include &lt;stdlib.h&gt; #include &lt;string.h&gt; #include &lt;sys/wait.h&gt; #include &lt;unistd.h&gt; #include &lt;fcntl.h&gt; #include &lt;sys/stat.h&gt; #include &lt;sys/types.h&gt; #include &lt;sys/shm.h&gt; #include &lt;semaphore.h&gt; #include &lt;sys/mman.h&gt; 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) &lt; 0) { perror("my_pipe"); exit(1); } if (fork()) { rb = my_read(spd[0], buff, sizeof(buff)); if (rb &gt; 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; } </code></pre> <p>Is using on an <strong>Anonymous-pipe</strong> that's implemented using shared memory library. </p> <p>When I put the <code>1st</code> command of <code>fork()</code> , as above , then my program is working as expected , all the <code>hello-world</code>-s are presented . </p> <p>But when I put the <code>fork</code> after <code>initPipe()</code> , there's deadlock , and the program hangs : </p> <pre><code>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) &lt; 0) { perror("my_pipe"); exit(1); } // from here the same as above } </code></pre> <p>I think that the initialization stage for the <code>fork()</code> is happening only once , and not twice , as in the first <code>main()</code> . </p> <p>I guess that there's something wrong with the writing/reading stage , but I can't seem to find the exact source . </p> <p>I'd appreciate for your help with the matter</p> <p>Thanks </p> <p><strong>EDIT:</strong></p> <p>the struct in the H. file :</p> <pre><code>struct PipeShm { int init; int flag; sem_t *mutex; char * ptr1; char * ptr2; int status1; int status2; int semaphoreFlag; }; </code></pre> <p>this is initPipe:</p> <pre><code>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 } </code></pre> <p>this is my_pipe():</p> <pre><code>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; } </code></pre> <p>This is the reading :</p> <pre><code>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, &amp;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; } </code></pre> <p>and this is the writing :</p> <pre><code>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; } </code></pre> <p>The processes are hanging like that - in the second main (this is the output on the console): </p> <pre><code>my_read - wait 0 my_read - proceed Output:hello world! // here it just gets stuck </code></pre> <p>And in the first main , the output in console is: </p> <pre><code>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 </code></pre>
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    1. COAre you deadlocking on the actual call to `fork()`, or are you saying that you're just not printing your lines afterwards via the pipes? Have you tried simply checking to see if the call to `fork` is made successively by outputting a statement to `stderr` immediately after the `fork` call ?
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      1. USJason
    2. CO@Jason: I'm saying that in the `2nd` main , only once the `hello-world` is presented , and the program continues , but nothing is happening . I thing that the read/writing is just hanging in there and waiting for something that would never happen .
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      1. USron
    3. COToo hard to guess what's going on without looking at what initPipes() does; I'm assuming that the two forks are what you really intended. Have you tried using a debugger to see where the processes are hung?
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      1. POProgram goes into a deadlock when invoking fork()
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