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POReleasing Shared Memory
 

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    <p>So the program below accesses a piece of shared memory at location "shmid" and attaches it to pointer "total." A child process is then created that accesses and modifies this piece of memory and then once complete, another child process is created and does the same, and then a 3rd child. Once these 3 children have finished modifying the value and finished executing, the parent process releases the shared memory and then the program exits. I was under the impression that this "shmctl" block would have the desired effect, but it doesn't seem to. </p> <pre><code>if ((shmctl (shmid, IPC_RMID, (struct shmid_ds *) 0)) == -1) { perror ("shmctl"); exit (-1); } </code></pre> <p>I don't believe it does, because following this block I have the following which during execution printed out the same value as before. Doesn't this indicate that the pointer is still accessing the same bit of memory, or am I mistaken?</p> <pre><code>printf("value after memory release:%d\n", total-&gt;value); </code></pre> <p>Thank you for your input!</p> <pre><code>#include &lt;stdio.h&gt; #include &lt;sys/types.h&gt; #include &lt;sys/ipc.h&gt; #include &lt;sys/shm.h&gt; #include &lt;stdlib.h&gt; #include &lt;unistd.h&gt; /* change the key number */ #define SHMKEY ((key_t) 5600) //7890 typedef struct { int value; } shared_mem; shared_mem *total; /*----------------------------------------------------------------------* * This function increases the value of shared variable "total" * by one all the way to 100000 *----------------------------------------------------------------------*/ void process1 () { int k = 0; while (k &lt; 100000) { k++; total-&gt;value = total-&gt;value + 1; } printf ("From process1 total = %d\n", total-&gt;value); //process 1 prints out total and returns to main() function } /*----------------------------------------------------------------------* * This function increases the vlaue of shared memory variable "total" * by one all the way to 170000 *----------------------------------------------------------------------*/ void process2 () { int k = 0; while (k &lt; 170000) { k++; total-&gt;value = total-&gt;value + 1; } printf ("From process2 total = %d\n", total-&gt;value); //process 2 prints out total and returns to main() function } /*----------------------------------------------------------------------* * This function increases the vlaue of shared memory variable "total" * by one all the way to 200000 *----------------------------------------------------------------------*/ void process3 () { int k = 0; while (k &lt; 200000) { k++; total-&gt;value = total-&gt;value + 1; } printf ("From process3 total = %d\n", total-&gt;value); //process 3 prints out total and returns to main() function } /*----------------------------------------------------------------------* * MAIN() *----------------------------------------------------------------------*/ int main() { int shmid; int pid1; int pid2; int pid3; int ID; int status; char *shmadd; shmadd = (char *) 0; /* Create and connect to a shared memory segmentt*/ if ((shmid = shmget (SHMKEY, sizeof(int), IPC_CREAT | 0666)) &lt; 0) { perror ("shmget"); exit (1); } if ((total = (shared_mem *) shmat (shmid, shmadd, 0)) == (shared_mem *) -1) { perror ("shmat"); exit (0); } total-&gt;value = 0; if ((pid1 = fork()) == 0) //first child created { process1(); //first child process begins } while((ID = wait(&amp;status)) != -1) //parent loops until 1st child is finished printf("child %d is finished\n", ID); //parent prints out returned value after child is finished if ((pid1 != 0) &amp;&amp; ((pid2 = fork()) == 0)) //second child created { process2(); //second child process begins } while((ID = wait(&amp;status)) != -1) //parent loops until 2nd child is finished printf("child %d is finished\n", ID); //parent prints out returned value after child is finished if ((pid1 != 0) &amp;&amp; (pid2 != 0) &amp;&amp; ((pid3 = fork()) == 0)) //third child created { process3(); //third child process begins } while((ID = wait(&amp;status)) != -1) //parent loops until 3rd child is finished printf("child %d is finished\n", ID); //parent prints out returned value after child is finished if ((pid1 != 0) &amp;&amp; (pid2 != 0) &amp;&amp; (pid3 != 0)) { if ((shmctl (shmid, IPC_RMID, (struct shmid_ds *) 0)) == -1) { perror ("shmctl"); exit (-1); } printf ("\t\t End of Program.\n"); //prints after all children have finished printf("value after memory release:%d\n", total-&gt;value); } return 0; } /***** Note: loop for parent to wait for child processes to finish and print ID of each child*****/ </code></pre>
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    1. PO
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    1. This table or related slice is empty.
    1. PTQuestion
    1. USTshepang
    1. USNicholasNickleby
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    1. This table or related slice is empty.
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      1. POReleasing Shared Memory
      1. USSibi Rajasekaran
      1. VTFavorite
    2. VO
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      1. VTUpMod
    3. VO
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      1. VTUpMod
    1. COIs this supposed to be tagged with C++? C would have been more relevant?
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      1. USSteve Folly
    2. COMy bad. Thanks for catching that.
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      1. POReleasing Shared Memory
      1. USNicholasNickleby
 

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