Posix threads synchronisation with semaphores

This is a discussion on Posix threads synchronisation with semaphores within the C Programming forums, part of the General Programming Boards category; Hi all, can someone explain for me the output of this program : Code: #include <stdio.h> #include <unistd.h> #include <stdlib.h> ...

  1. #1
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    Posix threads synchronisation with semaphores

    Hi all,

    can someone explain for me the output of this program :
    Code:
    #include <stdio.h>
    #include <unistd.h>
    #include <stdlib.h>
    #include <string.h>
    #include <pthread.h>
    #include <semaphore.h>
    
    void *thread_function(void *arg);
    sem_t bin_sem;
    
    #define WORK_SIZE 1024
    char work_area[WORK_SIZE];
    
    int main() {
       int res;
       pthread_t a_thread;
       void *thread_result;
    
       res = sem_init(&bin_sem, 0, 0);
       if (res != 0) {
           perror("Semaphore initialization failed");
           exit(EXIT_FAILURE);
       }
       res = pthread_create(&a_thread, NULL, thread_function, NULL);
       if (res != 0) {
           perror("Thread creation failed");
           exit(EXIT_FAILURE);
       }
    
       printf("Input some text. Enter 'end' to finish\n");
       while(strncmp("end", work_area, 3) != 0) {
         if (strncmp(work_area, "FAST", 4) == 0) {
           sem_post(&bin_sem);
           strcpy(work_area, "Wheeee...");
         } else {
           fgets(work_area, WORK_SIZE, stdin);
         }
         sem_post(&bin_sem);
       }
    
       printf("\nWaiting for thread to finish...\n");
       res = pthread_join(a_thread, &thread_result);
       if (res != 0) {
           perror("Thread join failed");
           exit(EXIT_FAILURE);
       }
       printf("Thread joined\n");
       sem_destroy(&bin_sem);
       exit(EXIT_SUCCESS);
    }
    
    void *thread_function(void *arg) {
       sem_wait(&bin_sem);
       while(strncmp("end", work_area, 3) != 0) {
           printf("You input %d characters\n", strlen(work_area) -1);
           sem_wait(&bin_sem);
       }
       pthread_exit(NULL);
    }
    when you compile :
    Code:
    gcc -Wall -D_REENTRANT thread1.c -o thread1 -lpthread
    and run it :
    Code:
    ./thread1
    Input some text. Enter 'end' to finish
    hello
    You input 5 characters
    i m a thread
    You input 12 characters
    FAST
    You input 8 characters
    You input 8 characters
    You input 8 characters
    end
    
    Waiting for thread to finish...
    Thread joined
    and this is not clear for me.

    Thanks for your help.

    Regards.

  2. #2
    cas
    cas is offline
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    In the future, please explain what is not clear. It's good that you showed the output of your program, but you did not explain why it is incorrect. I can only assume that you're confused about the fact that FAST results in "You printed.." being shown 3 times, but I really don't know.

    I'll have to assume that's the case. When you type FAST, you cause sem_post() to be called three times, which causes sem_wait() to wake up three times.

    a) You type in FAST: the sem_post() after your if/else block is called.
    b) Your strncmp() (which could just be a strcmp()) matches FAST, so sem_post() in that block is called.
    c) The sem_post() from (a) is called again

  3. #3
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    Thanks cas for the reply, i'm sorry about explaining what is not clear. But i'm very confused about this output:

    Code:
    FAST
    You input 8 characters
    You input 8 characters
    You input 8 characters
    I was expecting just one time this sequence but i got it three times so it's not clear at all.

    Thanks again.

    Regards.

  4. #4
    Registered User Codeplug's Avatar
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    Officially, you have undefined behavior due to unsynchronized access to "work_area".

    >> I was expecting just one ... but i got it three times ...
    You see the message 3 times because main() called sem_post() 3 times. cas identified all 3 calls in "a) b) c)" of post #2.

    gg

  5. #5
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    Thanks for your reply Codeplug but i don't understand the call in a) and c) from the reply of cas? When i type FAST i increment the semaphore so the thread_function executes and ouput 3!!! right?

    Thanks.

  6. #6
    Registered User Codeplug's Avatar
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    1) "FAST" is entered, fgets() returns, POST
    2) loop to top of while loop, work_area == "FAST", POST
    3) bottom of while loop is reached, POST

    There are your 3 sem_post() calls.

    4) loop to top of while loop, call fgets() and wait for input

    In the meantime, thread_function() processes all 3 sem_post() calls, and see's "Wheeee...".

    gg

  7. #7
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    That's much better , thank you very much codeplug.

    Regards.

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