Thread: Some confusion about POSIX threads

  1. #1
    [](){}(); manasij7479's Avatar
    Join Date
    Feb 2011
    Chennai, India

    Some confusion about POSIX threads

    I decided to learn threading for a program of mine which seems (I may be wrong) to scream its necessity.
    I thought threads were supposed to be executed in parallel in machines having more than one core.
    So why am I getting the following output from the following program?
    [I left out joining for now...Does that play a role here ?]
    #define _REENTRANT
    using namespace std;
    void* foo(void* arg);
    int main()
        int return_value;
        pthread_t reference;
        return_value= pthread_create(&reference,NULL,foo,NULL);
            cout<<"Creation Failed";
            return 1;
        int x(10);
        return 0;
    void* foo(void* arg)
        //cout<<"Thread Running.."<<endl;
        int x(10);
        return (void*)(arg);
    The output is
    instead of my expected jumble of 9s and 8s. Why is it so ?

  2. #2
    spurious conceit MK27's Avatar
    Join Date
    Jul 2008
    segmentation fault
    You want to use pthread_detach() after pthread_create.

    Also: there are no guarantees about the order of events with separate threads. Do not even presume that you will get:


    You are just as likely to get 999999888888 again, or 888888999999. If you need to guarantee a sequence, you will need to use mutex locks. Without locking, threads are fine for doing things that do not have to happen in any particular order (because they won't).
    C programming resources:
    GNU C Function and Macro Index -- glibc reference manual
    The C Book -- nice online learner guide
    Current ISO draft standard
    CCAN -- new CPAN like open source library repository
    3 (different) GNU debugger tutorials: #1 -- #2 -- #3
    cpwiki -- our wiki on sourceforge

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