Vlákna a semafory – C / C++ – Fórum – Programujte.com

Ahoj,

pracuji na vícevláknové aplikaci a narazil jsem na problém v následujícím kódu. Funkce pocitej nedela vlastne nic jineho, nez ze zvetsi ukazatel o jednicku a precte z pole[ukazatel]. Je tam i semaphor, aby si vlákna nemohla měnit hodnotu data->ukazatel, ale i přesto to nefunguje.

#include <pthread.h>

#include <stdio.h>

#include <stdlib.h>

#include <semaphore.h>

sem_t mutex; 



typedef struct DATA{

    int ukazatel;

    int ** pole;

    } DATA;

void * pocitej(void * arg){

    DATA * data = (DATA *) arg;

    int * cislo;

    while (1){

        sem_wait(&mutex);

        cislo = data->pole[data->ukazatel];

        data->ukazatel = data->ukazatel + 1;

        

        if (data->ukazatel > 100){

            sem_post(&mutex);

            break;

            }

        sem_post(&mutex);        

        

        }    

    return NULL;

    }

void vlakna(int threads){

    int ** pole = (int **)malloc(101 * sizeof(int *));

    int i;

    int * cislo;

    pthread_t ** vlakna = (pthread_t **) malloc(threads * sizeof(pthread_t *));

    pthread_attr_t attr;

    sem_init(&mutex, 0, 1); 

    pthread_attr_init(&attr);

    pthread_attr_setdetachstate(&attr, PTHREAD_CREATE_JOINABLE);

    for (i = 0; i != 100; i++){

        pole[i] = (int *) malloc(sizeof(int));

        (*pole[i]) = i;

        }

    pole[i] = NULL;

    DATA * data = (DATA *) malloc(sizeof(DATA));

    data->ukazatel = 0;

    data->pole = pole;

    for (i = 0; i != threads; i++){

        vlakna[i] = (pthread_t *) malloc(sizeof(pthread_t));

        pthread_create(vlakna[i], &attr, pocitej, (void *) data);

        } 

    for (i = 0; i != threads; i++){

        pthread_join(*(vlakna[i]), NULL);

        }    

    for (i = 0; i != 100; i++){

        free(pole[i]);

        }

    free(pole);

    for (i = 0; i != threads; i++){

        free(vlakna[i]);

        }

    free(vlakna);

    free(data);

    pthread_attr_destroy(&attr);

    }

int main(int argc, char *argv[]){

    vlakna(5);

    return 1;

    }

gcc -o a.out test.c -lpthread -lrt

valgrind ./a.out

==29337== Memcheck, a memory error detector

==29337== Copyright (C) 2002-2010, and GNU GPL'd, by Julian Seward et al.

==29337== Using Valgrind-3.6.0.SVN-Debian and LibVEX; rerun with -h for copyright info

==29337== Command: ./a.out

==29337== 

==29337== Thread 3:

==29337== Invalid read of size 8

==29337==    at 0x4008DA: pocitej (in /home/jakub/prog/osy1/a.out)

==29337==  Address 0x55d6368 is 0 bytes after a block of size 808 alloc'd

==29337==    at 0x4C2815C: malloc (vg_replace_malloc.c:236)

==29337==    by 0x400942: vlakna (in /home/jakub/prog/osy1/a.out)

==29337==    by 0x400B66: main (in /home/jakub/prog/osy1/a.out)

==29337== 

==29337== 

==29337== HEAP SUMMARY:

==29337==     in use at exit: 0 bytes in 0 blocks

==29337==   total heap usage: 113 allocs, 113 frees, 2,664 bytes allocated

==29337== 

==29337== All heap blocks were freed -- no leaks are possible

==29337== 

==29337== For counts of detected and suppressed errors, rerun with: -v

==29337== ERROR SUMMARY: 4 errors from 1 contexts (suppressed: 4 from 4)

...

if (data->ukazatel > 101-pocet_vlaken){

...

neni to treba v tomdle:

    for (i = 0; i != 100; i++){

        pole[i] = (int *) malloc(sizeof(int));

        (*pole[i]) = i;

        }

Dokonce víc než 0 až 100. Každé to vlákno vesele načte hodnotu a posune ukazatel a až pak ho testuje.

To nervak : Nj, s vláknama začínám :) díky, už to funguje

To Blujacker : todle byla jen prace s pameti.. vlakna s tim nic nemely :D

nj, ale kdyby to nebyla vláknová aplikace, tak se kód za tu podmínku nedostane :-)

To Blujacker : no kdyby si ty podminky udelal stejne, tak te ani obycejna aplikace nezachrani :D