POSIX Thread. Why is the producer-consumer program terminated?

May 10, 2022

I tried to write a simple C producer-consumer program using POSIX Thread, a library for working with threads.

The code has a global queue from where threads exchange information. The queue has been tested and should work correctly. There are also two threads involved:

Producer. Generates a random number and puts it in a queue;
Consumer. Retrieves a number from the queue and prints it to the screen.

main.c:


#include <stdio.h>
#include <pthread.h>
#include <time.h>
#include "queue.h"

pthread_mutex_t mutex;
pthread_cond_t pcond;
pthread_cond_t ccond;

static const unsigned int X = 10;

void* producer(void* args)
{
    for (int i = 0; i < X; i++)
    {
        pthread_mutex_lock(&mutex);
        while (queue_is_full())
            pthread_cond_wait(&pcond, &mutex);
        if (queue_is_empty())
            pthread_cond_signal(&ccond);
        queue_enqueue(rand() % (9999 + 1 - 0) + 0);
        pthread_mutex_unlock(&mutex);
    }
    return NULL;
}

void* consumer(void* args)
{
    for (int i = 0; i < X; i++)
    {
        pthread_mutex_lock(&mutex);
        while (queue_is_empty())
            pthread_cond_wait(&ccond, &mutex);
        if (queue_is_full())
            pthread_cond_signal(&pcond);
        printf("%i", queue_dequeue());
        pthread_mutex_unlock(&mutex);
    }
    return NULL;
}

int main(void)
{
    pthread_mutex_init(&mutex, NULL);
    pthread_cond_init(&pcond, NULL);
    pthread_cond_init(&ccond, NULL);
    
    pthread_t thProducer, thConsumer;
    pthread_create(&thProducer, NULL, producer, NULL);
    pthread_create(&thConsumer, NULL, consumer, NULL);
    
    pthread_join(thProducer, NULL);
    pthread_join(thConsumer, NULL);
    return 0;
}

This program terminates upon startup. The message Application.exe has stopped working. is issued by the system immediately after starting the program. How can I fix the program?

queue.c:

#include "queue.h"
 
#define CAPACITY 2048
 
static int* queue;
static int head;
static int tail;
 
void queue_init()
{
    queue = malloc(CAPACITY * sizeof(int));
    head = -1;
    tail = 0;
}
 
void queue_enqueue(int value)
{
    if (queue_is_full())
        exit(1);
    queue[tail] = value;
    if (head == -1)
        head = 0;
    if (tail == CAPACITY - 1)
        tail = 0;
    else
        tail++;
}
 
int queue_dequeue()
{
    if (queue_is_empty())
        exit(1);
    int item = queue[head];
    if (head != CAPACITY - 1)
        head++;
    else
        head = 0;
    if (head == tail)
    {
        head = -1;
        tail = 0;
    }
    return item;
}
 
bool queue_is_full()
{
    return head == tail;
}
 
bool queue_is_empty()
{
    return head == -1;
}
 
int queue_size()
{
    if (head != -1)
    {
        return head < tail ? tail - head : (CAPACITY - head) + tail;
    }
    return 0;
}

>Solution :

gcc’s -fsanitize=address finds the direct problem immediately (and so should any debugger).

AddressSanitizer:DEADLYSIGNAL
=================================================================
==1==ERROR: AddressSanitizer: SEGV on unknown address 0x000000000000 (pc 0x000000401436 bp 0x7fbd0f7fde40 sp 0x7fbd0f7fde30 T2)
==1==The signal is caused by a READ memory access.
==1==Hint: address points to the zero page.
    #0 0x401436 in queue_dequeue /app/example.c:56
    #1 0x401596 in consumer /app/example.c:99
    #2 0x7fbd139c4608 in start_thread (/lib/x86_64-linux-gnu/libpthread.so.0+0x8608)
    #3 0x7fbd138e9162 in __clone (/lib/x86_64-linux-gnu/libc.so.6+0x11f162)

So it’s a NULL pointer dereference at this line: