Why do I keep getting segmentation fault in my C program

Question

I've been trying to implement thread synchronization on C. However, I keep getting the segmentation fault when my invoke the function that I want the thread to execute. So anyone can suggest the solution on for this problem?

Here is my code

#include <stdio.h>
#include <stdlib.h>
#include <pthread.h>
#include <semaphore.h>
#include <fcntl.h>
#include <unistd.h>
#include <sys/stat.h>

#define N 5
#define M 3
#define LEFT (robot_id - 1) % N
#define RIGHT (robot_id + 1) % N

pthread_t robots_id[N];
sem_t simulations[M];
pthread_mutex_t sever_mutex;

void Learning(int robot_id)
{
    printf("learning robot = %d\n", robot_id);
}

void *robotAct(void *id)
{
    int *robot_id = id;
    printf("robot id = %d\n", robot_id);
    Learning(*robot_id);
}

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

    E = atoi(argv[1]);
    T = atoi(argv[2]);

    printf("Initializing Robot!\n");

    //Initializes the simulations
    for (int i = 0; i < M; i++)
    {
        sem_init(&simulations[i], 0, 0);
    }

    //Initializes the robots
    for (int i = 0; i < N; i++)
    {
        printf("Robot %d is created\n", i + 1);
        pthread_create(&robots_id[i], NULL, robotAct, (void *)i + 1);
    }

    sleep(T);

    printf("Terminating Robots\n");
    for (int i = 0; i < N; i++)
    {
        pthread_cancel(robots_id[i]);
    }
    printf("Termination is completed!\n");
    printf("-------Report-------------\n");
    //getReport();
    return 0;
}

Here is my result that I keep getting

Initializing Robot!
Robot 1 is created
Robot 2 is created
Robot 3 is created
robot id = 1
robot id = 2
Robot 4 is created
robot id = 3
[1]    54477 segmentation fault  ./project 5 10

Answer 1

The main issue is explained in my comment:

You're not passing a valid pointer to the thread function. You sort of, mostly, almost get away with the misuse of it in the printf() call in robotAct(); you emphatically do not get away with it in the call to Learning() where you dereference the invalid non-pointer.

A solution is to create an array of integers in the main program which holds robot ID numbers (int id[N];). Then, initialize each element and pass &id[i] to pthread_create().

You should not print addresses with the %d format (even though it works on 32-bit systems; it does not work on 64-bit systems). The correct technique is to use %p to format the address. Or, in this case, print the integer and not the address using *robot_id.

The code that follows has minimal adaptations to the original code and has not been compiled or tested (there could be problems outside the lines changed):

#include <stdio.h>
#include <stdlib.h>
#include <pthread.h>
#include <semaphore.h>
#include <fcntl.h>
#include <unistd.h>
#include <sys/stat.h>

#define N 5
#define M 3
#define LEFT (robot_id - 1) % N
#define RIGHT (robot_id + 1) % N

pthread_t robots_id[N];
sem_t simulations[M];
pthread_mutex_t sever_mutex;

void Learning(int robot_id)
{
    printf("learning robot = %d\n", robot_id);
}

void *robotAct(void *id)
{
    int *robot_id = id;
    printf("robot id = %d\n", *robot_id);        // Changed
    Learning(*robot_id);
    return 0;                                    // Added
}

int main(int argc, char *argv[])
{
    int E, T;
    int id[N];                                   // Added

    E = atoi(argv[1]);
    T = atoi(argv[2]);

    printf("Initializing Robot!\n");

    //Initializes the simulations
    for (int i = 0; i < M; i++)
    {
        sem_init(&simulations[i], 0, 0);
    }

    //Initializes the robots
    for (int i = 0; i < N; i++)
    {
        printf("Robot %d is created\n", i + 1);
        id[i] = i + 1;                                          // Added
        pthread_create(&robots_id[i], NULL, robotAct, &id[i]);  // Changed
    }

    sleep(T);

    printf("Terminating Robots\n");
    for (int i = 0; i < N; i++)
    {
        pthread_cancel(robots_id[i]);
    }
    printf("Termination is completed!\n");
    printf("-------Report-------------\n");
    //getReport();
    return 0;
}

Avoid using pthread_cancel() for ending the threads; the threads should terminate under control. For example, there might be a flag that you set in the main thread to indicate that the threads should cease, and they'd check that periodically. Normally, pthread_join() is used to clean up the completed threads.

For future posts, please read about how to create an MCVE (Minimal, Complete, Verifiable Example). There are parts of the code shown that are not relevant to the problem — the mutex and the semaphores, for example, are not really used.