Seg Fault when reading from file, and using pthread_create

Question

Whenever I run my code I get through 4 iterations of reading the file and creating a pthread until it segfaults with ID 11.

The segfault is caused by my print ln: printf("%s %s\\n", "Calling lab_manifes_alignment with package", *package); But why does this cause a segfault?

Halp?

#include <stdio.h>
#include <string.h>
#include <pthread.h>
#include <stdlib.h>

pthread_mutex_t mutex;

FILE *packageList;

void *submitPackage(void * packageReq){
    char ** package = packageReq;
    strcat(packageReq, " | sh lab_manifest_alignment.sh");
    printf("%s %s\n", "Calling lab_manifes_alignment with package", *package);
    system(packageReq);
    return NULL;
}


int main(){
    int numThreads;
    pthread_t threads[numThreads];

    //Init mutex
    if(pthread_mutex_init(&mutex, NULL)){
            fprintf(stderr, "Error initializing mutex");
    }

    int rc;
    FILE *packageList = fopen("package_list.txt", "r");
    if(packageList == NULL){
            fprintf(stderr, "ERROR: cannot open file.\n");
            return 1;
    }

    int i = 0;
    char line[128];

    while ( fgets ( line, sizeof line, packageList ) != NULL ){ 
    /* read a line spawn as many threads as needeed*/
        printf("%s %d, %s\n", "line: ",i, line);
        rc = pthread_create(&(threads[i]), NULL, submitPackage, line);

        if(rc){
            printf("ERROR: return code from pthread_create() is %d\n", rc);
            exit(EXIT_FAILURE);
        }
        i++;
    }

    numThreads = i;

    for(i = 0; i < numThreads; i++){
            pthread_join(threads[i], NULL);
    }

    fclose(packageList);
    return 0;

}

Answer 1

I think problem in here:

char ** package = packageReq;

Try to change:

char ** package = &packageReq;

Answer 2

pthread_t threads[numThreads]; numThreads is uninitialized here, you should choose a maximum value of threads or allocate it dynamically.

fgets ( line, sizeof line, packageList ) reads 128 bytes into line (the size of the array), but strcat(packageReq, " | sh lab_manifest_alignment.sh"); adds something behind it. This is the reason for your segfault. You should increase the size of the array and decrease the size parameter in fgets .

The next iteration in your main thread overwrites your line array, while the threads working with it. You should use a 2D array or allocate a buffer in each iteration and free it in the thread after working with it. Each thread must get his own buffer, not everyone the same.

char ** package = packageReq; should be char *package = packageReq; and remove the * at the printf .