Question

I need several global pointers to be shared among a few files – the pointers are essentially arrays of double whose lengths are only determined at runtime.

I include here the pieces of the code that caused the issue. This is not the exact code, but it illustrates all the points precisely:

foo.h

#ifndef FOOH
#define FOOH

/* ------------------
COMMON VARIABLES
---------------------*/

// create_bundles.c
extern double *all_bundle;

/* ------------------
COMMON FUNCTIONS
---------------------*/

// create_bundles.c
void create_bundles(int num_firm);

// memory_allocation.c
void allocate_memory(int num_firm, int num_bundle);
void clean_memory(void);
#endif

create_bundles.c

#include "foo.h"
extern double *all_bundle;

void create_bundles(int num_firm) {
int i;
    for (i = 0; i < num_firm; i++) {
        all_bundle[i] = 1
    }

memory_allocation.c

#include "foo.h"

// create_bundles.c
double *all_bundle = NULL;

void allocate_memory(int num_firm, int num_bundle) {
    all_bundle = calloc(num_bundle * num_firm, sizeof(double));
}
void clean_memory(void) {
    free(all_bundle);
}

main.c

#include "foo.h"
void main(int num_firm, int num_bundle) {
    allocate_memory(num_firm, num_bundle);
    create_bundles(num_firm);
    clean_memory();
} 

What happened is that if I print out all_bundle[i] it’ll all be 0, and then it’ll give me a segmentation error.

Why the error and how to fix it?

Answer 1

The problem is not in global pointer, but something else. Keep looking for the problem in your common code. I hope you are trying to print contents of all_bundle array before calling clean_memory. I have edited your code a little bit and it works great without any segmentation errors and prints 1.0000. Here it is, take a look:
foo.h:

#ifndef FOOH
#define FOOH

// create_bundles.c
extern double *all_bundle;

// create_bundles.c
void create_bundles(int num_firm);

// memory_allocation.c
void allocate_memory(int num_firm, int num_bundle);
void clean_memory(void);

#endif

memory_allocation.c:

#include <stdlib.h>

#include "foo.h"

double *all_bundle = 0;

void allocate_memory(int num_firm, int num_bundle) {
    all_bundle = calloc(num_bundle * num_firm, sizeof(double));
}
void clean_memory(void) {
    free(all_bundle);
}

create_bundles.c:

#include "foo.h"

void create_bundles(int num_firm) {
    int i;
    for (i = 0; i < num_firm; i++) {
        all_bundle[i] = 1;
    }
}

main.c:

#include <stdio.h>
#include "foo.h"

int main(int argc, char *argv[]) {
    allocate_memory(100, 1);
    create_bundles(100);

    {
        int i;
        for(i = 0; i < 100; ++i)
            printf("%f\n", all_bundle[i]);
    }

    clean_memory();
    return 0;
}