Question

I’m having a real difficult time getting this code to work. I’m trying to pass an array by reference to a function, in order to modify it in that function. Then I need the modifications to be handed back to the original caller function.

I have searched here for a similar problem, but couldn’t find anything that can run successfully like the way I want to do it.

Here’s my code, I would really appreciate any help. Thanks a lot:

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

#define SIZE_OF_VALUES 5
#define SIZE_OF_STRING 100

void set_values(char **values);

void set_values(char **values)
{
    *values = malloc(sizeof(char)*SIZE_OF_VALUES);
    for (int i = 0; i < (sizeof(char)*SIZE_OF_VALUES); i++) {
        values[i] = malloc(sizeof(char)*SIZE_OF_STRING);
        values[i] = "Hello";
        //puts(values[i]); //It works fine here.
    }
}

int main (int argc, const char * argv[])
{
    char *values;
    set_values(&values);

    for (int i = 0; i < (sizeof(char)*SIZE_OF_VALUES); i++) {
        puts(values[i]); //It does not work!
    }

    return 0;
}

Answer 1

There are several problems with your code:

1. You should have three-level pointers – void set_values(char ***values), read it as a “a reference (first *) to an array (second *) of char* (third *)”

2. Each element in *values should be a pointer (char*) not char, so you need:

   *values = malloc(sizeof(char*)*SIZE_OF_VALUES);
   

3. You are leaking memory, first mallocing then assigning literal, and additionally not dereferencing values, you need either:

   (*values)[i] = "Hello";
   

   or

   (*values)[i] = strdup("Hello");  // you will have to free it later
   

   or

   (*values)[i] = malloc(sizeof(char)*SIZE_OF_STRING); // you will have to free this as well
   strcpy((*values)[i], "Hello");
   

4. In your main, you should declare char **values; as it is a pointer to an array of char* (character string/array).

5. In you loop you are incorrectly multiplying indices by sizeof, index is counted in elements not in bytes. Thus, you need:

   for (int i = 0; i < SIZE_OF_VALUES; i++)
   

6. Don’t forget to free the memory at the end.