Hi I need help with my C assignment. I need to modify a dynamic 2d array, so I pass a pointer to it to function, unfortunately it seems to not modify it but only copy it and leave it untouched, I have tried everything, can someone help please?

int getUniqueRelMembersCount(universe_t u, relation_t rel, char **members)
{
    int memberCount = 0;

    //count and dynamically populate passed 2d array with unique members of rel

    return memberCount;
}

How I call the function:

char **members = malloc(sizeof(members));
int len = getUniqueRelMembersCount(universe, relations.relations[0], members);
printf("%d\n", len);
printf("%s", members[0]);
for (int i = 0; i < len; i  )
{
     free(members[i]);
}

free(members);

output:

3

Segmentation fault

CodePudding user response：

Despite having the same name, the variable named members in your function and the variable named members in the caller are different variables.

You change members in the function (members = realloc(...)), but don't propagate this change to the caller.

Simplified demo:

void f(int members) {
   printf("%d\n", members);   // 2
   members = 3;
   printf("%d\n", members);   // 3
}

int main(void) {
   int members = 2;
   printf("%d\n", members);   // 2
   f(members);
   printf("%d\n", members);   // 2
   return 0;
}

You could return the modified value. In this case, it's better to pass a pointer to the variable.

int f(int *membersPtr) {
   printf("%d\n", *membersPtr);   // 2
   *membersPtr = 3;
   printf("%d\n", *membersPtr);   // 3
}

int main(void) {
   int members = 2;
   printf("%d\n", members);   // 2
   f(&members);
   printf("%d\n", members);   // 3
   return 0;
}

In your full program, a pointer to the variable would have type char***.

CodePudding user response：

Your following code shouldn't work:

char **members = malloc(sizeof(members));
int len = getUniqueRelMembersCount(universe, relations.relations[0], members);

because:

1. you cannot use 'member' value to initialize 'member';
2. malloc returns signle pointer, but not double pointer.

Try this:

int members_count = 10;
char *members = malloc(members_count);
int len = getUniqueRelMembersCount(universe, relations.relations[0], &members);