Question

I am working through Learn C The Hard Way and am stumped on something. I’ve written a simplified version of the problem I am running into to make it easier to get down to it:

#include <stdlib.h>

#define GROUP_SIZE 10
#define DATA_SIZE 64

struct Dummy {
    char *name;
};

struct Group {
    struct Dummy **dummies;
};

int main() {
    struct Group *group1 = malloc(sizeof(struct Group));
    group1->dummies = malloc(sizeof(struct Dummy) * GROUP_SIZE);
    struct Dummy *dummy1 = group1->dummies[3];

    // Why does this seg fault?
    dummy1->name = (char *) malloc(DATA_SIZE);

    return 0;
}

When I try to set the name pointer on one of my dummies I get a seg fault. Using valgrind it tells me this is uninitialized space. Why is this?

Answer 1

Your use of dummies appears inconsistent with its declaration. From the way you use the dummies field it appears that dummies was intended as an array of Dummy structs, not an array of arrays of Dummy structs. If this is the case, change your declaration to this:

struct Group {
    struct Dummy *dummies; // Single asterisk
};

Then change your usage as follows:

struct Dummy *dummy1 = &group1->dummies[3];

Of course this assumes that GROUP_SIZE is four or more.