How work with malloc and array of struct pointer

Question

I would like to try allocating my first 10 structs. We have to store an array of struct pointer at the heap but I don't know what i am doing wrong

my struct looks like this

typedef struct TEST
{
    char* first_string;
    char* second_string;
} test;

what i have written:

test **string_pair;

string_pair = malloc(10 * sizeof(test *));
  if(string:pair == NULL)
  {
    free(string_pair);
    exit(4);
  }

This works fine However later I am trying in an for loop to allocate my strings

string_pair[count]->first_string = malloc(10 * sizeof(char)); // note that count is currently 0
  if(string_pair[count]->first_string == NULL)
  {
    free(string_pair[count]->first_string);
    free(string_pair[count]);
    exit(4);
  }

And I get an segmentation fault

Before anyone suggests we are not allowed to simply use an array of structs we have to do it the complicated way.

Thx in advance

Answer 1

You allocated an array of pointers.

But you did not allocate memory for the actual structures, and you did not initialize the pointers you allocated.

This line:

string_pair = malloc(10 * sizeof(test *));

Allocates array of 4 or 8 (depending on your architecture) byte variables that can store memory addresses.

It does not allocate struct TEST . The pointers will contain garbage (possibly invalid) addresses until you initialize them.

You are suppose to do it by calling malloc for struct TEST , and placing the resulting pointer in the array.

Otherwise, when your code reaches this line:

string_pair[count]->first_string

it will look for struct TEST at some random address, possibly 0, and try to read its first_string field, which will cause segmentation fault.

Answer 2

You are getting a segmentation fault because you are allocating an array of pointers but not allocating the structure itself.

With string_pair = malloc(10 * sizeof(test *)); , you allocate string_pair (a pointer to a pointer) to 10 pointers (pointing to a test structure).

Then, with string_pair[count]->first_string = malloc(10 * sizeof(char)); , you allocate the first_string member of string_pair[count] to a string of 10 characters. The -> token means to access the member in string_pair[count] . However, the actual structure is not allocated.

You need to allocate string_pair[count] , with something like string_pair[count] = malloc(sizeof(test)); .

I'd recommend just using a test* for your array, and allocating it with test* string_pair = malloc(10 * sizeof(test));.