C struct problem

Question

I am a C beginner, and I am curious why this gives me a Seg Fault everytime:

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

struct Wrapper {
  int value;
};

int main () {
  struct Wrapper *test;
  test->value = 5;

  return 0;
}

I know I don't fully understand pointers yet, but I thought that

struct_ptr->field

is the same as

(*struct_ptr).field

so trying to make an assignment right to the field should be ok. This works like expected:

struct Wrapper test;
test.value = 5;

but I am curious why using the pointer causes a Seg Fault.

I am on Ubuntu 9.04 (i486-linux-gnu), gcc version 4.4.1

Answer 1

You didn't assign the pointer to anything. It's an uninitialized pointer pointing to who knows what, so the results are undefined.

You could assign the pointer to a dynamically created instance, like this:

int main () {
  struct Wrapper *test;
  test = (struct Wrapper *) malloc(sizeof(struct Wrapper));
  test->value = 5;
  free(test);

  return 0;
}

EDIT: Realized this was C, not C++. Fixed code example accordingly.

Answer 2

You need to create an instance of Wrapper first:

struct Wrapper *test;
test = new struct Wrapper;
test->Value = 5;

Good luck.

Answer 3

You are using an uninitialised pointer, hence the segfault. Catching this kind of error is possible, if you turn on some more warnings, using -Wall for example

You need to use -Wall in conjonction with some optimisation (-On) for the warning to appear. For instance, compiling your code with

gcc -Wall -O2 -c test.c

resulted in the following error message :

test.c: Dans la fonction «main» :
test.c:10: attention : «test» is used uninitialized in this function

While using french word, this compiler message is not an insult but a warning ;) See below for a code allocating memory for your test pointer

int main () {
  struct Wrapper *test;
  test = malloc(sizeof(struct Wrapper))
  if(test == NULL) {
  /* error handling */
  }
  test->value = 5;
  free(test)

  return 0;
}