seg. fault when allocating memory via a pointer inside a struct

elmafiacs · 02-19-2005, 08:18 AM

Why this doesn't work?

Code:

struct small {
  int num;
};

struct big {
  struct small *small_thing;
};

void f(struct small *s)
{
  s = (struct small *) malloc(sizeof(struct small));
  memset(&s, 0, sizeof(struct small));
  s->num = 42;
}

main()
{
  struct big *big_thing;

  big_thing = (struct big *) malloc(sizeof(struct big));
  memset(&big_thing, 0, sizeof(struct big));

  f(big_thing->small_thing);
  printf("%i\n", big_thing->small_thing->n);
}

Thanks in advance.

Hivemind · 02-19-2005, 08:26 AM

A few things.

1. Don't cast what malloc() returns.

2. main() returns int and the return value is not implicit.

3. in function f() the pointer is passed by value. You need f to take a struct small** for it to work as expected (and you will also have to alter the body of fand how you call it as well).

4. Post code inside [.code][./code] blocks (remove the dot).

aluser · 02-19-2005, 08:27 AM

Quote:

void f(struct small *s)
{
s = (struct small *) malloc(sizeof(struct small));
memset(s, 0, sizeof(struct small));
s->num = 42;
}

Since variables are passed by value, this function doesn't change the value of s in the caller. All it does is leak some memory. You could write it like

Code:

struct small * f(void) {
    struct small *ret;
    ret = malloc(sizeof(struct small));
    memset(ret, 0, sizeof(struct small));
    ret->num = 42;
    return ret;
}

and say

Code:

big_think->small_thing = f();

ech, keyboard lag. other dude got it

elmafiacs · 02-20-2005, 07:19 AM

I have fixed the code, but now when I do:
[code]
struct small {
int num;
};

struct big {
struct small *small_thing;
};

void f(struct small **s)
{
s = malloc(sizeof(struct small));
memset(s, 0, sizeof(struct small));
s->num = 42;
}

main()
{
struct big *big_thing;

big_thing = malloc(sizeof(struct big));
memset(&big_thing, 0, sizeof(struct big));

f(big_thing->small_thing);
printf("%i\n", big_thing->small_thing->n);
}

elmafiacs · 02-20-2005, 07:26 AM

I think I have fixed the code:

Code:

struct small {
  int num;
};

struct big {
  struct small *small_thing;
};

void f(struct small **s)
{
  s = malloc(sizeof(struct small));
  memset(s, 0, sizeof(struct small));
  s->num = 42; 
}

main()
{
  struct big *big_thing;

  big_thing = malloc(sizeof(struct big));
  memset(&big_thing, 0, sizeof(struct big));

  f(&big_thing->small_thing);
  printf("%i\n", big_thing->small_thing->n);
}

But when I do:

Code:

 s->num = 42;

the compiler fails with error 'request for member 'num' in something not an structure or union'.

I tried this way:

Code:

void f(struct small **s)
{
  struct small *s_ptr;
  s_ptr->num = 42;
  memcpy(s, &s_ptr, sizeof(struct small));
}

I'm not sure, but I think that the second function is unnecessary. So, how to fix the first function?