Question

I have a question regarding this code. I write this code in my framework, and it caused the framework crashed. But when I rewrite this code below in a single file, but it works just fine. I was just wondering, is the code below is correct for memory allocation and freeing it? (especially for the part of msg->context_var.type = f;)
Thank you

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


typedef struct
{
  int value;
  int price;
  int old;
} type_t;


typedef struct {
  type_t            *type;  
} context_t;


typedef struct {
  context_t context_var; 
} send_request;



void send_Message(send_request *msg)
{   

type_t *f = 0;
f = malloc(sizeof(f));
msg->context_var.type = f;
msg->context_var.type->price = 1;
msg->context_var.type->value = 100;
msg->context_var.type->old =120;


printf("value of %d/n", msg->context_var.type->price);
free(f);
}

int main()
{   

    send_request *msg = 0;
    msg = (send_request *) malloc(sizeof(send_request));

    send_Message(msg);
    free(msg);
    return 0;
}

Answer 1

It’s wrong.

f = malloc(sizeof(f)); /* Wrong */
f = malloc(sizeof(*f)); /* Better ? */

sizeof(f) will give you the size of a pointer on your machine; sizeof(*f) will give you the size of the object pointed to.

EDIT As requested by @Perception

When you allocate less than you need you’re eliciting Undefined Behavior. Anything can happen (even the desired behavior) and it all depends on the platform, the environment (the moon phase, etc).

msg->context_var.type->value = 100; /* Writes beyond what's allocated. */

So, depending on the memory layout of the “framework” this might simply overwrite some memory and “work”, or it could crash. Frankly I prefer when it crashes straight away.