Allocating memory for a Structure in C

Question

I'm tasked to create a program which dynamically allocates memory for a structure. normally we would use

x=malloc(sizeof(int)*y);

However, what do I use for a structure variable? I don't think its possible to do

struct st x = malloc(sizeof(struct)); 

Could someone help me out? Thanks!

Answer 1

My favorite:

#include <stdlib.h>

struct st *x = malloc(sizeof *x); 

Note that:

• x must be a pointer
• no cast is required
• include appropriate header

Answer 2

You're not quite doing that right. struct st x is a structure, not a pointer. It's fine if you want to allocate one on the stack. For allocating on the heap, struct st * x = malloc(sizeof(struct st));.

Answer 3

struct st* x = malloc( sizeof( struct st ));

Answer 4

It's exactly possible to do that - and is the correct way

Assuming you meant to type

struct st *x = malloc(sizeof(struct st)); 

ps. You have to do sizeof(struct) even when you know the size of all the contents because the compiler may pad out the struct so that memebers are aligned.

struct tm {
  int x;
  char y;
}

might have a different size to

struct tm {
  char y;
  int x;
}

Answer 5

This should do:

struct st *x = malloc(sizeof *x); 

Answer 6

struct st *x = (struct st *)malloc(sizeof(struct st));