CODEWITHSUNDEEP
csizeof
Khamidulla
Khamidulla

Reputation: 2975

sizeof of structure and pointer to structure?

I have following code example:

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

int main(int argc, char **argv)
{
    uint32 *data;

    printf("%lu\n", sizeof(uint32_t));
    printf("%lu\n", sizeof(uint32_t*));
    printf("%lu\n", sizeof(*data));

}

**OUTPUT**:
4  // <-This is size uint32_t structure 
8  // <-Here I understand it is size of pointer which is equal to 8 
4  // <-What does it mean?

Does sizeof(*data) is correct way to allocate memory in malloc(sizeof(*data))? I saw this kind of memory allocation in library source code and I cannot understand if somebody can explain I will be were glad. And feel free to correct title of question. Because I am not aware what does it mean. And if there is already same question asked before just point to this question in comments I will just close question. Thank you in advance.

Upvotes: 3

Views: 1209

Answers (3)

virtus
virtus

Reputation: 291

sizeof is a unary operator. It returns (calculates) size of it's argument in bytes. Argument to sizeof can be a type or an expression.

Frist two lines of your example provide a type as an argument to sizeof while the third line provides the expression.

Expression you have provided dereferences a pointer to uint32. Because final type of that expression is of type uint32 and it's size on your machine is 4 bytes, that's what you get.

Upvotes: 2

n0p
n0p

Reputation: 3496

Well it seems quite logic to me: you are asking about the size of *data: since it is a pointer on uint32_t that you dereferencing you get an uint32_t, hence the 4

uint32_t * data; // pointer on uint32_t
uint32_t deref_data = *data; // uint32_t

sizeof(*data) == sizeof(*(uint32_t*)) == sizeof(uint32_t)

About the malloc you can just:

uint32_t * data = malloc(sizeof(uint32_t));

Upvotes: 4

Alec Teal
Alec Teal

Reputation: 5918

It means whatever data points to is 4 chars (usually bytes) big.

sizeof is an operator, sizeof(thing) is how many chars "thing" is big.

data points to a uint32, so it is 8 chars big (64 bits), and the thing it points to 4 chars (32 bits). so sizeof(data) = sizeof((uint32*)) = sizeof(uint32) basically.

Upvotes: 0

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