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C: Which data type sizes are correct?

  1. Why do C and gdb output something different?

  2. And how some data type sizes can even be equal? At least wikipedia says that all of these have different value ranges.

I have a 64-bit machine.

#include <stdio.h>

int main() {

        printf("int: %d, long int: %d, long long int: %d\n", sizeof(int), sizeof(long int), sizeof(long long int));
}

$ gcc -g test.c
$ ./a.out
int: 4, long int: 8, long long int: 8

$ gdb -q
(gdb) p sizeof(int)
$1 = 4
(gdb) p sizeof(long int)
$2 = 4
(gdb) p sizeof(long long int)
$3 = 8
Solution

  • The output from the C code without GDB is defined by the compiler.

    This post can help you with figuring out the sizes according to your machine:

    Size of integer in C

    on a 64 bit windows the size of long int is 4 bytes (like you got) while on Mac this type is 8 bytes.

    if you want precision while writing c code you should use "uintX_t" types from the stdint built in module.

    for example: "int8_t, uint32_t ..."

    This makes sure you will get the expected sizes regardless of the compiler or the machine you run on.