Question

#include <stdio.h>
typedef struct {
    short x,y;
    char type;
} Tile;

int main(int argc, const char *argv[])
{
    printf("%d\n",sizeof(short));
    printf("%d\n",sizeof(char));
    printf("%d\n",sizeof(Tile));
    return 0;
}

The output is:

2
1
6

I expected sizeof(Tile) to be 5, instead of 6. Is this a well-defined behaviour that structs add one extra byte of memory usage, or is it implementation dependant?

Answer 1

It’s because of padding (kind of like rounding).

for example:

struct example1
{
    char a;
    int b;
    char c;
}

struct example2
{
    char a;
    char b;
    int c;
}

will likely differ in size, first will have size of 12B, second will likely only eat 8B (arch and compiler dependant).

Edit: gcc does padding by size of biggest memeber of struct.

Gcc can minimize this behavior by option -fpack-struct, however this may not be best idea ever, it could even backfire (network protocol implementantion is first thing that pops into my mind).