Structure

[primal socket]

Can anyone explain why
sizeof(int) = 4
sizeof(char) = 1
but for

{
int i;
char c;
}

sizeof(struct s) = 8

[harsh prairie]

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[dusk nexus]

That's because of padding.
You can however remove that by using the __attribute__((__packed__)):

;compile

#include <stdio.h>

struct __attribute__((__packed__)) foo {
    int i;
    char c;
} x;

int main() {
    printf("sizeof(x) = %d\n", sizeof(x));
}

[vestal quartz]

Program Output

sizeof(x) = 5

Samu#6425 | 56ms | c | x86-64 gcc 12.2 | godbolt.org

[dusk nexus]

More information about the topic: https://stackoverflow.com/questions/4306186/structure-padding-and-packing

[cobalt lava]

if you're gonna talk about "removing" padding, should also mention why it exists I think

including alignment constraints

[dusk nexus]

I was just gonne give that example, then link to the SO post

[cobalt lava]

fair enough

[primal socket]

@dusk nexus so padding is a bad thing or it can be ignored?

[dusk nexus]

Just ignore it for now.

Padding exists to properly align memory because access to unaligned memory is slower than it is to aligned memory (and some CPU architectures won't even allow unaligned memory access).

The tradeoff when you pack a struct is that each individual struct will consume less memory, however the access to the structs' elements may be slower.

[primal socket]

Kk. Thanks

!solved