Question

How does C find the size of an array at runtime? Where is the information about the array size or bounds stored ?

Answer 1

sizeof(array) is implemented entirely by the C compiler. By the time the program gets linked, what looks like a sizeof() call to you has been converted into a constant.

Example: when you compile this C code:

#include <stdlib.h> #include <stdio.h> int main(int argc, char** argv) {     int a[33];     printf('%d\n', sizeof(a)); } 

you get

    .file   'sz.c'     .section        .rodata .LC0:     .string '%d\n'     .text .globl main     .type   main, @function main:     leal    4(%esp), %ecx     andl    $-16, %esp     pushl   -4(%ecx)     pushl   %ebp     movl    %esp, %ebp     pushl   %ecx     subl    $164, %esp     movl    $132, 4(%esp)     movl    $.LC0, (%esp)     call    printf     addl    $164, %esp     popl    %ecx     popl    %ebp     leal    -4(%ecx), %esp     ret     .size   main, .-main     .ident  'GCC: (GNU) 4.1.2 (Gentoo 4.1.2 p1.1)'     .section        .note.GNU-stack,'',@progbits 

The $132 in the middle is the size of the array, 132 = 4 * 33. Notice that there’s no call sizeof instruction – unlike printf, which is a real function.