Object files

Learn the structure of object files (.o).

What is an object file?

An object file (.o) is the relocatable machine code the compiler produces from a source file. It is not yet executable; the linker combines several object files into the final executable.

An object file has three main sections: .text (machine instructions), .data (initialized global variables) and .bss (uninitialized global variables). Each section is written with relative addresses; the linker assigns final addresses later. The object file also contains a symbol table and relocation entries; these let the linker do its job.

Object file sections

The sections of an object file hold different kinds of content. The .text section contains machine instructions; it is usually read-only. The .data section holds initialized global variables; for example int counter = 42; goes here.

The .bss section holds uninitialized or zero-initialized variables; for example int total;. BSS takes no space in the file; only its size is recorded. This split lets the linker and loader place each section in the right memory region. Sections are standardized in file formats such as ELF.

Exercise Object file simulation

Define the global int counter = 42; (this goes to .data). Write int get_counter(void) that returns counter (this goes to .text). Write int main(void) and print with printf("Sayaç: %d\n", get_counter()).

Show example solution

#include <stdio.h>

int counter = 42;

int get_counter(void) {
    return counter;
}

int main(void) {
    printf("Sayaç: %d\n", get_counter());
    return 0;
}

Test yourself

What does a .o file contain?
- Only source code
- An executable binary
- Relocatable machine code
- Just plain text
Show answer

Relocatable machine code — A .o file is relocatable code the linker will combine.

Complete the section name

int counter = 42;  /* .____ bölümünde */

Show answer

int counter = 42;  /* .data bölümünde */

Match section to content

text

Machine instructions
data

Initialized data
bss

Uninitialized data
rodata

Read-only constant

Next lesson: Symbol resolution

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