Introduction to C · Lesson 3 of 35

How a C Program Is Compiled and Run

3 min read Updated Nov 7, 2026 Saurabh Samir

On this page

The four stages
The preprocessor at work
Three kinds of errors
A program in memory
Practice
Summary

Typing gcc hello.c -o hello looks like one step, but four tools run behind it. Understanding them explains confusing errors ("undefined reference" vs "undeclared"), how header files work, and why C programs are fast. It's also a favourite viva and interview question.

In this lesson

The four stages: preprocess, compile, assemble, link
Seeing each stage with gcc flags
Compile errors vs link errors vs runtime errors
How a running program is laid out in memory

The four stages

From hello.c to a running program

Preprocessorhandles #include and #define → expanded source (hello.i)
Compilerchecks syntax and types, translates C to assembly (hello.s)
Assemblerturns assembly into machine code → object file (hello.o)
Linkerjoins object files with libraries (printf lives in libc) → executable
Loaderthe OS loads the executable into memory and calls main

You can stop gcc after any stage to look at the result:

gcc -E hello.c -o hello.i     # preprocess only: see what #include pasted in
gcc -S hello.c -o hello.s     # compile to assembly
gcc -c hello.c -o hello.o     # compile + assemble: object file, not yet runnable
gcc hello.o -o hello          # link into an executable

The preprocessor at work

#include <stdio.h>
#define PI 3.14159
#define SQUARE(x) ((x) * (x))

int main(void) {
    double r = 2.0;
    printf("Area = %.2f\n", PI * SQUARE(r));   // becomes 3.14159 * ((r) * (r))
    return 0;
}

Output

Area = 12.57

The preprocessor does plain text substitution before the compiler sees the code: PI is replaced by 3.14159 everywhere. Chapter 12 covers macros and their traps in detail.

Three kinds of errors

Error type	Found by	Example
Compile-time (syntax/type)	compiler	missing semicolon, undeclared variable, wrong argument type
Link-time	linker	undefined reference to 'calculate' – declared but never defined, or a library not linked (`-lm`)
Runtime	running the program	division by zero, reading outside an array, a segmentation fault
Logical	you, by testing	the program runs but gives the wrong answer

#include <stdio.h>

int square(int n);          // declaration: promises the function exists

int main(void) {
    printf("%d\n", square(7));
    return 0;
}

int square(int n) {         // definition: the actual code (remove it → linker error)
    return n * n;
}

Output

A program in memory

When the loader starts your program, it gives it memory divided into regions. You'll meet each of them in later chapters:

Memory layout of a running C program (high addresses at the top)

Stacklocal variables and function calls; grows and shrinks automatically
↓ free space ↑the stack and heap grow towards each other
Heapmemory you allocate with malloc and release with free
BSSglobal and static variables without an initial value (set to 0)
Datainitialised global and static variables
Text (code)the machine instructions; usually read-only

#include <stdio.h>
#include <stdlib.h>

int counter = 10;          // data segment
int unset;                 // BSS: starts at 0

int main(void) {
    int local = 5;                         // stack
    int *dynamic = malloc(sizeof *dynamic);  // heap
    if (dynamic == NULL) return 1;
    *dynamic = 42;
    printf("counter=%d unset=%d local=%d dynamic=%d\n", counter, unset, local, *dynamic);
    free(dynamic);
    return 0;
}

Output

counter=10 unset=0 local=5 dynamic=42

Practice

Quick check

What are the four stages of compilation?

Preprocessing, compilation, assembly and linking.

"undefined reference to sqrt" – which stage, and the fix?

The linker; link the maths library with -lm.

Which gcc flag stops after creating an object file?

-c.

Where do local variables live?

On the stack.

Summary

gcc runs the preprocessor, compiler, assembler and linker in turn.
Compile errors come from syntax/types, link errors from missing definitions or libraries.
A running program has text, data, BSS, heap and stack regions.

Interview practice: C language basics interview questions.

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