Multiple modules

Learn to combine multiple modules in one project.

Multiple modules

A big C project splits into modules. Each module has a .h and a .c file. For example vector.h/vector.c and matrix.h/matrix.c. main.c includes both headers and calls functions.

Compile: gcc -c vector.c and gcc -c matrix.c produce separate .o files, then gcc -o app main.o vector.o matrix.o links them. A Makefile automates these steps. Modular layout cuts compile time; only the changed module recompiles. This is the standard structure of real-world C projects.

Module design

A good module carries one responsibility. The vector module only does vector ops; matrix does matrix ops. The header exposes the public API; the .c hides implementation detail. static makes internal helpers invisible outside the file.

Dependencies between modules should be one-directional when possible. For example matrix may depend on vector, but not the other way. Circular dependency is a design smell; split modules. This discipline makes maintenance easier in big projects.

Exercise Simulate a two-module program

Simulate two modules in one file: write int vec_add(int a, int b) { return a + b; } and int vec_mul(int a, int b) { return a * b; }. In int main(void) add printf("%d \n", vec_add(2, 3)); and printf("%d\n", vec_mul(4, 5));. Don't forget <stdio.h>.

Show example solution

#include <stdio.h>

int vec_add(int a, int b) {
    return a + b;
}
int vec_mul(int a, int b) {
    return a * b;
}

int main(void) {
    printf("%d \n", vec_add(2, 3));
    printf("%d\n", vec_mul(4, 5));
    return 0;
}

Test yourself

Match module files

vector.h

Vector declarations
vector.c

Vector definitions
vector.o

Compiled object
main.c

Program entry
What does gcc -c do?
- Produces an executable
- Performs linking
- Includes headers
- Only compiles, makes .o
Show answer

Only compiles, makes .o — -c compiles only, no linking; leaves a .o file.

Complete the link command

gcc -o app main.o vector.o ____.o

Show answer

gcc -o app main.o vector.o matrix.o

Next lesson: Summary - Multi-file

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