Guia de C · 4/6

Modules and Libraries

Por enquanto, este capítulo está disponível apenas em inglês.

C has no module system or package manager built into the language. Instead, you split a program into header files (.h) holding declarations and source files (.c) holding implementations, compile each source file separately, and let the linker stitch the results together. This chapter covers that split, separate compilation, static and shared libraries, and the basics of Make and CMake.

Splitting headers and sources

A header contains only what other files need to know: prototypes, type definitions, and constants. Function bodies live in the source file. An include guard keeps the header harmless if it ends up included twice in the same translation unit.

// mathutil.h
#ifndef MATHUTIL_H
#define MATHUTIL_H

#include <stddef.h>

typedef struct {
    double min, max, mean;
} Stats;

int clamp(int value, int lo, int hi);
Stats compute_stats(const double *values, size_t count);

#endif // MATHUTIL_H

// mathutil.c
#include "mathutil.h"

static double sum(const double *values, size_t count) {   // private to this file
    double total = 0.0;
    for (size_t i = 0; i < count; i++) total += values[i];
    return total;
}

int clamp(int value, int lo, int hi) {
    if (value < lo) return lo;
    if (value > hi) return hi;
    return value;
}

Stats compute_stats(const double *values, size_t count) {
    Stats s = { values[0], values[0], 0.0 };   // assumes count >= 1
    for (size_t i = 1; i < count; i++) {
        if (values[i] < s.min) s.min = values[i];
        if (values[i] > s.max) s.max = values[i];
    }
    s.mean = sum(values, count) / (double)count;
    return s;
}

// main.c
#include <stdio.h>
#include "mathutil.h"

int main(void) {
    double data[] = {3.0, 9.5, 1.5, 6.0};
    Stats s = compute_stats(data, sizeof data / sizeof data[0]);
    printf("min %.1f, max %.1f, mean %.2f\n", s.min, s.max, s.mean);
    printf("clamp: %d\n", clamp(120, 0, 100));
    return 0;
}

A few rules of thumb:

Use #include <...> for system headers and #include "..." for your own.
Marking a function static gives it internal linkage, so nothing outside the file can see it. Hide anything that is not part of the public interface.
To share a global variable, declare it in the header as extern int counter; and define it as int counter = 0; in exactly one source file.
#pragma once is not standard C, but every major compiler supports it as an alternative to include guards.

Separate compilation and linking

Compile each source file to an object file with -c, then link the objects. When you edit one file, only that file needs recompiling.

gcc -std=c17 -Wall -Wextra -c mathutil.c -o mathutil.o
gcc -std=c17 -Wall -Wextra -c main.c -o main.o
gcc main.o mathutil.o -o stats
./stats

Dúvidas sobre este capítulo?

Kind	Linux	macOS	Windows	Behavior
Static	`.a`	`.a`	`.lib`	Copied into the executable at link time
Shared	`.so`	`.dylib`	`.dll`	Loaded at run time and shared between programs

Modules and Libraries

Splitting headers and sources

Separate compilation and linking

Static and shared libraries

Make and CMake

Summary

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