Linking & Libraries

/* ===== mathlib.h ===== */
/* Header guard — prevents double inclusion */
#ifndef MATHLIB_H
#define MATHLIB_H

/* Declaration only — no function body here */
/* extern is implicit for function declarations */
int add(int a, int b);
int multiply(int a, int b);
double power(double base, int exp);

#endif /* MATHLIB_H */


/* ===== mathlib.c ===== */
/* This file provides the DEFINITIONS (actual code) */
#include "mathlib.h"  /* include our own header */

int add(int a, int b) {
    return a + b;
}

int multiply(int a, int b) {
    return a * b;
}

double power(double base, int exp) {
    double result = 1.0;
    for (int i = 0; i < exp; i++) {
        result *= base;
    }
    return result;
}


/* ===== main.c ===== */
#include <stdio.h>
#include "mathlib.h"  /* get declarations so compiler checks our calls */

int main(void) {
    printf("add(3, 4)        = %d\n",   add(3, 4));
    printf("multiply(6, 7)   = %d\n",   multiply(6, 7));
    printf("power(2.0, 10)   = %.0f\n", power(2.0, 10));
    return 0;
}

# Build object files for library components
gcc -c mathlib.c -o mathlib.o
gcc -c strutils.c -o strutils.o

# Create static library archive (convention: name starts with 'lib', ends with '.a')
ar rcs libmathlib.a mathlib.o strutils.o
# r = insert/replace members
# c = create archive if it doesn't exist
# s = write an object-file index (speeds up linking)

# Inspect the archive
ar t libmathlib.a
# Output:
# mathlib.o
# strutils.o

# Link the library into a program
# -L.        = look for libraries in the current directory (.)
# -lmathlib  = link against libmathlib.a (strips 'lib' prefix and '.a' suffix)
gcc -o calc main.c -L. -lmathlib

# Run the program
./calc

# View symbols inside the library
nm libmathlib.a
# T = defined symbol (Text section = code)
# U = undefined symbol (must be resolved by the linker)
# Output includes lines like:
# 0000000000000000 T add
# 0000000000000020 T multiply

# Step 1: compile with -fpic (position-independent code — required for shared libs)
gcc -c -fpic mathlib.c -o mathlib.o

# Step 2: create shared library from the PIC object
gcc -shared -o libmathlib.so mathlib.o

# Step 3: link the program against the shared library
gcc -o calc main.c -L. -lmathlib

# Step 4a: run — if libmathlib.so is NOT in a standard library path, set env var
LD_LIBRARY_PATH=. ./calc

# Step 4b: alternatively, install to system library path (needs root) and refresh cache
# sudo cp libmathlib.so /usr/local/lib/
# sudo ldconfig

# Check what shared libraries 'calc' depends on
ldd ./calc
# Output (example):
#   linux-vdso.so.1 => (0x00007ffce6bfe000)
#   libmathlib.so => ./libmathlib.so (0x00007f4a3c123000)
#   libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00007f4a3bf00000)

CC      = gcc
CFLAGS  = -Wall -Wextra -g -I./include
LDFLAGS = -L./lib
LIBS    = -lmathlib

# Default target: build the executable
all: calc

# Link: combine main.o with the library
calc: main.o lib/libmathlib.a
	$(CC) $(CFLAGS) -o $@ $< $(LDFLAGS) $(LIBS)

# Compile main.c to main.o
main.o: src/main.c include/mathlib.h
	$(CC) $(CFLAGS) -c src/main.c -o $@

# Compile library source files
lib/mathlib.o: src/mathlib.c include/mathlib.h
	$(CC) $(CFLAGS) -c src/mathlib.c -o $@

# Build static library from object files
lib/libmathlib.a: lib/mathlib.o
	ar rcs $@ $^

# Clean all generated files
clean:
	rm -f main.o lib/*.o lib/libmathlib.a calc

.PHONY: all clean