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Question

I have a project I'm working on that currently has a matrix.c file containing a some functions and has a corresponding header matrix.h . I made a shared library libccalc.so using matrix.c and another file. I am trying to create a directory in /usr/local/lib and a directory in /usr/local/include , both called ccalc to house the respective .so and .h files which can then later be used to compile programs using the functionality of libccalc.so . However, when I do this I am getting an error.

To be precise, matrix.c contains functions:
Matrix *mat_create(int rows, int cols) ,
void mat_fill(Matrix *mat, double *entries) and
void mat_print(Matrix *mat)
which are declared in matrix.h . I place the files in their respective directories as explained above, I run ldconfig /usr/local/lib/ccalc and I make a new file test.c in some other directory with the following:

// test.c

#include <stdio.h>
#include "matrix.h"

int main() {
    Matrix *m = mat_create(2, 2);
    double entries[4] = {1, 2, 3 ,4};
    mat_fill(m, entries);

    mat_print(m);

    return 0; 
}

matrix.h contains the following:

// matrix.h

#ifndef MATRIX_H
#define MATRIX_H

#define MAX_SIZE 50

typedef struct Matrix Matrix;

struct Matrix {
    int rows;
    int cols;
    double *data;
};

Matrix *mat_create(int n, int p);
void mat_fill(Matrix *mat, double *entries);
void mat_print(Matrix *mat);

#endif

When I enter the command: gcc -I/usr/local/include/ccalc -L/usr/local/lib/ccalc -lccalc test.c -o test , I get the error:

/usr/bin/ld: /tmp/ccesD44J.o: in function `main':
test.c:(.text+0x26): undefined reference to `mat_create'
/usr/bin/ld: test.c:(.text+0x71): undefined reference to `mat_fill'
/usr/bin/ld: test.c:(.text+0x7d): undefined reference to `mat_print'
collect2: error: ld returned 1 exit status

However, when I place libccalc.so and matrix.h in /usr/local/lib and /usr/local/include , enter ldconfig and enter the command gcc -L/usr/local/lib/ccalc -lccalc test.c -o test , and run ./test , it works perfectly

Can someone tell me what I am doing wrong?

Answer 1

The order of program arguments to gcc matters a lot. Please read the documentation about Invoking GCC .

Also, test is often a builtin command. See test(1) .

So (on Linux) rename test.c as prog.c then compile your program with

   gcc -Wall -Wextra -g prog.c -L/usr/local/lib/ccalc -lccalc -o prog

Then use strace(1) and gdb(1) to understand the behavior of prog , and ldd(1) , objdump(1) with readelf(1) to analyze prog . See also elf(5) and ld.so(8) .

Regarding building shared libraries, read Drepper's paper How to write shared libraries

You probably want to compile matrix.c into libcalc.so using

gcc -fPIC -Wall -O -shared -g matrix.c -o libcalc.so

but you might need other program arguments.

Regarding matrix operations, this answer could be inspirational.