How to solve "undefined reference to function" error?

Question

I have to build a library for a school project and use that library in a small aplication. Now, I have made the XPM_lib.h and XPM_lib.c files and also my test.c file. But when I try to compile my test.c file I get an "undefined reference to initXPM_image" error.

My files are (XPM_lib.h):

#ifndef LIBRARYXPM_H_INCLUDED
#define LIBRARYXPM_H_INCLUDED



#include<stdio.h>
#include<stdlib.h>
#include<malloc.h>
#define HEADER "/*XPM*/"
#define STRING_BEGIN "static char *egc[] = {\n\n /*width ,  height , ncolors ,        charsperpixel */ \n "
#define STRING_END "\n }"
#define STRING_SEPARATOR ","
#define STRING_COLORS "/* colors #RRGGBB */"
#define STRING_PIXELS "/* pixels */"


struct Color{

        unsigned char r,g,b;
        char *color_char;
        char key[2];
};   


struct XPM_image {

        unsigned int width;
        unsigned int height;
        unsigned int no_colors; // number of colors used

        unsigned char char_per_pixel; // number of characters to describe a pixel

        struct Color *color_table; // table containing the colors
        unsigned int **image_data; // contains indices from color_table

} ;


// functions first problem , lab 1

extern void initXPM_image (struct XPM_image *image, unsigned int width, unsigned int      height, unsigned char char_per_pixel, unsigned int no_colors );

extern void freeXPM_image (struct XPM_image *image);

extern void setXPMpixel (struct XPM_image *image, unsigned int poz_x, unsigned int poz_y, unsigned int index );

extern void setXPMcolor (struct XPM_image *image, unsigned int index, unsigned char r, unsigned char g, unsigned char b, char *pattern);

extern void writeToFile (struct XPM_image *image , char *pathname);



#endif // LIBRARYXPM_H_INCLUDED

and (XPM_lib.c) :

    #include "XPM_lib.h"

// initializeaza spatiul pentru imagine si tabela de culori si seteaza proprietatiile de baza
void initXPM_image (struct XPM_image *image, unsigned int width, unsigned int height, unsigned char char_per_pixel, unsigned int no_colors )
{
    image->width = width;
    image->height = height;
    image->char_per_pixel = char_per_pixel;
    image->no_colors = no_colors;

    if ( ( image->color_table =(struct Color*) malloc ( image->no_colors * sizeof(struct Color)) == NULL ))
        {
            printf("Eroare in initXPM_image la alocarea color_table");
            exit (0);

        }



    if ( ( image->image_data = (unsigned int **) malloc (height * sizeof(unsigned int *))) == NULL )
    {

        printf("Eroare in initXPM_image la alocarea image_data (height)");
        exit (0);

    }

    int i;
    for (  i = 0 ; i < height ; i++ )
    {
        if ( (image->image_data[i] = (unsigned int *) malloc (width * sizeof (unsigned int ))) == NULL )
        {
            printf( "Eroare in initXPM_image la alocarea image_data (width)");
            exit (0);
        }

    }



}


void setXPMcolor (struct XPM_image *image, unsigned int index, unsigned char r, unsigned char g, unsigned char b, char *pattern)
{

    image->color_table[index].r = r;
    image->color_table[index].g = g;
    image->color_table[index].b = b;

    image->color_table[index].color_char = pattern;

}

void setXPMpixel (struct XPM_image *image, unsigned int poz_x, unsigned int poz_y, unsigned int index )
{

    image->image_data[poz_x][poz_y] = index; // pun index-ul culorii din tabela de culori din imagine

}

void writeToFile (struct XPM_image *image , char *pathname)
{

    FILE *f;

    if (( f = fopen(pathname,"wt")) == NULL)
    {
        printf("Eroare la deschiderea fisierului!");
        exit(0);

    }

    fprintf( f ,"%s\n%s",  HEADER , STRING_BEGIN);

    fprintf( f , "\" %d %d %d %d\"", image->width, image->height , image->no_colors , image->char_per_pixel);

    // colors
    fprintf( f, "%s \n" , STRING_COLORS);
    int i;
    for ( i = 0 ; i < image->no_colors ; i++ )
        {
            printf("\"%s c #%.02X%.02X%.02X\",\n" , image->color_table[i].color_char , image->color_table[i].r , image->color_table[i].g , image->color_table[i].b);
        }


}

The test.c file is :

#include "lib/XPM_lib.h"


#define WIDTH 50
#define HEIGHT 50
#define COLORS 50


int main ()
{

    char first_char = 'a';
    struct XPM_image *image;

    if ((image = (struct XPM_image*) malloc (sizeof(struct XPM_image))) == NULL )
    {
        printf("Eroare la alocarea XPM_image\n");
        exit(0);

    }

    initXPM_image (image, WIDTH, HEIGHT,1, COLORS);

    int i,j;
    for (  i = 0 ; i < COLORS; i++ )
    {
        setXPMcolor(image, i, 255*i/(COLORS-1),0,0,&first_char);
        first_char++;
    }

    for ( i = 0 ; i < WIDTH ; i++ )
        for ( j = 0 ; j < HEIGHT ; j++ )
            setXPMpixel(image,i,j,i);

    writeToFile(image,"imagine.xpm");
    return 0;

}

Now, I can't use and IDE to set up a project for this so I have to compile it manually using gcc. To compile, I did:

gcc -c .\lib\XPM_lib.c
gcc -c test.c
gcc -o program .\lib\XPM_lib.o test.o 

( I forgot to mention !) My directory structure :

.\lib\XPM_lib.h 
.\lib\XPM_lib.c
.\test.c

Answer 1

You need to make sure gcc can "see" all the required header files to ensure proper compilation. In order to do this you should include the lib directory into your path when building:

gcc -I ./lib -c XPM_lib.c test.c
gcc -o program XPM_lib.o test.o

Note:

"undefined reference to..." means even though you included the header file, but you don't tell the compiler to link with the library. These aren't "undeclared", they're "undefined". That's why it doesn't find symbols.