undefined reference BUT header is there

Question

I am trying to run the ezsift library example. The example has name "image_match.cpp".

Here is the code

image_match.cpp

#include <iostream>
#include <vector>
#include <stdio.h>
#include <stdlib.h>
#include <ctype.h>

#include "../ezsift.h"

using namespace std;

#define USE_FIX_FILENAME 0
int main(int argc, char ** argv)
{
#if USE_FIX_FILENAME
    char * file1 = "img1.pgm";
    char * file2 = "img2.pgm";
#else
    if (argc != 3)
    {
        printf("Please input two image filenames.\n");
        printf("usage: image_match img1 img2\n");
        return -1;
    }
    char file1[255];
    char file2[255];
    memcpy(file1, argv[1], sizeof(char) * strlen(argv[1]));
    file1[strlen(argv[1])] = 0;
    memcpy(file2, argv[2], sizeof(char) * strlen(argv[2]));
    file2[strlen(argv[2])] = 0;
#endif

    // Read two input images
    ImageObj<uchar> image1, image2;
    if(image1.read_pgm(file1) != 0)
    {
        printf("Failed to open input image1!\n");
        return -1;
    }

    if(image2.read_pgm(file2) != 0)
    {
        printf("Failed to open input image2!\n");
        return -1;
    }
    printf("Image 1 loaded. Image size: %d x %d\n", image1.w, image1.h);
    printf("Image 2 loaded. Image size: %d x %d\n", image2.w, image2.h);

    // Double the original image as the first octive.
    double_original_image(true);

    // Detect keypoints
    list<SiftKeypoint> kpt_list1, kpt_list2;
    printf("\nSIFT detection on image 1 ...\n");
    sift_cpu(image1, kpt_list1, true);
    printf("# keypoints in image1: %d\n", kpt_list1.size());

    printf("\nSIFT detection on image 2 ...\n");
    sift_cpu(image2, kpt_list2, true);
    printf("# keypoints in image2: %d\n", kpt_list2.size());

    // Save keypoint list, and draw keypoints on images.
    char filename[255];
    sprintf(filename, "s_A_keypoints.ppm");
    draw_keypoints_to_ppm_file(filename, image1, kpt_list1);
    export_kpt_list_to_file("s_A_keypoints.key", kpt_list1, true);

    sprintf(filename, "s_B_keypoints.ppm");
    draw_keypoints_to_ppm_file(filename, image2, kpt_list2);
    export_kpt_list_to_file("s_B_keypoints.key", kpt_list2, true);

    // Match keypoints.
    list<MatchPair> match_list;
    match_keypoints(kpt_list1, kpt_list2, match_list);

    // Draw result image.
    sprintf(filename, "s_A_B_matching.ppm");
    draw_match_lines_to_ppm_file(filename, image1, image2, match_list);
    printf("# of matched keypoints: %d\n", match_list.size());

    return 0;
}

ezsift.h

#ifndef EZSIFT_H
#define EZSIFT_H

#include "util/image.h"
#include "util/img_io.h"
#include <vector>
#include <list>
.
.
.
#endif

#define DEGREE_OF_DESCRIPTORS (128)
// Enable doubling of original image.
void double_original_image(bool doubleFirstOctave);

// Detect keypoints and extract descriptor.
int sift_cpu(
    const ImageObj<uchar> &image, 
    std::list<SiftKeypoint> & kpt_list, 
    bool bExtractDescriptors);

// Match keypoints from two keypoint lists.
int match_keypoints(
    std::list<SiftKeypoint> & kpt_list1,
    std::list<SiftKeypoint> & kpt_list2,
    std::list<MatchPair> & match_list);

.
.
.
#endif

(I will not include all the code here, I just wanted to give an idea. It is ease to download the source and see the files that I am talking about)

Also there is a "ezsift.cpp" file with more files in the "util" folder

I'm trying to run it from the terminal. I go to the directory and type "gcc image_match.cpp" but is says that double_original_image, sift_cpu (and all other functions in the header file) are not defined.

more specifically is gives this error:

image_match.cpp:(.text+0x1c7): undefined reference to `double_original_image(bool)'
image_match.cpp:(.text+0x20d): undefined reference to `sift_cpu(ImageObj<unsigned char> const&, std::list<_SiftKeypoint, std::allocator<_SiftKeypoint> >&, bool)'

(these are not the only error of course. It gives same error for all the functions)

I then tried "gcc ../ezsift.cpp image_match.cpp" but the same error.

What am I doing wrong?? I am running ubuntu 14.04.

Answer 1

(expending Cameron's comment into a tested answer)

You need to compile both ezsift.cpp and img_io.cpp with your program (image_match.cpp), while providing the location of the header files (-I\path\to\directory). Here is a working snippet, assuming you are in the examples directory:

g++ image_match.cpp  ../ezsift.cpp ../util/img_io.cpp -I.. -o image_match