In function _start , 'undefined reference to main' , opencv, panorama stitching - c++

Question

I was trying the sample codes under opencv3.0.0-alpha while I encountered the following error:
ps#hp-pavilion:~/cvit/opencv_projects$ make stitch
g++ `pkg-config --cflags opencv` -o stitch stitch.cpp `pkg-config --libs opencv`
/usr/lib/gcc/x86_64-linux-gnu/5/../../../x86_64-linux-gnu/crt1.o: In function `_start':
/build/glibc-qbmteM/glibc-2.21/csu/../sysdeps/x86_64/start.S:114: undefined reference to `main'
collect2: error: ld returned 1 exit status
makefile:5: recipe for target 'stitch' failed
make: *** [stitch] Error 1
I had simply copy-pasted the stitching.cpp file from opencv/samples/cpp and renamed and placed it as stitch.cpp in my projects folder where I also have my makefile. The makefile looks like :
CFLAGS = `pkg-config --cflags opencv`
LIBS = `pkg-config --libs opencv`
% : %.cpp
g++ $(CFLAGS) -o $# $< $(LIBS)
I compile .cpp files for instance temp.cpp simply by
make temp
and it works perfectly every time. But with this particular stitching code, the error pops-up each time. Here is the sample code -
#include <iostream>
#include <fstream>
#include "opencv2/imgcodecs.hpp"
#include "opencv2/highgui.hpp"
#include "opencv2/stitching.hpp"
using namespace std;
using namespace cv;
bool try_use_gpu = false;
vector<Mat> imgs;
string result_name = "stitch_result.jpg";
void printUsage();
int parseCmdArgs(int argc, char** argv);
int main(int argc, char* argv[])
{
int retval = parseCmdArgs(argc, argv);
if (retval) return -1;
Mat pano;
Stitcher stitcher = Stitcher::createDefault(try_use_gpu);
Stitcher::Status status = stitcher.stitch(imgs, pano);
if (status != Stitcher::OK)
{
cout << "Can't stitch images, error code = " << int(status) << endl;
return -1;
}
imwrite(result_name, pano);
return 0;
}
void printUsage()
{
cout <<
"Rotation model images stitcher.\n\n"
"stitching img1 img2 [...imgN]\n\n"
"Flags:\n"
" --try_use_gpu (yes|no)\n"
" Try to use GPU. The default value is 'no'. All default values\n"
" are for CPU mode.\n"
" --output <result_img>\n"
" The default is 'result.jpg'.\n";
}
int parseCmdArgs(int argc, char** argv)
{
if (argc == 1)
{
printUsage();
return -1;
}
for (int i = 1; i < argc; ++i)
{
if (string(argv[i]) == "--help" || string(argv[i]) == "/?")
{
printUsage();
return -1;
}
else if (string(argv[i]) == "--try_use_gpu")
{
if (string(argv[i + 1]) == "no")
try_use_gpu = false;
else if (string(argv[i + 1]) == "yes")
try_use_gpu = true;
else
{
cout << "Bad --try_use_gpu flag value\n";
return -1;
}
i++;
}
else if (string(argv[i]) == "--output")
{
result_name = argv[i + 1];
i++;
}
else
{
Mat img = imread(argv[i]);
if (img.empty())
{
cout << "Can't read image '" << argv[i] << "'\n";
return -1;
}
imgs.push_back(img);
}
}
return 0;
}
Edit : I just tried running the sample code from the samples folder itself and it works. The makefile runs perfectly without any error if I place it in the opencv/samples/cpp folder , but doesn't when I copy - paste it to another location.

Answer 1

The error means that the linker can't find your main function. Even though stitch.cpp defines main (I assume), the linker can't find it. The reason is unclear, because of the way you constructed your Makefile. I would make these changes:
use CXXFLAGS and/or CPPFLAGS for C++, because CFLAGS is for C.
expand the pkg-config output in make, so you can see what your compiler is being asked to do.
Your Makefile would then look like this:
CXXFLAGS = $(shell pkg-config --cflags opencv)
LIBS = $(shell pkg-config --libs opencv)
% : %.cpp
g++ $(CXXFLAGS) -o $# $^ $(LIBS)
I would like to think that with these changes the actual source of the problem will become apparent.