OpenCV - C program not compiling

Technical Issues and Assistance Programming
,
#1

Hi, I'm having trouble compiling a C code using OpenCV library, this is my main.c file.

#include <opencv/cv.h>

int main()
{
    CvCapture* webcam = cvCreateCameraCapture(0);

    return 0;
}

This is the error I get when I compile the program with gcc

$ gcc main.c 
main.c: In function ‘main’:
main.c:5:5: error: unknown type name ‘CvCapture’; did you mean ‘CvCmpFunc’?
     CvCapture* webcam = cvCreateCameraCapture(0);
     ^~~~~~~~~
     CvCmpFunc
main.c:5:25: warning: implicit declaration of function ‘cvCreateCameraCapture’; did you mean ‘cvCreateGraph’? [-Wimplicit-function-declaration]
     CvCapture* webcam = cvCreateCameraCapture(0);
                         ^~~~~~~~~~~~~~~~~~~~~
                         cvCreateGraph
main.c:5:25: warning: initialization of ‘int *’ from ‘int’ makes pointer from integer without a cast [-Wint-conversion]

It doesn't recognize the typedef struct CvCapture even though I included opencv/cv.h.

Here's the documentation I referenced for cvCreateCameraCapture(): https://docs.opencv.org/3.4/dd/d01/group__videoio__c.html

I installed the OpenCV library using Pamac Manager, so the header files are in /usr/include/opencv/ and shared object files are in /usr/lib/.

Any help is appreciated!

#2 
CvCapture *webcam = cvCreateCameraCapture(0);
#3

I'm still getting the same error. Both ways of declaring pointer variables should be the same.

#4

hmm, check for the include file if you can find the structure definition and the function prototype. This is odd. I am installing the package from the repos now. There is also a package with samples "opencv-samples". Unfortunately my wifi is slow now (used up my highspeed with phone).
Edit: Better yet, grep for the definitions in the opencv include directory with *.h

#5

Ok, I ran grep -rnw '/usr/include/opencv2/' -e 'CvCapture' to search the folder for the definition and found the structure definition in /usr/include/opencv2/videoio/videoio_c.h. It compiles now but I get a bunch more undefined references

$ gcc main.c 
/usr/bin/ld: /tmp/cc39Gglx.o: in function `cvDecRefData':
main.c:(.text+0xa24): undefined reference to `cvFree_'
/usr/bin/ld: main.c:(.text+0xaad): undefined reference to `cvFree_'
/usr/bin/ld: /tmp/cc39Gglx.o: in function `cvGetRow':
main.c:(.text+0xbc5): undefined reference to `cvGetRows'
/usr/bin/ld: /tmp/cc39Gglx.o: in function `cvGetCol':
main.c:(.text+0xbf3): undefined reference to `cvGetCols'
/usr/bin/ld: /tmp/cc39Gglx.o: in function `cvReleaseMatND':
main.c:(.text+0xc0d): undefined reference to `cvReleaseMat'
/usr/bin/ld: /tmp/cc39Gglx.o: in function `cvSubS':
main.c:(.text+0xd6a): undefined reference to `cvAddS'
/usr/bin/ld: /tmp/cc39Gglx.o: in function `cvCloneSeq':
main.c:(.text+0xdbc): undefined reference to `cvSeqSlice'
/usr/bin/ld: /tmp/cc39Gglx.o: in function `cvSetNew':
main.c:(.text+0xe38): undefined reference to `cvSetAdd'
/usr/bin/ld: /tmp/cc39Gglx.o: in function `cvGetSetElem':
main.c:(.text+0xefb): undefined reference to `cvGetSeqElem'
/usr/bin/ld: /tmp/cc39Gglx.o: in function `cvReadIntByName':
main.c:(.text+0xfa2): undefined reference to `cvGetFileNodeByName'
/usr/bin/ld: /tmp/cc39Gglx.o: in function `cvReadRealByName':
main.c:(.text+0x103d): undefined reference to `cvGetFileNodeByName'
/usr/bin/ld: /tmp/cc39Gglx.o: in function `cvReadStringByName':
main.c:(.text+0x10c1): undefined reference to `cvGetFileNodeByName'
/usr/bin/ld: /tmp/cc39Gglx.o: in function `cvReadByName':
main.c:(.text+0x1104): undefined reference to `cvGetFileNodeByName'
/usr/bin/ld: main.c:(.text+0x111a): undefined reference to `cvRead'
/usr/bin/ld: /tmp/cc39Gglx.o: in function `main':
main.c:(.text+0x1178): undefined reference to `cvCreateCameraCapture'
collect2: error: ld returned 1 exit status
#6

Well, I don't even get this far. According to the cv.h file you should include this:

//CV_WARNING("This is a deprecated opencv header provided for compatibility. Please include a header from a corresponding opencv module")

#include "opencv2/core/core_c.h"
#include "opencv2/imgproc/imgproc_c.h"
#include "opencv2/photo/photo_c.h"
#include "opencv2/video/tracking_c.h"
#include "opencv2/objdetect/objdetect_c.h"

replace the quotes with < and >

#7

this should be your programm:

#include <opencv2/core/core_c.h>
#include <opencv2/imgproc/imgproc_c.h>
#include <opencv2/photo/photo_c.h>
#include <opencv2/video/tracking_c.h>
#include <opencv2/objdetect/objdetect_c.h>
#include <opencv2/videoio/videoio_c.h>

int main()
{
    CvCapture *webcam = cvCreateCameraCapture(0);

    return 0;
}

The error you get are linker errors. You need to bind the correct Library like such with the -L option of gcc. I do not know which of this libs its.

#8

This is how far i came:

$ gcc -lopencv_videoio opencv.c 
/usr/bin/ld: /tmp/ccHPAbQm.o: undefined reference to symbol 'cvArcLength'
/usr/bin/ld: /usr/lib/libopencv_imgproc.so.3.4: error adding symbols: DSO missing from command line
collect2: error: ld returned 1 exit status
#9

I got it to compile with no errors, I referenced this site http://answers.opencv.org/question/67705/undefined-reference-error-while-compiling-a-c-program-on-raspberry-pi/.

The command I used to compile it:

gcc main.c -lopencv_core -lopencv_imgproc -lopencv_highgui -lopencv_videoio
#10

Just wanted to tell you. This is my command:

$ gcc -l opencv_videoio -l opencv_imgproc -l opencv_core opencv.c
#11

Oh that works too. Yours has less library bindings. :slight_smile:

Thanks for the help Calvous. This is my first post on the forums. I appreciate that you took your time on this issue!

#12

Just a little tip: This libraries are stripped an therefore cannot be scanned with the nm command:

$ nm /usr/lib/libopencv_videoio.so
nm: /usr/lib/libopencv_videoio.so: no symbols

In this case I am using brute force:

$ grep CvCapture /usr/lib/libopencv_*.so
Binary file /usr/lib/libopencv_videoio.so matches

This way you can find undefined references in shared librarys. Not nice but saves some time to search for it in documentations :wink:

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