c++ - Opencv: Null pointer (NULL array pointer is passed) in cvGetMat -

i'm getting error when trying capture video webcam using opencv. code i'm using:

#include "opencv2/opencv.hpp"  using namespace cv; using namespace std;  int main( int argc, const char** argv ) {     cvcapture *capture = cvcapturefromcam(0);     iplimage *frame;     cvnamedwindow("test");      while ( 1)     {         frame = cvqueryframe(capture) ;         cvshowimage("test",  frame);         int key = cvwaitkey(1);         if ( key == 27 ) break;   // esc key pressed     }      // memory deallocation     cvreleasecapture(&capture);     cvdestroywindow("test");      return 0; } 

the error occur @ cvshowimage("text", frame):

opencv error: null pointer (null array pointer passed) in cvgetmat, file /opt/local/var/macports/build/_opt_mports_dports_graphics_opencv/opencv/work/opencv-2.4.6.1/modules/core/src/array.cpp, line 2382 libc++abi.dylib: terminate called throwing exception (lldb)

what error , how can fix it?

first should check whether there device @ index 0, this

  if (!capture)   {       // print error , exit       cout << "error: capture null!\n";       return -1;   } 

secondly, :

while ( capture) {      frame = cvqueryframe(capture) ;      cvshowimage("test",  frame);      int key = cvwaitkey(1);      if ( key == 27 ) break;   // esc key pressed } 

hope helps