printf output is too late

asked May 18 '13

andrempunkt
11 ●1 ●2

updated May 18 '13

Mathieu Barnachon

4678 ●18 ●53 http://www.math-barnac...

Hi, i have the following code:

#include "highgui.h"
#include "stdio.h"
using namespace std;

const char* name="Example1";

int main(int argc, char** argv) {
    IplImage* img = cvLoadImage(argv[1]);
    cvNamedWindow( name, CV_WINDOW_AUTOSIZE);
    cvShowImage( name, img);
    printf("HelloWorld...");
    cvWaitKey(0);
    cvReleaseImage(&img);
    cvDestroyWindow (name);
}

It is all working but the "HelloWorld" is not written until I press a key. I don't understand why it is not written before.

ciao, Andre

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answered May 18 '13

Guanta
6736 ●6 ●25 ●79

The reason is that stdout is buffered. If you want to print it directly, write to stderr instead of stdout (if you really want to use C instead of C++ this would work with fprintf(stderr, "HelloWorld...")); or flush the buffer, should be working with a simple newline at the end: "HelloWorld\n".

Note: I highly recommend to switch to C++! OpenCV has mainly switched to it and you will have it a lot easier w. coding (std::cerr and std::cout ftw, and of course cv::Mat instead of IplImage/CvMat)!

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thanks for the fast answer, worked perfect :)

andrempunkt (May 18 '13)edit

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answered Jun 10 '13

SR
1560 ●1 ●14 ●29

Guanta tells the right cause.

However, personally I would discourage the use of C++ streams. Mostly because these can be quite unreadable and most important these are not thread-safe in multi-threaded enviroments. That is, your program may crash spuriously because their internal state may get corrupted.

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Hehe, good point! Streams definitely have their drawbacks, guess I've advertised them in my post too much :)

Guanta (Jun 11 '13)edit

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