How to use flock()

Question

I want to use system locks to avoid race conditions. One process being an opencv program saving captured image to a .jpg file. The other process being browser fetching the same image file from the server. I want to avoid race condition between these two processes. Am i using the flock function right?

My opencv code is:

#include<sys/file.h> 
#include<fcntl.h>
#include<string.h>
#include "cv.h"
#include "highgui.h"
#include <stdio.h>

int main() 
{
    int fd1,fd2;
    struct flock lock;
    fd1=open("a1.jpg",O_WRONLY);
    fd2=open("a2.jpg",O_WRONLY);
     lock.l_type=F_UNLCK;
     fcntl(fd1,F_SETLKW,&lock);

    CvCapture* capture = cvCaptureFromCAM(-1);
    if ( !capture )
    {

            fprintf( stderr, "ERROR: capture is NULL \n" );
            getchar();
            return -1;
    }

    cvNamedWindow( "mywindow", CV_WINDOW_AUTOSIZE );
    while ( 1 )
    {
            IplImage* frame = cvQueryFrame( capture );
            if ( !frame )

                    fprintf( stderr, "ERROR: frame is null...\n" );
                    getchar();
                    break;
            }
            cvShowImage( "mywindow", frame );
            if(flock(fd1,LOCK_EX)==0)
            {
            printf("A1 is locked");
            cvSaveImage("a1.jpg",frame,0);
            flock(fd1,LOCK_UN);
            printf("A1 unlocked");
            }
            else
            {
            flock(fd2,LOCK_EX);
            cvSaveImage("a2.jpg",frame,0);
            flock(fd2,LOCK_UN);
            printf("A2 is unlocked");
            }
            if ( (cvWaitKey(500) & 255) == 27 )
                    break;
    }
    cvReleaseCapture( &capture );
    cvDestroyWindow( "mywindow" );
    close(fd1);
    close(fd2);
    return 0;

}

And my .php file is

<?php
$dest='a.jpg'; 
$src1='a1.jpg';
$src2='a2.jpg';
if(flock($fp1,LOCK_EX))
{
    echo "a1";
    $img=imagecreatefromjpeg($src1);
    $copy=imagejpeg($img,$dest);
    imagedestroy($img);
    flock($fp1,LOCK_UN);
}
else
{
    echo "A2";
    flock($fp2,LOCK_EX);
    $img=imagecreatefromjpeg($src2);
    $copy=imagejpeg($img,$dest);
    imagedestroy($img);

    flock($fp2,LOCK_UN);
}
?>

This php is called by the client side using AJAX.

In which sense are you uncertain about the rightness of the usage? Have you tested your code and you get wired results? Can you elaborate a bit? — hakre, Feb 26 '12 at 11:38
If flock is successful it should return 0 right, jowever it is not returning any value. Also when i try to access the file by some other process i can access it. So it means that i'm not getting a lock on that file. — user927774, Feb 26 '12 at 11:43
Is that inside the C part or the PHP part? http://php.net/flock — hakre, Feb 26 '12 at 11:49
You do realize that locks created with flock are *advisory* locks? A lock won't block another process unless that process also tries to lock the file. — John, Feb 26 '12 at 13:14
Also, if it does not return 0, what does it return? The return type for flock is int so it will always return an integer. — John, Feb 26 '12 at 13:22
@user927774 your code doesn't check all the the return values of flock. How do you know it's not returning any value ? You also seem to have a fcntl call in there, fcntl locks and flock locks does not interact with eachother. — nos, Feb 27 '12 at 20:52

score 0 · Answer 1 · answered Feb 27 '12 at 17:56

flock(2) blocks if you are asking for an exclusive lock and it is unavailable. It returns 0 when all is behaving normally -- it will return -1 if there is an error, which you can then examine using perror(3) or the errno variable.

It appears that your C code is at least vaguely following the correct protocol -- getting an exclusive lock on a shared file before writing it and then unlocking it -- but I cannot understand what your code is trying to do overall, so it is difficult to understand whether or not your code is doing the right thing.

How to use flock()

1 Answers1