__BEGIN__; __END__是opencv中的一种错误处理机制，它可以防止非法的内存释放，以及内存泄露

void cvResizeDCT( CvMat* input_array, CvMat* output_array )

{

    CvMat* temp_array = 0; // declare pointer that should be released anyway.

    CV_FUNCNAME( "cvResizeDCT" ); // declare cvFuncName

    __BEGIN__; // start processing. There may be some declarations just after

    // this macro, but they could not be accessed from the epilogue.

    if( !CV_IS_MAT(input_array) || !CV_IS_MAT(output_array) )

        // use CV_ERROR() to raise an error

        CV_ERROR( CV_StsBadArg,

        "input_array or output_array are not valid matrices" );

    // some restrictions that are going to be removed later, may be checked

    // with CV_ASSERT()

    CV_ASSERT( input_array->rows == 1 && output_array->rows == 1 );

    // use CV_CALL for safe function call

    CV_CALL( temp_array = cvCreateMat( input_array->rows,

        MAX(input_array->cols,

        output_array->cols),

        input_array->type ));

    if( output_array->cols > input_array->cols )

        CV_CALL( cvZero( temp_array ));

    temp_array->cols = input_array->cols;

    CV_CALL( cvDCT( input_array, temp_array, CV_DXT_FORWARD ));

    temp_array->cols = output_array->cols;

    CV_CALL( cvDCT( temp_array, output_array, CV_DXT_INVERSE ));

    CV_CALL( cvScale( output_array,

        output_array,

        1./sqrt((double)input_array->cols*output_array->cols), 0 ));

    __END__; // finish processing. Epilogue follows after the macro. 

    // release temp_array. If temp_array has not been allocated

    // before an error occured, cvReleaseMat

    // takes care of it and does nothing in this case.

    cvReleaseMat( &temp_array );

}

int main( int argc, char** argv )

{

    CvMat* src = cvCreateMat( 1, 512, CV_32F );

#if 1 /* no errors */

    CvMat* dst = cvCreateMat( 1, 256, CV_32F );

#else

    CvMat* dst = 0; /* test error processing mechanism */

#endif

    cvSet( src, cvRealScalar(1.), 0 );

#if 0 /* change 0 to 1 to suppress error handler invocation */

    cvSetErrMode( CV_ErrModeSilent );

#endif

    cvResizeDCT( src, dst ); // if some error occurs, the message

    // box will popup, or a message will be

    // written to log, or some user-defined

    // processing will be done

    if( cvGetErrStatus() < 0 )

        printf("Some error occured" );

    else

        printf("Everything is OK" );

    return 0;

}

if ( !flag )        

{            

       fclose(fp);          

       return 0;       

 }