Go4Expert - fwrite slowdown problem

Hi,

I have a probelm with fwrite function. I use it to write large chunk of binary data to 24-bit bitmap(.bmp) files of resolution 2400x1500.
In the program below, I read an image 1700.bmp and write the same to 1.bmp. I perform this infinite number of times. At the beginning, the writting is fast. But after about 25 images, the writting becomes too slow. When I remove the fwrite statement, the program executes very fast. So fwrite is the culprit. But I have to make the program run fast and at the same speed. Why is this problem caused and how to rectify it? I have copied the program below:

Code:

#include <stdio.h>

#include<conio.h>

#include<stdint.h>

struct MagicNumber

{

    unsigned char type[2]; 

}BmpMN;



struct INFOHEADER

{

    unsigned int sizeByte; 

    unsigned short reserved1; 

    unsigned short reserved2; 

    unsigned int offsetbits; 

    unsigned int sizeHeader; 

    int width;

    int height;

    unsigned short planes; 

    unsigned short bitcount; 

    unsigned int compression; 

    unsigned int sizeImage; 

    int xpelspermeter;

    int ypelspermeter;

    unsigned int colorsused;

    unsigned int colorsimportant;

} BitmapInfoHeader; 



int main()

{

    FILE *ip,*op;

    int i=1;    

    uint8_t buff[1500][2400*3];

    while(1)

    {

        printf("Reading %d\n",i);

        ip=fopen("op/1700.bmp","rb");

        op=fopen("op/1.bmp","wb");

        fread(&BmpMN,sizeof(BmpMN),1,ip);      

           fread(&BitmapInfoHeader,sizeof(BitmapInfoHeader),1,ip);

        fread(&buff,sizeof(buff),1,ip);

        fwrite(&BmpMN,sizeof(BmpMN),1,op);      

           fwrite(&BitmapInfoHeader,sizeof(BitmapInfoHeader),1,op);

        //This statement is executed slower from 26th image onwards

        fwrite(&buff,sizeof(buff),1,op);        

        fclose(ip);        

        fclose(op);

        i++;

        fflush(&buff);

    }

    return 0;

}

Thank you