bmp file read and write

#include <stdlib.h>
#include <stdio.h>
#include <math.h>



typedef struct _lbheader{
    unsigned short identifier;      // 0x0000
    unsigned int filesize;          // 0x0002
    unsigned int reserved;          // 0x0006
    unsigned int bitmap_dataoffset; // 0x000A
    unsigned int bitmap_headersize; // 0x000E
    unsigned int width;             // 0x0012
    unsigned int height;            // 0x0016
    unsigned short planes;          // 0x001A
    unsigned short bits_perpixel;   // 0x001C
    unsigned int compression;       // 0x001E
    unsigned int bitmap_datasize;   // 0x0022
    unsigned int hresolution;       // 0x0026
    unsigned int vresolution;       // 0x002A
    unsigned int usedcolors;        // 0x002E
    unsigned int importantcolors;   // 0x0032
    unsigned int palette;           // 0x0036
} __attribute__((packed,aligned(1))) lbheader;

void updateData(int pt_x, int pt_y, char* pt);

int n_weight;
int n_height;

int main(){
	
	FILE* fp = fopen("512_color_lena.bmp", "rb");
	FILE* fp_output = fopen("output.bmp", "wb");

	lbheader bmp_head;
	fread(&bmp_head, sizeof(lbheader), 1, fp);
	n_weight = bmp_head.width;
	n_height = bmp_head.height;
	printf("%d * %d \n", n_weight, n_height);

	char* pData = new char[n_weight*3*n_height];
	fread(pData, sizeof(unsigned char), n_weight*3*n_height, fp);

	
	for(int x = 0; x < bmp_head.width; x++){
		for(int y = 0; y < bmp_head.height; y++){
			updateData(x, y, pData);

		}
	}
	fwrite(&bmp_head, sizeof(lbheader), 1, fp_output);
	fwrite(pData, sizeof(unsigned char), bmp_head.width*3*bmp_head.height, fp_output);
	delete [] pData;
	fclose(fp);
	fclose(fp_output);
	

	return 0;
}

void updateData(int pt_x, int pt_y, char* pt){
	int x = n_weight/2 - pt_x;
	int y = pt_y - n_height/2;
	

	double r = sqrt(pow(x, 2)+pow(y, 2));

	int n_offset = ((n_height - pt_x)*n_weight + pt_y)*3;
	if (r > 256){
		*(pt+n_offset) = 255;
		*(pt+n_offset+1) = 255;
		*(pt+n_offset+2) = 255;
	}



}