Problem with converting colour image to greyscale

Question

The following code is to accept an colour image as input and write a greyscale image but it didn't work as planned.
#include<stdio.h>
#include<stdint.h>
#include<math.h>
typedef uint8_t Byte;
typedef struct
{
Byte rgbtBlue;
Byte rgbtGreen;
Byte rgbtRed;
}rgbtriple;
int greyscale(int width,int height);
int main()
{
greyscale(1920,1280);
}
int greyscale(int width,int height)
{
rgbtriple sample[height][width];
FILE *fp=fopen("/content/sample_1920×1280.bmp","rb");
FILE *fp1=fopen("/content/newimage1.bmp","wb");
if(fp!=NULL && fp1!=NULL)
{
int i=0,j=0;
Byte average=0;;
for(i=0;i<height;i++)
{ for(j=0;j<width;j++)
{
fread(&sample[i][j], sizeof(sample[i][j]),1,fp);
average=(sample[i][j].rgbtRed+sample[i][j].rgbtGreen+sample[i][j].rgbtBlue)/3;
sample[i][j].rgbtRed=sample[i][j].rgbtGreen=sample[i][j].rgbtBlue=average;
fwrite(&sample[i][j], sizeof(sample[i][j]),1,fp1);
} }}
else{printf("Can'topenfile");}}

Answer 1

If the file is a standard bmp file then it has a header section that comes before the pixel data. See https://en.wikipedia.org/wiki/BMP_file_format.