C program for course work that reads a bitmap file and then writes copy of the bitmap file while altering the red pixels to reveal a hidden message.

whodunit.c

#include <stdio.h>

#include <stdlib.h>

#include <string.h>



#include "bmp.h"



int main (int argc, char *argv[])

{

    if (argc != 3)

    {

        fprintf(stderr, "Usage: ./whodunit infile outfile\n");

        return 1;

    }



    // remember filenames

    char *infile = argv[1];

    char *outfile = argv[2];



    // open input file

    FILE *inptr = fopen(infile, "r");

    if (inptr == NULL)

    {

        fprintf(stderr, "Could not open %s.\n", infile);

        return 2;

    }



    FILE *outptr = fopen(outfile, "w");

    if (outptr == NULL)

    {

        fclose(inptr);

        fprintf(stderr, "Could not create %s.\n", outfile);

        return 3;

    }



    // read infile's BITMAPFILEHEADER

    BITMAPFILEHEADER bf;

    fread(&bf, sizeof(BITMAPFILEHEADER), 1, inptr);



    // read infile's BITMAPINFOHEADER

    BITMAPINFOHEADER bi;

    fread(&bi, sizeof(BITMAPINFOHEADER), 1, inptr);



    // ensure infile is (likely) a 24-bit uncompressed BMP 4.0

    if (bf.bfType != 0x4d42 || bf.bfOffBits != 54 || bi.biSize != 40 ||

        bi.biBitCount != 24 || bi.biCompression != 0)

    {

        fclose(outptr);

        fclose(inptr);

        fprintf(stderr, "Unsupported file format.\n");

        return 4;

    }



    // write outfile's BITMAPFILEHEADER

    fwrite(&bf, sizeof(BITMAPFILEHEADER), 1, outptr);



    // write outfile's BITMAPINFOHEADER

    fwrite(&bi, sizeof(BITMAPINFOHEADER), 1, outptr);



    // determine padding for scanlines

    int padding = (4 - (bi.biWidth * sizeof(RGBTRIPLE)) % 4) % 4;



    // iterate over infile's scanlines

    for (int i = 0, biHeight = abs(bi.biHeight); i < biHeight; i++)

    {

        // iterate over pixels in scanline



        for (int j = 0; j < bi.biWidth; j++)

        {

            // temporary storage

            RGBTRIPLE triple;



            // read RGB triple from infile

            fread(&triple, sizeof(RGBTRIPLE), 1, inptr);

            //TODO

            if (triple.rgbtBlue == 0x00 && triple.rgbtGreen == 0x00 && triple.rgbtRed == 0xff)

            triple.rgbtBlue = 0xff; 

            triple.rgbtGreen = 0xff;

            // write RGB triple to outfile

            fwrite(&triple, sizeof(RGBTRIPLE), 1, outptr);

        }

        // skip over padding, if any

        fseek(inptr, padding, SEEK_CUR);



        // then add it back (to demonstrate how)

        for (int k = 0; k < padding; k++)

        {

            fputc(0x00, outptr);

        }

    }

    // printf("Number of pure red pixels: %i\n", count);

    // close infile

    fclose(inptr);



    // close outfile

    fclose(outptr);



    // success

    return 0;

}