How do I properly read and write Binary files? (C++)

Question

In this program, I am reading, manipulating, and writing binary, bitmap files. I am trying to use C-style files for both reading and writing these files. I am also dynamically allocating the memory onto the heap using new() and delete in order to then write that block of memory to another file. I think I have a problem when I read and also write the binary files within my for loop. Somehow I overwrite by File and Info header after taking a look at my output file. I just need help understanding why this is happening. All my other code should be fine.

#include <cstdint>
#include <cstdio>

#pragma pack(push, 2)
struct BitmapFileHeader {

    uint16_t type;
    uint32_t size;
    uint16_t reserved_1;
    uint16_t reserved_2;
    uint32_t offset;

};
struct BitmapInfoHeader {

    uint32_t size;
    uint32_t width;
    uint32_t height;
    uint16_t planes;
    uint16_t bitcount;
    uint32_t compression;
    uint32_t imagesize;
    uint32_t x_pixels_per_meter;
    uint32_t y_pixels_per_meter;
    uint32_t color_used;
    uint32_t color_important;
};
#pragma pack(pop)

struct Pixel {
    uint8_t blue;
    uint8_t green;
    uint8_t red;
};

int main(int argc, char* argv[])
{
    if(argc != 3) {
        printf("Usage : %s input_file output_file\n", argv[0]);
        return 1;
    }

    FILE *fin;
    FILE *fout;
    BitmapFileHeader bfh;
    BitmapInfoHeader bih;

    fin = fopen(argv[1], "rb");

    if (nullptr == fin) {
        perror(argv[1]);
        return -1;
    }

    if (sizeof(BitmapFileHeader) != fread(
        &bfh,
        1,
        sizeof(bfh),
        fin
    )) {
        printf("Unable to read bitmap file header. \n");
        return -2;
    }

    if (sizeof(BitmapInfoHeader) != fread(
        &bih,
        1,
        sizeof(bih),
        fin
    )) {
        printf("Unable to read bitmap info header. \n");
        return -3;
    }

    printf("Size of File Header = %lu\n", sizeof(BitmapFileHeader));

    int8_t first = (bfh.type >> 8) & 0xff;
    int8_t second = bfh.type & 0xff;

    if ( (first != 'M') && (second != 'B') ){
        printf("Input file is not a Bitmap file. \n");
        return -4;
    }

    printf("File type = %c%c\n", first, second);
    printf("File size = %u\n", bfh.size);
    printf("File offset = %u\n", bfh.offset);
    printf("File width = %u\n", bih.width);
    printf("Info size = %u\n", bih.size);

    uint32_t padding_bytes = 0;
    uint32_t row_bytes_final = bih.width * sizeof(Pixel);
    uint32_t row_bytes_initial = row_bytes_final;

    do{
        uint32_t rem = row_bytes_final % 4;

        if (rem != 0) {
            row_bytes_final += 1;
        }

        padding_bytes = row_bytes_final - row_bytes_initial;

    } while( (row_bytes_final % 4) != 0);


    fseek(fin, bfh.offset, SEEK_SET);

    Pixel *p = new Pixel[bih.height * bih.width];

    for (uint32_t i = 0; i < (bih.height); i++) {

        for (uint32_t j = 0; j < bih.width; j++) {
            fread(&p[i], 1, sizeof(p), fin);
            i++;
            //Something I don't understand is wrong here. 
        }

        fseek(fin, padding_bytes, SEEK_CUR);

    }

    fclose(fin);


    fout = fopen(argv[2], "wb");

    if(nullptr == fout) {
        perror(argv[2]);
        return -5;
    }

    if( sizeof(BitmapFileHeader) != fwrite(
    &bfh, 
    1, 
    sizeof(bfh), 
    fout
    )) {
        printf("Unable to write bitmap file header.\n");
        return -6;
    }

    if( sizeof(BitmapInfoHeader) != fwrite(
        &bih, 
        1, 
        sizeof(bih), 
        fout
        )) {
            printf("Unable to write bitmap info header.\n");
            return -7;
        }

    fseek(fout, bfh.offset, SEEK_SET);

    for (uint32_t i = 0; i < (bih.height) ; i++) {

        for (uint32_t j = 0; i < bih.width; j++) {
            fwrite(&p[i], 1, sizeof(p), fout);
            i++;
            //same problem
        }

        fseek(fout, padding_bytes, SEEK_CUR);

    }

    fclose(fout);
    delete p;


    //fseek(fin, bfh.offset, SEEK_SET);
    //Pixel p;
    //fread(&p, 1, sizeof(p), fin);
    //printf("R = %u, G = %u, B = %u\n", p.red, p.green, p.blue);
    return 0;

}

How am I overwriting my headers for my output file if I am moving my file pointer to the offset?

Answer 1

I think fread(&p[i], 1, sizeof(p), fin); should be fread(&p[i], 1, sizeof(Pixel), fin);

Sizeof a pointer is (based on architecture) generally either 4 or 8 bytes. your pixel class is 3 bytes (with #pragma pack(push, 1)).

May be use fread(&p[i], sizeof(Pixel), bih.width, fin); Be aware though, ordering and padding can vary on different architecture.

Answer 2

I see the following problems in your code:

Problem 1

You are using the wrong argument to sizeof in

fread(&p[i], 1, sizeof(p), fin);

and

fwrite(&p[i], 1, sizeof(p), fout);

They need to be sizeof(p[i]) , sizeof(p[0]) , or sizeof(Pixel) . I prefer sizeof(p[0]) since it works regardless of the type of p[0] .

Problem 2

You have a typo in

for (uint32_t j = 0; i < bih.width; j++) {

It needs to be

for (uint32_t j = 0; j < bih.width; j++) {
//                  ^^^ Fix

Problem 3

You are not using the right index to read into p and write from p .

for (uint32_t i = 0; i < bih.height; i++) {
   for (uint32_t j = 0; j < bih.width; j++) {

      // Using p[i] is not correct. You need to use the right index.
      // sizeof(p[0]) is just as good as sizeof(p[index]) here.

      uint32_t index = i*bih.width + j;
      fread(&p[index], 1, sizeof(p[0]), fin);
   }
   fseek(fin, padding_bytes, SEEK_CUR);
}

The loop for writing needs to be fixed similarly.

for (uint32_t i = 0; i < bih.height; i++) {
   for (uint32_t j = 0; j < bih.width; j++) {
      uint32_t index = i*bih.width + j;
      fwrite(&p[index], 1, sizeof(p[0]), fout);
   }
   fseek(fout, padding_bytes, SEEK_CUR);
}