Define image datatype and image_t in C language

// read the image
BMP readBMP(string filename) {
    BMP image;
    int i;
    string fileName = filename;
    FILE *f = fopen(fileName.c_str(), "rb");
    fread(image.header, sizeof(unsigned char), 54, f); // read the 54-byte header

    // extract image height and width from header
    image.width = *(int *) &image.header[18];
    image.height = *(int *) &image.header[22];
    cout<<"inage width: "<< image.width <<", image height: "<< image.height <<endl;

    image.size = 3 * image.width * image.height;
    image.pixels = new unsigned char[image.size]; // allocate 3 bytes per pixel
    fread(image.pixels, sizeof(unsigned char), image.size, f); // read the rest of the data at once
    fclose(f);

    // bmp stors data in BGR format, so changing the BGR to RGB.
    for (i = 0; i < image.size; i += 3) {
        unsigned char tmp = image.pixels[i];
        image.pixels[i] = image.pixels[i + 2];
        image.pixels[i + 2] = tmp;
    }
    return image;
}

// write image
void writeBMP(string filename, BMP image) {
    string fileName = filename;
    FILE *out = fopen(fileName.c_str(), "wb");
    fwrite(image.header, sizeof(unsigned char), 54, out);
    int i;
    unsigned char tmp;
    for (i = 0; i < image.size; i += 3) {
        tmp = image.pixels[i];
        image.pixels[i] = image.pixels[i + 2];
        image.pixels[i + 2] = tmp;
    }
    fwrite(image.pixels, sizeof(unsigned char), image.size, out); // read the rest of the data at once
    fclose(out);
}