saving simple 32-bit RGBA top-down bmp file

write_bmp.c

#include <stdarg.h>
#include <stdio.h>

static void write(FILE* f, const char* fmt, ...)
{
  va_list ap;
  va_start(ap, fmt);
  char ch;
  while ((ch = *fmt++))
  {
    if (ch == '0')
    {
      fputc((char)0, f);
    }
    else if (ch == 'o')
    {
      fputc((char)0, f);
      fputc((char)0, f);
    }
    else if (ch == 'O')
    {
      fputc((char)0, f);
      fputc((char)0, f);
      fputc((char)0, f);
      fputc((char)0, f);
    }
    else if (ch == '1')
    {
      int x = va_arg(ap, int);
      fputc((char)x, f);
    }
    else if (ch == '2')
    {
      int x = va_arg(ap, int);
      fputc((char)x, f);
      fputc((char)(x >> 8), f);
    }
    else if (ch == '4')
    {
      int x = va_arg(ap, int);
      fputc((char)x, f);
      fputc((char)(x >> 8), f);
      fputc((char)(x >> 16), f);
      fputc((char)(x >> 24), f);
    }
  }
  va_end(ap);
}

#define RGBA(r,g,b,a) ( ((a) << 24) | ((b) << 16) | ((g) << 8) | ((r) << 0) )

static void write_bmp32(const char* fname, int width, int height, const void* data)
{
  FILE* f = fopen(fname, "wb");
  if (f)
  {
    int fheader = 14; // = sizeof(BITMAPFILEHEADER)
    int iheader = 108; // = sizeof(BITMAPV4HEADER)
    int dsize = width * height * 4;
    int fsize = fheader + iheader + dsize;
    int doffset = fheader + iheader;
    int planes = 1;
    int bitcount = 32;
    int compression = 3; // = BI_BITFIELDS
    int red = RGBA(255,0,0,0);
    int green = RGBA(0,255,0,0);
    int blue = RGBA(0,0,255,0);
    int alpha = RGBA(0,0,0,255);
    int colorspace = 0x57696E20; // = LCS_WINDOWS_COLOR_SPACE
    write(f, "11 4oo4 4442244OOOO 4444 4 OOOOOOOOOOOO",
      'B', 'M',
      fsize, doffset,
      iheader, width, -height, planes, bitcount, compression, dsize,
      red, green, blue, alpha,
      colorspace);
    fwrite(data, width * height * 4, 1, f);
    fclose(f);
  }
}

int main()
{
  unsigned data[4*2] = {
    RGBA(255,0,0,255), RGBA(0,255,0,255), RGBA(0,0,255,255), RGBA(255,255,255,255),
    RGBA(255,0,0,128), RGBA(0,255,0,128), RGBA(0,0,255,128), RGBA(255,255,255,128),
  };

  write_bmp32("test.bmp", 4, 2, data);
}