BITMAPFILEHEADER

BITMAPFILEHEADER is a fixed-size structure used in the Windows BMP file format to describe the overall file as a bitmap image. It constitutes the first header in a BMP file and is exactly 14 bytes long. The information contained in this header specifies how the rest of the file is organized, including where the pixel data begins and the total file size.

The header consists of five fields. bfType is a 2-byte value that identifies the file as a BMP; it must be the characters 'BM' (0x4D42 in little-endian form). bfSize is a 4-byte value representing the total size of the BMP file in bytes. bfReserved1 and bfReserved2 are 2-byte fields reserved for future use and should be zero in standard BMP files. bfOffBits is a 4-byte value that specifies the offset from the start of the file to the pixel data (the beginning of the bitmap). This offset is where the color table (if present) and the actual bitmap pixel array begin.

BITMAPFILEHEADER is stored in little-endian byte order on Windows systems. It is commonly followed by a DIB header (such as BITMAPINFOHEADER) that contains metadata about the image, including width, height, color depth, and compression. The bfOffBits value indicates where those pixel bytes start; for a typical 24-bit BMP with no color table, this is 54 bytes (14-byte file header plus a 40-byte BITMAPINFOHEADER). Different BMP variants or color table usages can alter the offset.

In practice, applications reading or writing BMP files use BITMAPFILEHEADER to validate the file type, determine the overall file size, and locate the pixel data efficiently. The structure is usually defined with 1-byte packing to ensure no padding affects the 14-byte length.