Byte alignment in C ++

# Pragma pack (n) C compiler alignment according to n Bytes;
# Use The Pragma pack () method to cancel custom settings;
# Pragma pack (push, 1)Sets the original alignment mode to the pressure stack, and sets the new method to a byte alignment.
# Restore The Pragma pack (POP) to its status

In this case, when reading BMP images in C ++ (windows is not required. h) when you write your own image headers without adding # pragme pack (push, 1) & # pragme pack (POP), you cannot read the image correctly,

 

 

# Pragma pack (push, 1) typedef struct tagbitmapfileheader {unsigned short bftype; // The bitmap file type must be 0x0000d, that is, the string "BM", that is, all "*. the first two bytes of the BMP file are "BM ". Unsigned long bfsize; // the size of the bitmap file, including the 14 bytes. Unsigned short bfreserved1; // reserved words for Windows. Unsigned short bfreserved2; // reserved words for Windows. Unsigned long bfoffbits; // The number of offset bytes from the file header to the actual bitmap data, and the length of the first three parts in Figure 1-7 .} Bitmapfileheader; typedef struct tabbitmapinfoheader {unsigned long bisize; // the length of this structure, which is 40 bytes. Long biwidth; // The width of the bitmap, in pixels. Long biheight; // The height of the bitmap, in pixels. Unsigned short biplanes; // The level of the target device, which must be 1. Unsigned short bibitcount; // The number of BITs occupied by each pixel. The value must be 1 (black and white image), 4 (16 color image), and 8 (256 color) and 24 (true color chart). The new BMP format supports 32-bit colors. Unsigned long bicompression; // bitmap compression type. Valid values include bi_rgb (uncompressed), bi_rle8, bi_rle4, and bi_bitfileds (Windows-defined constants ). Here we only discuss uncompress, that is, bicompression = bi_rgb. Unsigned long bisizeimage; // The number of bytes occupied by the actual bitmap data. The size of this value is described in Part 1 of the bitmap data. Long bixpelspermeter; // specify the horizontal resolution of the target device, in pixels/meters. Long biypelspermeter; // specifies the vertical resolution of the target device, in pixels/meters. Unsigned long biclrused; // number of colors actually used by the bitmap. If the value is zero, the number of colors used is the bibitcount power of 2. Unsigned long biclrimportant; // number of important colors in the bitmap display process. If the value is zero, all colors are considered important .} Bitmapinfoheader; typedef struct tagrgbquad {unsigned char rgbblue; // The Blue weight of the color; unsigned char rgbgreen; // The green weight of the color; unsigned char rgbred; // The Red weight of the color; unsigned char rgbreserved; // reserved byte .} Rgbquad; # pragma pack (POP)