C++ 为什么这个 .c 文件 #include 本身

该文件包括其自身，因此可以使用相同的源代码来为宏USIZE的特定值生成4组不同的函数.

这#include条指令实际上包含在#ifndef中，这将递归限制在单个级别:

#ifndef USIZE

// common definitions
...
//

#define VSENC vsenc
#define VSDEC vsdec

#define USIZE 8
#include "vsimple.c"
#undef USIZE

#define USIZE 16
#include "vsimple.c"
#undef USIZE

#define USIZE 32
#include "vsimple.c"
#undef USIZE

#define USIZE 64
#include "vsimple.c"
#undef USIZE

#else // defined(USIZE)

// macro expanded size specific functions using token pasting

...

#define uint_t TEMPLATE3(uint, USIZE, _t)

unsigned char *TEMPLATE2(VSENC, USIZE)(uint_t *__restrict in, size_t n, unsigned char *__restrict out) {
   ...
}

unsigned char *TEMPLATE2(VSDEC, USIZE)(unsigned char *__restrict ip, size_t n, uint_t *__restrict op) {
   ...
}

#endif

本模块中定义的函数包括

// vsencNN: compress array with n unsigned (NN bits in[n]) values to the buffer out. Return value = end of compressed output buffer out
unsigned char *vsenc8( unsigned char  *__restrict in, size_t n, unsigned char  *__restrict out);
unsigned char *vsenc16(unsigned short *__restrict in, size_t n, unsigned char  *__restrict out);
unsigned char *vsenc32(unsigned       *__restrict in, size_t n, unsigned char  *__restrict out);
unsigned char *vsenc64(uint64_t       *__restrict in, size_t n, unsigned char  *__restrict out);

// vsdecNN: decompress buffer into an array of n unsigned values. Return value = end of compressed input buffer in
unsigned char *vsdec8( unsigned char  *__restrict in, size_t n, unsigned char  *__restrict out);
unsigned char *vsdec16(unsigned char  *__restrict in, size_t n, unsigned short *__restrict out);
unsigned char *vsdec32(unsigned char  *__restrict in, size_t n, unsigned       *__restrict out);
unsigned char *vsdec64(unsigned char  *__restrict in, size_t n, uint64_t       *__restrict out);

它们都是从vsimple.c中的两个函数定义展开的:

unsigned char *TEMPLATE2(VSENC, USIZE)(uint_t *__restrict in, size_t n, unsigned char *__restrict out) {
   ...
}

unsigned char *TEMPLATE2(VSDEC, USIZE)(unsigned char *__restrict ip, size_t n, uint_t *__restrict op) {
   ...
}

TEMPLATE2和TEMPLATE3宏在conf.h中定义为

#define TEMPLATE2_(_x_, _y_) _x_##_y_
#define TEMPLATE2(_x_, _y_) TEMPLATE2_(_x_,_y_)

#define TEMPLATE3_(_x_,_y_,_z_) _x_##_y_##_z_
#define TEMPLATE3(_x_,_y_,_z_) TEMPLATE3_(_x_, _y_, _z_)

这些宏是通过令牌粘贴创建标识符的classic 预处理器构造.TEMPLATE2和TEMPLATE2_通常称为GLUE和XGLUE.

函数模板的开头是:

unsigned char *TEMPLATE2(VSENC, USIZE)(uint_t *__restrict in, size_t n, unsigned char *__restrict out) ...

它在第一个递归包含中被扩展为USIZE，定义为8:

unsigned char *vsenc8(uint8_t *__restrict in, size_t n, unsigned char *__restrict out) ...

第二个递归包含将USIZE定义为16，将模板扩展为:

unsigned char *vsenc16(uint16_t *__restrict in, size_t n, unsigned char *__restrict out) ...

另外两个包裹体定义了vsenc32和vsenc64.

预处理源代码的这种用法在单独的文件中更为常见:一个用于包含所有公共定义(特别是宏)的实例化部分，另一个用于代码和数据模板的单独文件，该文件多次包含在不同的宏定义中.

一个很好的例子是在QuickJS中从atom和opcode定义生成枚举、字符串和 struct array.