Why does this .c file #include itself?
vsimple.c
#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
CodePudding user response：
The file includes itself so the same source code can be used to generate 4 different sets of functions for specific values of the macro USIZE.
The #include directives are actually encapsulated in an #ifndef:
#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
The functions defined in this module are
// 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);
They are all expanded from the two function definitions in vsimple.c:
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) {
...
}
The TEMPLATE2 and TEMPLATE3 macros are defined in conf.h as
#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_)
So unsigned char *TEMPLATE2(VSENC, USIZE)(uint_t *__restrict in, size_t n, unsigned char *__restrict out) is expanded as unsigned char *vsenc8(uint8_t *__restrict in, size_t n, unsigned char *__restrict out) in the first inclusion, vsenc16(uint16_t *__restrict in, ... in the second, etc.
This usage of preprocessed source code is more common with separate files: one for the instantiating part that has all the common definitions, especially the macros, and a separate file for the code and data templates, which is included multiple times with different macro definitions.
A good example is the generation of enums, string and structures arrays from atom and opcode definitions in QuickJS.