What does the include in the middle of code in C?

Question

Please, could you tell me what does the code below do?

...code...
#include file.h
...code...

I was used to put includes a the beggining of

Answer 1

As everyone tells you #include can be used every where (provided it is on its own logicial line). And there are cases where you want to #include several times the same file. Read first the X macro wikipage, and the C preprocessor wikipage.

And I have a concrete example in my MELT monitor (related to MELT ...).

I have a predef-monimelt.h (generated) file containing lines like
MOM_PREDEFINED_NAMED( name, id,hash) e.g.

MOM_PREDEFINED_NAMED(GET,_9dsak0qcy0v_1c5z9th7x3i,1573018885)
MOM_PREDEFINED_NAMED(HEAD,_47fatww79x6_vh8ap22c0ch,3922245622) 
MOM_PREDEFINED_NAMED(web_handler,_7sav6zery1v_24sa6jwwu6c,2339220870)
#undef MOM_PREDEFINED_NAMED

My monimelt.h file (a real header file) define external pointers and an enum, so has notably:

// declare the predefined
#define MOM_PREDEFINED_NAMED(Name,Id,H) extern momitem_t* mom_named__##Name;
#include "predef-monimelt.h"

/// declare the hash of the predefined as an enum
#define MOM_PREDEFINED_NAMED(Name,Id,H) mom_hashname__##Name = H,
enum {
#include "predef-monimelt.h"
};

My main.c file contains notably a routine :

  // if this routine is compiled, we are sure that all predefined hashes
  // are unique
  const momitem_t *
  mom_predefined_item_of_hashcode (momhash_t h) {
    switch (h) {
  #define MOM_PREDEFINED_NAMED(Nam,Id,Hash) case Hash: return mom_named__##Nam;
  #include "predef-monimelt.h"
    default:
    return NULL;
     }
  }

but my items.c includes the predef-monimelt.h file twice (to create the predefined items at initialization, and to define their variables):

 void mom_create_predefined_items (void) {
   int nbnamed = 0;
 #define MOM_PREDEFINED_NAMED(Nam,Id,H) do { \
   mom_named__##Nam = mom_make_item_of_identcstr(#Id); \
   mom_named__##Nam->i_space = momspa_predefined; \
   mom_register_item_named_cstr (mom_named__##Nam, #Nam); \
   nbnamed ++; \
 } while(0);
 #include "predef-monimelt.h"
 } // end of mom_create_predefined_items

 // declare the predefined
 #define MOM_PREDEFINED_NAMED(Nam,Id,H) momitem_t* mom_named__##Nam;
 #include "predef-monimelt.h"

FWIW, the MELT monitor is GPLv3+ licensed software