C Preprocessor metaprogramming and other useful abuses

The C Preprocessor is very mighty.

This here basically implements a NOTEMPTY macro and on top of that a FORGE macro, which is able to variably iterate over a variable number of arguments.

This is based on the EVAL idiom, which is very CPU intensitive!

Note that this is based on ideas presented elsewhere. Please see comments for details.

You probably need a modern C18 GCC, as ellipsis variables are used heavily.

Usage

git submodule add https://github.com/hilbix/cppmeta.git
ln -s --relative cppmeta/cppmeta.h h/

In your source then:

#include "h/cppmeta.h"

Example

/* A,B,C,D,..   => A B , C D , ..       */
#define Otwo(A...)              EVAL(FORGE(Otwo,A))
#define Otwo_0(A,B,C...)
#define Otwo_1(A,B,C...)        A B
#define Otwo_2(A,B,C...)        C
#define Otwo_3(...)             ,
#define Otwo_4()

/* A,B,C,D,..   => B , D , ..   */
#define O2nd(A...)              EVAL(FORGE(O2nd,A))
#define O2nd_0(A,B,C...)
#define O2nd_1(A,B,C...)        B
#define O2nd_2(A,B,C...)        C
#define O2nd_3(...)             ,
#define O2nd_4()

/* Create our own checked wrappers from the prototypes
 * void Ofunctionname()
 * has the same calling signature as
 * int functioname()
 * but OOPSes when an error occurs.
 */
#define C(A,B...)       A(B)
#define O(A,B...)       static void O##A(Otwo(B)) { if (C(A,O2nd(B))) OOPS(#A "() failed"); }
#define I(A,B...)       static void O##A(Otwo(B)) { while (C(A,O2nd(B))) { if (errno!=EINTR) OOPS(#A "() failed"); } }
#define R(X,A,B...)     static X O##A(Otwo(B)) { X _; _=C(A,O2nd(B)); if (!_) OOPS(#A "() failed"); return _; }

You can use this to create some standard wrapper routines, which stay very closely to the real function definition prototype:

int sigemptyset (sigset_t * set);
O(  sigemptyset, sigset_t *,set);

int sigaction( int signum, const struct sigaction * act, struct sigaction * oldact);
O(  sigaction, int,signum, const struct sigaction *,act, struct sigaction *,oldact);

Or functions which must retry on EINTR:

int tcgetattr(int fd, struct termios * termios_p);
I(  tcgetattr,int,fd, struct termios *,termios_p);

int tcsetattr(int fd, int optional_actions, const struct termios * termios_p);
I(  tcsetattr,int,fd, int,optional_actions, const struct termios *,termios_p);

Or prevent the NULL on error for functions returning pointers:

  struct tm * gmtime_r(const time_t * timep, struct tm * result);
R(struct tm *,gmtime_r,const time_t *,timep, struct tm *,result);

  void * malloc(size_t size);
R(void *,malloc,size_t,size);

For this all you have to do is to stick to the type,name idiom. typedef is your friend.

Links

• https://github.com/pfultz2/Cloak/
• https://web.archive.org/web/20180815204201/http://saadahmad.ca/cc-preprocessor-metaprogramming-2/
• https://gustedt.wordpress.com/2010/06/08/detect-empty-macro-arguments/

FAQ

WTF why?

• Because I needed it.

License?

• My part of the code is free as in free beer, free speech and free baby.
• But I'm not sure about the code references used to assemble these sources.

Standard?

• No standard. Probably C18. Perhaps GCC only.

Contact?

• Open Issue on GH
• Eventually I listen

Contrib?

• Send PR on GH
• Eventually I listen
• Waive all Copyright, else I will not pull!

Names?

• Warning! The names might change to some better scheme
• However currently I have no good idea
• git is your friend, submodules can keep it compatible.