Commit 4541a590 authored by Nick Mathewson's avatar Nick Mathewson 🦀
Browse files

lib/cc: Define the standard C macro pasting trick in one place. 

This file is a workaround for the issue that if you say `a ## b` to
create a token that is the name of a macro, the C preprocessor won't
expand that macro.  So you can't say this:

#define FOO__SQUARE(x) ((x)*(x))
#define FOO__CUBE(x)   ((x)*(x)*(x))
#define FOO(func, x)   FOO__##func(x)

Instead, the standard C trick is to add a layer of indirection:

#define PASTE(a,b)     PASTE__(a,b)
#define PASTE__(a,b)   a ## b

#define FOO__SQUARE(x) ((x)*(x))
#define FOO__CUBE(x)   ((x)*(x)*(x))
#define FOO(func, x)   PASTE(FOO__, func)(x)

We should use this kind of trick sparingly, since it gets confusing.
parent 45698e89

src/lib/cc/include.am

+1 −0
Original line number Diff line number Diff line
@@ -3,4 +3,5 @@ 
noinst_HEADERS += \

	src/lib/cc/compat_compiler.h \

	src/lib/cc/ctassert.h \

	src/lib/cc/tokpaste.h \

	src/lib/cc/torint.h

src/lib/cc/tokpaste.h

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+30 −0
Original line number Diff line number Diff line 
/* Copyright (c) 2001 Matej Pfajfar.

 * Copyright (c) 2001-2004, Roger Dingledine.

 * Copyright (c) 2004-2006, Roger Dingledine, Nick Mathewson.

 * Copyright (c) 2007-2019, The Tor Project, Inc. */

/* See LICENSE for licensing information */



/**

 * @file tokpaste.h

 * @brief Token-pasting macros.

 **/



#ifndef TOR_LIB_CC_TOKPASTE_H

#define TOR_LIB_CC_TOKPASTE_H



/**

 * Concatenate `a` and `b` in a way that allows their result itself to be

 * expanded by the preprocessor.

 *

 * Ordinarily you could just say `a ## b` in a macro definition.  But doing so

 * results in a symbol which the preprocessor will not then expand.  If you

 * wanted to use `a ## b` to create the name of a macro and have the

 * preprocessor expand _that_ macro, you need to have another level of

 * indirection, as this macro provides.

 **/

#define PASTE(a,b) PASTE__(a,b)



/** Helper for PASTE(). */

#define PASTE__(a,b) a ## b



#endif /* !defined(TOR_LIB_CC_TOKPASTE_H) */