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

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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.
This commit is contained in:
Nick Mathewson 2019-10-28 09:05:02 -04:00
parent 45698e89ed
commit 4541a59019
2 changed files with 31 additions and 0 deletions
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1
src/lib/cc/include.am
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@ -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

30
src/lib/cc/tokpaste.h Normal file
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@ -0,0 +1,30 @@
/* 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) */