commit 4541a590192a97d56c7a303492a2bc1bbeacb441 Author: Nick Mathewson nickm@torproject.org Date: Mon Oct 28 09:05:02 2019 -0400

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. --- src/lib/cc/include.am | 1 + src/lib/cc/tokpaste.h | 30 ++++++++++++++++++++++++++++++ 2 files changed, 31 insertions(+)

diff --git a/src/lib/cc/include.am b/src/lib/cc/include.am index 1aa722dd8..d2a415e95 100644 --- a/src/lib/cc/include.am +++ b/src/lib/cc/include.am @@ -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 diff --git a/src/lib/cc/tokpaste.h b/src/lib/cc/tokpaste.h new file mode 100644 index 000000000..e7ddbffc6 --- /dev/null +++ b/src/lib/cc/tokpaste.h @@ -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) */