mdw@git.distorted.org.uk Git - finally/blob - finally.h

   1 /* -*-c-*-
   2  *
   3  * Arrange to have code executed when a function ends
   4  *
   5  * (c) 2023 Straylight/Edgeware
   6  */
   7
   8 /*----- Licensing notice --------------------------------------------------*
   9  *
  10  * This file is part of the `Finally' package.
  11  *
  12  * Finally is free software: you can redistribute it and/or modify it
  13  * under the terms of the GNU Library General Public License as published
  14  * by the Free Software Foundation; either version 2 of the License, or
  15  * (at your option) any later version.
  16  *
  17  * Finally is distributed in the hope that it will be useful, but WITHOUT
  18  * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
  19  * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU Library General Public
  20  * License for more details.
  21  *
  22  * You should have received a copy of the GNU Library General Public
  23  * License along with Finally.  If not, write to the Free Software
  24  * Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307,
  25  * USA.
  26  */
  27
  28 #ifndef FINALLY_H
  29 #define FINALLY_H
  30
  31 /*----- Compatibility machinery -------------------------------------------*/
  32
  33 /* Some preliminary hacks for detecting compiler versions. */
  34 #ifdef __GNUC__
  35 #  define FINALLY__GCC_P(maj, min)                                      \
  36         (__GNUC__ > (maj) || (__GNUC__ == (maj) && __GNUC_MINOR__ >= (min)))
  37 #else
  38 #  define FINALLY__GCC_P(maj, min) 0
  39 #endif
  40
  41 #ifdef __clang__
  42 #  define FINALLY__CLANG_P(maj, min)                                    \
  43         (__clang_major__ > (maj) || (__clang_major__ == (maj) &&        \
  44                                      __clang_minor__ >= (min)))
  45 #else
  46 #  define FINALLY__CLANG_P(maj, min) 0
  47 #endif
  48
  49 /* If configuration machinery hasn't determined a flavour, then we'll take a
  50  * rough guess based on compiler versions.
  51  */
  52 #if !defined(FINALLY_CONFIG_FLAVOUR) && \
  53         !defined(__clang__) && FINALLY__GCC_P(3, 3)
  54 #  define FINALLY_CONFIG_FLAVOUR GCC_NESTED_FUNCTIONS
  55 #endif
  56
  57 #if !defined(FINALLY_CONFIG_FLAVOUR)
  58 #  define FINALLY_CONFIG_FLAVOUR NIL
  59 #endif
  60
  61 /*----- Macros provided ---------------------------------------------------*/
  62
  63 /* Before we start, a note about compatibility.  We're using pretty esoteric
  64  * compiler features here, and not all compilers support them.  I'm most
  65  * interested in GCC, which will work fine.  This isn't going to work for
  66  * other compilers, but lots of them try to impersonate GCC, and it's just
  67  * not worth the effort to try to see through their lies.
  68  *
  69  * So the rules are: if you include this header file, you've either already
  70  * made an effort to check that it's likely to work (e.g., by using the
  71  * provided Autoconf macro), or you're willing to put up with whatever
  72  * wreckage you end up with if the compiler doesn't actually like what we're
  73  * saying here.
  74  */
  75
  76 /* And a utility for pasting tokens after macro expansion. */
  77 #define FINALLY__GLUE(x, y) FINALLY__DOGLUE(x, y)
  78 #define FINALLY__DOGLUE(x, y) x##y
  79
  80 /* Now, we need a way to make a temporary name which isn't likely to conflict
  81  * with anything else.
  82  */
  83 #define FINALLY__TMP(name) FINALLY__GLUE(_finally__##name##__, __LINE__)
  84
  85 /* And some other tricks which we may or may not need. */
  86 #if defined(__cplusplus) && __cplusplus >= 201703
  87 #  define FINALLY__IGNORABLE fucksocks
  88 #elif FINALLY__GCC_P(2, 5) || FINALLY__CLANG_P(3, 3)
  89 #  define FINALLY__IGNORABLE __attribute__((__unused__))
  90 #endif
  91 #ifndef FINALLY__IGNORABLE
  92 #  define FINALLY__IGNORABLE
  93 #endif
  94
  95 /* Flavour selection machinery. */
  96 #define FINALLY__FLAVOUR_NIL -1
  97 #define FINALLY__FLAVOUR_GCC_NESTED_FUNCTIONS 1
  98
  99 #define FINALLY__SELECTED_FLAVOUR                                       \
 100         FINALLY__GLUE(FINALLY__FLAVOUR_, FINALLY_CONFIG_FLAVOUR)
 101
 102 /* @FINALLY(code)@
 103  * @FINALLY_TAGGED(tag, code)@
 104  *
 105  * This macro may be placed anywhere within a function where a declaration is
 106  * acceptable -- so at the head of a block in C89, or anywhere between
 107  * statements in C99 or later.  It causes @code@ to be executed when control
 108  * leaves the enclosing scope.  If it's placed at top-level within a
 109  * function, for example, then the @code@ is executed when the function
 110  * returns.
 111  *
 112  * There may be multiple @FINALLY@ invocations within a scope; they are
 113  * executed in reverse order when control leaves the scope.
 114  *
 115  * Due to technical limitations, it's forbidden to have two @FINALLY@
 116  * invocations on the same source line.  This would seem to stymie writing a
 117  * macro which expands to two invocations of @FINALLY@, which would be useful
 118  * if those invocations were to end up in different scopes.  Such a macro can
 119  * still be written using @FINALLY_TAGGED@ instead, which takes an additional
 120  * @tag@ argument, which may be any identifier: the rule then becomes that a
 121  * source line may not contain two invocations of @FINALLY_TAGGED@ bearing
 122  * the same @tag@.  (It would be possible to overcome this limitation using
 123  * the @__COUNTER__@ GCC extension, but I'd prefer not to limit the potential
 124  * portability of this feature even further by insisting that something
 125  * analogous exist in every supported compiler, and it would do users a
 126  * disservice to create portability problems by having different rules on
 127  * different compilers.)
 128  */
 129 #if defined(__cplusplus) && __cplusplus >= 201103
 130   /* Oooh, we're being compiled by a C++ compiler.  There's no need to deploy
 131    * nonportable tricks, because we have an insane language that can already
 132    * do what we need.
 133    */
 134
 135    namespace finally {
 136      template<typename F> class Finally {
 137        F fn;
 138      public:
 139        Finally(F f) : fn{f} { ; }
 140        ~Finally() { fn(); }
 141      };
 142      template<typename F> Finally<F> finally(F &&fn)
 143        { return Finally<F>(fn); }
 144    }
 145
 146 #  define FINALLY_TAGGED(tag, body)                                     \
 147         FINALLY__IGNORABLE auto FINALLY__TMP(tag##__var) =              \
 148           finally::finally([&]{ body })
 149
 150 #elif FINALLY__GLUE(FINALLY__FLAVOUR_, FINALLY_CONFIG_FLAVOUR) == \
 151         FINALLY__FLAVOUR_GCC_NESTED_FUNCTIONS
 152    /* We're being compiled by GCC, or by something which wants to be mistaken
 153     * for GCC at any rate.  And GCC has nested functions.  So, for each
 154     * block, we'll define a dummy variable that we don't care about, and
 155     * attach a nested function as its cleanup handler which will execute our
 156     * cleanup code.
 157     */
 158
 159 #  define FINALLY_TAGGED(tag, body)                                     \
 160         __extension__ __inline__                                        \
 161           void FINALLY__TMP(tag##__fn)                                  \
 162             (const int __attribute__((__unused__)) *_finally__hunoz)    \
 163             { body }                                                    \
 164         __attribute__((__unused__, __cleanup__(FINALLY__TMP(tag##__fn)))) \
 165           int FINALLY__TMP(tag##__var)
 166
 167 #elif FINALLY__GLUE(FINALLY__FLAVOUR_, FINALLY_CONFIG_FLAVOUR) == \
 168         FINALLY__GLUE(FINALLY__FLAVOUR_, NIL)
 169    /* We don't have a flavour to support this environment. */
 170
 171 #  error "Compiler not supported.  This isn't going to work."
 172 #else
 173    /* This case analysis should be exhaustive. */
 174
 175 #  error "Internal error: `FINALLY_CONFIG_FLAVOUR' bungled."
 176 #endif
 177
 178 /* We now have `FINALLY_TAGGED'; defining `FINALLY' is easy.  The TAG here is
 179  * guaranteed not conflict with any call on `FINALLY_TAGGED', since TAGs are
 180  * required to be identifiers.
 181  */
 182 #define FINALLY(code) FINALLY_TAGGED(0, code)
 183
 184 /*----- That's all, folks -------------------------------------------------*/
 185
 186 #endif