X-Git-Url: http://erislabs.net/gitweb/?a=blobdiff_plain;f=lib%2Fverify.h;h=4ad780c8fa00839fe66d5202edf8afc9165af3a4;hb=6ae551d8480ab63f539bbd23d199f5bf4a9a1264;hp=c8066e91d96cdc4a5f0d883411148fcae11e411f;hpb=e086ed0fe9b4d5907fbed51c06cda1ac116b2d66;p=gnulib.git
diff --git a/lib/verify.h b/lib/verify.h
index c8066e91d..4ad780c8f 100644
--- a/lib/verify.h
+++ b/lib/verify.h
@@ -1,11 +1,11 @@
/* Compile-time assert-like macros.
- Copyright (C) 2005 Free Software Foundation, Inc.
+ Copyright (C) 2005-2006, 2009-2010 Free Software Foundation, Inc.
- This program is free software; you can redistribute it and/or modify
+ This program is free software: you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
- the Free Software Foundation; either version 2, or (at your option)
- any later version.
+ the Free Software Foundation; either version 3 of the License, or
+ (at your option) any later version.
This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
@@ -13,34 +13,151 @@
GNU General Public License for more details.
You should have received a copy of the GNU General Public License
- along with this program; if not, write to the Free Software Foundation,
- Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA. */
+ along with this program. If not, see . */
-/* Written by Paul Eggert and Jim Meyering. */
+/* Written by Paul Eggert, Bruno Haible, and Jim Meyering. */
#ifndef VERIFY_H
# define VERIFY_H 1
-/* Each of these macros verifies that its argument R is a nonzero
- constant expression. To be portable, R's type must be integer (or
- boolean). Unlike assert, there is no run-time overhead. */
+/* Each of these macros verifies that its argument R is nonzero. To
+ be portable, R should be an integer constant expression. Unlike
+ assert (R), there is no run-time overhead.
-/* A type that is valid if and only if R is a nonzero constant expression.
- The symbols verify_type__ and verify_error_if_negative_size__ are
- private to this header file. */
+ There are two macros, since no single macro can be used in all
+ contexts in C. verify_true (R) is for scalar contexts, including
+ integer constant expression contexts. verify (R) is for declaration
+ contexts, e.g., the top level.
-# define verify_type__(R) \
- struct { int verify_error_if_negative_size__ : (R) ? 1 : -1; }
+ Symbols ending in "__" are private to this header.
-/* Verify requirement R at compile-time, as a declaration. */
+ The code below uses several ideas.
-# define verify(R) \
- extern int (* verify_function__ (void)) [sizeof (verify_type__ (R))]
+ * The first step is ((R) ? 1 : -1). Given an expression R, of
+ integral or boolean or floating-point type, this yields an
+ expression of integral type, whose value is later verified to be
+ constant and nonnegative.
+
+ * Next this expression W is wrapped in a type
+ struct verify_type__ { unsigned int verify_error_if_negative_size__: W; }.
+ If W is negative, this yields a compile-time error. No compiler can
+ deal with a bit-field of negative size.
+
+ One might think that an array size check would have the same
+ effect, that is, that the type struct { unsigned int dummy[W]; }
+ would work as well. However, inside a function, some compilers
+ (such as C++ compilers and GNU C) allow local parameters and
+ variables inside array size expressions. With these compilers,
+ an array size check would not properly diagnose this misuse of
+ the verify macro:
+
+ void function (int n) { verify (n < 0); }
+
+ * For the verify macro, the struct verify_type__ will need to
+ somehow be embedded into a declaration. To be portable, this
+ declaration must declare an object, a constant, a function, or a
+ typedef name. If the declared entity uses the type directly,
+ such as in
+
+ struct dummy {...};
+ typedef struct {...} dummy;
+ extern struct {...} *dummy;
+ extern void dummy (struct {...} *);
+ extern struct {...} *dummy (void);
+
+ two uses of the verify macro would yield colliding declarations
+ if the entity names are not disambiguated. A workaround is to
+ attach the current line number to the entity name:
+
+ #define _GL_CONCAT0(x, y) x##y
+ #define _GL_CONCAT(x, y) _GL_CONCAT0 (x, y)
+ extern struct {...} * _GL_CONCAT (dummy, __LINE__);
+
+ But this has the problem that two invocations of verify from
+ within the same macro would collide, since the __LINE__ value
+ would be the same for both invocations. (The GCC __COUNTER__
+ macro solves this problem, but is not portable.)
+
+ A solution is to use the sizeof operator. It yields a number,
+ getting rid of the identity of the type. Declarations like
+
+ extern int dummy [sizeof (struct {...})];
+ extern void dummy (int [sizeof (struct {...})]);
+ extern int (*dummy (void)) [sizeof (struct {...})];
+
+ can be repeated.
+
+ * Should the implementation use a named struct or an unnamed struct?
+ Which of the following alternatives can be used?
-/* Verify requirement R at compile-time, as an expression.
- This macro can be used in some contexts where verify cannot, and vice versa.
- Return void. */
+ extern int dummy [sizeof (struct {...})];
+ extern int dummy [sizeof (struct verify_type__ {...})];
+ extern void dummy (int [sizeof (struct {...})]);
+ extern void dummy (int [sizeof (struct verify_type__ {...})]);
+ extern int (*dummy (void)) [sizeof (struct {...})];
+ extern int (*dummy (void)) [sizeof (struct verify_type__ {...})];
-# define verify_expr(R) ((void) ((verify_type__ (R) *) 0))
+ In the second and sixth case, the struct type is exported to the
+ outer scope; two such declarations therefore collide. GCC warns
+ about the first, third, and fourth cases. So the only remaining
+ possibility is the fifth case:
+
+ extern int (*dummy (void)) [sizeof (struct {...})];
+
+ * GCC warns about duplicate declarations of the dummy function if
+ -Wredundant_decls is used. GCC 4.3 and later have a builtin
+ __COUNTER__ macro that can let us generate unique identifiers for
+ each dummy function, to suppress this warning.
+
+ * This implementation exploits the fact that GCC does not warn about
+ the last declaration mentioned above. If a future version of GCC
+ introduces a warning for this, the problem could be worked around
+ by using code specialized to GCC, just as __COUNTER__ is already
+ being used if available.
+
+ #if 4 <= __GNUC__
+ # define verify(R) [another version to keep GCC happy]
+ #endif
+
+ * In C++, any struct definition inside sizeof is invalid.
+ Use a template type to work around the problem. */
+
+/* Concatenate two preprocessor tokens. */
+# define _GL_CONCAT(x, y) _GL_CONCAT0 (x, y)
+# define _GL_CONCAT0(x, y) x##y
+
+/* _GL_COUNTER is an integer, preferably one that changes each time we
+ use it. Use __COUNTER__ if it works, falling back on __LINE__
+ otherwise. __LINE__ isn't perfect, but it's better than a
+ constant. */
+# if defined __COUNTER__ && __COUNTER__ != __COUNTER__
+# define _GL_COUNTER __COUNTER__
+# else
+# define _GL_COUNTER __LINE__
+# endif
+
+/* Generate a symbol with the given prefix, making it unique if
+ possible. */
+# define _GL_GENSYM(prefix) _GL_CONCAT (prefix, _GL_COUNTER)
+
+/* Verify requirement R at compile-time, as an integer constant expression.
+ Return 1. */
+
+# ifdef __cplusplus
+template
+ struct verify_type__ { unsigned int verify_error_if_negative_size__: w; };
+# define verify_true(R) \
+ (!!sizeof (verify_type__<(R) ? 1 : -1>))
+# else
+# define verify_true(R) \
+ (!!sizeof \
+ (struct { unsigned int verify_error_if_negative_size__: (R) ? 1 : -1; }))
+# endif
+
+/* Verify requirement R at compile-time, as a declaration without a
+ trailing ';'. */
+
+# define verify(R) \
+ extern int (* _GL_GENSYM (verify_function) (void)) [verify_true (R)]
#endif