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verify: use _Static_assert if available * lib/verify.h (HAVE__STATIC_ASSERT): New macro. (verify_true, verify): Use it if available. This generates better diagnostics with GCC 4.6.0 and later.
author Paul Eggert <eggert@cs.ucla.edu>
date Tue, 05 Apr 2011 23:45:10 -0700
parents 97fc9a21a8fb
children bfd6bd3592ec
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1 /* Compile-time assert-like macros.
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2
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3 Copyright (C) 2005-2006, 2009-2011 Free Software Foundation, Inc.
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5 This program is free software: you can redistribute it and/or modify
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6 it under the terms of the GNU General Public License as published by
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7 the Free Software Foundation; either version 3 of the License, or
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8 (at your option) any later version.
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10 This program is distributed in the hope that it will be useful,
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11 but WITHOUT ANY WARRANTY; without even the implied warranty of
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12 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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13 GNU General Public License for more details.
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15 You should have received a copy of the GNU General Public License
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16 along with this program. If not, see <http://www.gnu.org/licenses/>. */
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18 /* Written by Paul Eggert, Bruno Haible, and Jim Meyering. */
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20 #ifndef VERIFY_H
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21 # define VERIFY_H 1
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23 /* Define HAVE__STATIC_ASSERT to 1 if _Static_assert works as per the
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24 C1X draft N1548 section 6.7.10. This is supported by GCC 4.6.0 and
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25 later, and its use here generates easier-to-read diagnostics when
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26 verify (R) fails.
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28 For now, use this only with GCC. Eventually whether _Static_assert
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29 works should be determined by 'configure'. */
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30 # if 4 < __GNUC__ || (__GNUC__ == 4 && 6 <= __GNUC_MINOR__)
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31 # define HAVE__STATIC_ASSERT 1
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32 # endif
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34 /* Each of these macros verifies that its argument R is nonzero. To
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35 be portable, R should be an integer constant expression. Unlike
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36 assert (R), there is no run-time overhead.
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38 There are two macros, since no single macro can be used in all
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39 contexts in C. verify_true (R) is for scalar contexts, including
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40 integer constant expression contexts. verify (R) is for declaration
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41 contexts, e.g., the top level.
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43 Symbols ending in "__" are private to this header.
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45 If _Static_assert works, verify (R) uses it directly. Similarly,
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46 verify_true (R) works by packaging a _Static_assert inside a struct
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47 that is an operand of sizeof.
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48
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49 The code below uses several ideas for C++ compilers, and for C
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50 compilers that do not support _Static_assert:
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52 * The first step is ((R) ? 1 : -1). Given an expression R, of
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53 integral or boolean or floating-point type, this yields an
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54 expression of integral type, whose value is later verified to be
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55 constant and nonnegative.
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57 * Next this expression W is wrapped in a type
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58 struct verify_type__ { unsigned int verify_error_if_negative_size__: W; }.
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59 If W is negative, this yields a compile-time error. No compiler can
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60 deal with a bit-field of negative size.
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61
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62 One might think that an array size check would have the same
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63 effect, that is, that the type struct { unsigned int dummy[W]; }
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64 would work as well. However, inside a function, some compilers
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65 (such as C++ compilers and GNU C) allow local parameters and
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66 variables inside array size expressions. With these compilers,
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67 an array size check would not properly diagnose this misuse of
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68 the verify macro:
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69
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70 void function (int n) { verify (n < 0); }
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71
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72 * For the verify macro, the struct verify_type__ will need to
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73 somehow be embedded into a declaration. To be portable, this
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