static_assert(3)

1static_assert(3)           Library Functions Manual           static_assert(3)
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NAME

6       static_assert, _Static_assert - fail compilation if assertion is false
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LIBRARY

9       Standard C library (libc)
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SYNOPSIS

12       #include <assert.h>
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14       void static_assert(scalar constant-expression, const char *msg);
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16       /* Since C23: */
17       void static_assert(scalar constant-expression);
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DESCRIPTION

20       This  macro is similar to assert(3), but it works at compile time, gen‐
21       erating a compilation error (with an optional message) when  the  input
22       is false (i.e., compares equal to zero).
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24       If the input is nonzero, no code is emitted.
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26       msg must be a string literal.  Since C23, this argument is optional.
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28       There's  a keyword, _Static_assert(), that behaves identically, and can
29       be used without including <assert.h>.
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RETURN VALUE

32       No value is returned.
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VERSIONS

35       In C11, the second argument (msg) was mandatory; since C23, it  can  be
36       omitted.
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STANDARDS

39       C11 and later.
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EXAMPLES

42       static_assert()  can't  be  used  in  some  places, like for example at
43       global scope.  For that, a macro must_be() can be written in  terms  of
44       static_assert().   The following program uses the macro to get the size
45       of an array safely.
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47           #include <assert.h>
48           #include <stddef.h>
49           #include <stdint.h>
50           #include <stdio.h>
51           #include <stdlib.h>
52           #include <string.h>
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54           /*
55            * This macro behaves like static_assert(), failing to
56            * compile if its argument is not true.  However, it always
57            * returns 0, which allows using it everywhere an expression
58            * can be used.
59            */
60           #define must_be(e)                                        \
61           (                                                         \
62               0 * (int) sizeof(                                     \
63                   struct {                                          \
64                       static_assert(e);                             \
65                       int  ISO_C_forbids_a_struct_with_no_members;  \
66                   }                                                 \
67               )                                                     \
68           )
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70           #define is_same_type(a, b)  \
71               __builtin_types_compatible_p(typeof(a), typeof(b))
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73           #define is_array(arr)       (!is_same_type((arr), &*(arr)))
74           #define must_be_array(arr)  must_be(is_array(arr))
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76           #define sizeof_array(arr)   (sizeof(arr) + must_be_array(arr))
77           #define nitems(arr)         (sizeof((arr)) / sizeof((arr)[0]) \
78                                        + must_be_array(arr))
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80           int     foo[10];
81           int8_t  bar[sizeof_array(foo)];
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83           int
84           main(void)
85           {
86               for (size_t i = 0; i < nitems(foo); i++) {
87                   foo[i] = i;
88               }
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90               memcpy(bar, foo, sizeof_array(bar));
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92               for (size_t i = 0; i < nitems(bar); i++) {
93                   printf("%d,", bar[i]);
94               }
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96               exit(EXIT_SUCCESS);
97           }
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