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The __builtin_counted_by_ref builtin is used on a flexible array
pointer and returns a pointer to the "counted_by" attribute's COUNT
argument, which is a field in the same non-anonymous struct as the
flexible array member. This is useful for automatically setting the
count field without needing the programmer's intervention. Otherwise
it's possible to get this anti-pattern:
ptr = alloc(<ty>, ..., COUNT);
ptr->FAM[9] = 42; /* <<< Sanitizer will complain */
ptr->count = COUNT;
To prevent this anti-pattern, the user can create an allocator that
automatically performs the assignment:
#define alloc(TY, FAM, COUNT) ({ \
TY __p = alloc(get_size(TY, COUNT)); \
if (__builtin_counted_by_ref(__p->FAM)) \
*__builtin_counted_by_ref(__p->FAM) = COUNT; \
__p; \
})
The builtin's behavior is heavily dependent upon the "counted_by"
attribute existing. It's main utility is during allocation to avoid
the above anti-pattern. If the flexible array member doesn't have that
attribute, the builtin becomes a no-op. Therefore, if the flexible
array member has a "count" field not referenced by "counted_by", it
must be set explicitly after the allocation as this builtin will
return a "nullptr" and the assignment will most likely be elided.
---------
Co-authored-by: Bill Wendling <isanbard@gmail.com>
Co-authored-by: Aaron Ballman <aaron@aaronballman.com>
24 lines
761 B
C
24 lines
761 B
C
// RUN: %clang_cc1 -triple x86_64-unknown-linux -ast-print %s -o - | FileCheck %s
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typedef unsigned long int size_t;
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int global_array[42];
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int global_int;
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struct fam_struct {
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int x;
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char count;
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int array[] __attribute__((counted_by(count)));
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};
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// CHECK-LABEL: void test1(struct fam_struct *ptr, int size) {
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// CHECK-NEXT: size_t __ignored_assignment;
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// CHECK-NEXT: *_Generic(__builtin_counted_by_ref(ptr->array), void *: &__ignored_assignment, default: __builtin_counted_by_ref(ptr->array)) = 42;
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void test1(struct fam_struct *ptr, int size) {
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size_t __ignored_assignment;
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*_Generic(__builtin_counted_by_ref(ptr->array),
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void *: &__ignored_assignment,
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default: __builtin_counted_by_ref(ptr->array)) = 42; // ok
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}
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