| Issue |
160666
|
| Summary |
[libc++] Internal failure in std::format_to when passing a string of size being a multiple of 256
|
| Labels |
libc++
|
| Assignees |
|
| Reporter |
mk-huawei
|
This program, when compiled with DEBUG hardening `-O0 -std=c++2c -stdlib=libc++ -DNDEBUG -D _LIBCPP_HARDENING_MODE=_LIBCPP_HARDENING_MODE_DEBUG` provokes assertion failure in clang-20 and clang-21 and current trunk (2025-09-24, clang version 22.0.0git (https://github.com/llvm/llvm-project.git 89d2b7e08ec7571a53f4b1257df0f4d07f5cd6ce)).
Godbolt: https://godbolt.org/z/9MPhM7vr5
Only sizes being multiples of 256 fail, as it seems, formatting is done in chunks of 256 bytes and then `__size_ == __capacity_` (== 256) (cf. https://github.com/llvm/llvm-project/blob/main/libcxx/include/__format/buffer.h#L211).
The problem doesn't happen with `std::format` or size not being multiples of 256 (certainly due to `__container_inserter_buffer` having this size, cf. https://github.com/llvm/llvm-project/blob/main/libcxx/include/__format/buffer.h#L461).
```
#include <print>
#include <string>
int main() {
std::string vsmall(255, 'b');
std::string v(512, 'b'); // any multiple of 256 fails
std::string vlarge(257, 'b');
std::string buf;
std::ignore = std::format("'{}'", v); // OK
std::format_to(std::back_inserter(buf), "'{}'", vsmall); // OK
std::format_to(std::back_inserter(buf), "'{}'", vlarge); // OK
// BAD: assertion in __format/buffer.h:211:
std::format_to(std::back_inserter(buf), "'{}'", v);
return 0;
}
```
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