This ASSERT should fail now, right?

Remove parens for single statement conditionals. https://llvm.org/docs/CodingStandards.html#don-t-use-braces-on-simple-single-statement-bodies-of-if-else-loop-statements

We do have a check. Same for the test too.

K.3.8.3.3.3: > If there is a runtime-constraint violation, s[0] is set to a null character if s is not a null pointer and maxsize is not equal zero and is not greater than RSIZE_MAX But what's funny to me is that the sentence before (K.3.8.3.3.2) literally says maxsize shall not be less than 26. So if we first check that maxsize is not less than 26, then it would implicitly mean maxsize is not zero. So if you implement my above check, I think we can elide this `buffer_size != 0` check here.

K.3.8.3.3.2: > maxsize shall not be less than 26 and shall not be greater than RSIZE_MAX. So we're missing a comparison against 26 (and a test for `buffer_size` being 26 or less).

```suggestion time_t t = 2147483648; int result = LIBC_NAMESPACE::ctime_s(buffer, sizeof(buffer), &t); ```

```suggestion // 2038-01-19 03:14:07. Test with a valid buffer size. time_t t = 2147483647; int result = LIBC_NAMESPACE::ctime_s(buffer, sizeof(buffer), &t); ```

```suggestion // 1970-01-01 00:00:00. Test with a valid buffer size. time_t t = 0; int result = LIBC_NAMESPACE::ctime_s(buffer, sizeof(buffer), &t); ```

```suggestion time_t t = 0; int result = LIBC_NAMESPACE::ctime_s(buffer, 0, &t); ```

```suggestion int result = LIBC_NAMESPACE::ctime_s(nullptr, 0, nullptr); ```

Oh, I missed this before. I didn't spot which feature from C++20 you were using in the unit test. Can you remove this, or point out which feature you needed this change for?

```suggestion (char *buffer, rsize_t buffer_size, const time_t *t_ptr)) { // TODO(https://github.com/llvm/llvm-project/issues/115907): invoke constraint handler if (buffer == nullptr || t_ptr == nullptr) ```

Don't we need to zero the first element of `buffer` in this case?

What happens if `buffer` is `nullptr` (while the other params are ok)? We'll end up dereferencing it, which we do not want to do. So I think you should just start out with: ```c++ if (buffer == nullptr || t_ptr == nullptr) return EINVAL; ``` then move on to validating the other inputs.

What are you using from stddef.h? You should be using proxy headers (hdr/) here.

```suggestion #include "size_t.h" ```

~Is this needed since it is just returning an error?~ I did not think about the C standard. Sorry about that.

this typedef should be in a proper type header, not here.

K.3.8.3.3 describes `ctime_s`: > errno_t ctime_s(char *s, rsize_t maxsize, const time_t *timer); 2 Neither s nor timer shall be a null pointer. maxsize shall not be less than 26 and shall not be greater than RSIZE_MAX. 3 If there is a runtime-constraint violation, s[0] is set to a null character if s is not a null pointer and maxsize is not equal zero and is not greater than RSIZE_MAX. So setting `buffer[0] = '\0'` should be handled in this PR. Please do so, then add a test.

``` /android0/llvm-project/libc/src/time/ctime_s.cpp:23:13: error: comparison between pointer and integer ('time_t (*)(time_t *) noexcept' (aka 'long (*)(long *) noexcept') and 'int') 23 | *time > cpp::numeric_limits<int32_t>::max()) { | ~~~~~ ^ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ```

``` /android0/llvm-project/libc/src/time/ctime_s.h:17:27: error: unknown type name 'size_t'; did you mean 'time_t'? 17 | int ctime_s(char *buffer, size_t buffer_size, const time_t *t_ptr); | ^~~~~~ | time_t ``` Make sure to `#include <stddef.h>`.

`time_t` may not be a 32 bit number, and making this check may cause problems in approximately 14 years: https://en.wikipedia.org/wiki/Year_2038_problem

Missing `const` qualifier?

use `"hdr/errno_macros.h"` instead.

It's llvm style to omit `{}` for single statement conditionals; please omit them. https://llvm.org/docs/CodingStandards.html#don-t-use-braces-on-simple-single-statement-bodies-of-if-else-loop-statements