[libc++] Add a missing include in string.h

[atetubou]

stddef.h is necessary for size_t in explicit Clang modules build.

[llvmbot]

@llvm/pr-subscribers-libcxx

Author: Takuto Ikuta (atetubou)

Changes

stddef.h is necessary for size_t in explicit Clang modules build.

---

Full diff: https://github.com/llvm/llvm-project/pull/135134.diff

1 Files Affected:

• (modified) libcxx/include/string.h (+2)

diff --git a/libcxx/include/string.h b/libcxx/include/string.h
index 6bdcd6a6eecbd..c61fda32d76f8 100644
--- a/libcxx/include/string.h
+++ b/libcxx/include/string.h
@@ -64,6 +64,8 @@ size_t strlen(const char* s);
 #    include_next <string.h>
 #  endif
 
+#  include <stddef.h> 
+
 // MSVCRT, GNU libc and its derivates may already have the correct prototype in
 // <string.h>. This macro can be defined by users if their C library provides
 // the right signature.

[github-actions]

✅ With the latest revision this PR passed the C/C++ code formatter.

[frederick-vs-ja]

How about including <__cstddef/size_t.h> which looks smaller?

Suggested change

	# include <stddef.h>
	# include <__cstddef/size_t.h>

[atetubou]

As this file uses bare size_t instead of std::size_t, I think <stddef.h> is a corret header to include here.

[ldionne]

I agree, I don't think we can use __cstddef/size_t.h because it doesn't provide the right declaration (conceptually). In practice, of course, they're all the same type in the end.

This makes me wonder whether we should introduce an internal alias for decltype(sizeof(int)), something like __libcxx_size_t and then do namespace std { using size_t = ::__libcxx_size_t; }. However, until we have a reason to do that, I think including <stddef.h> here is fine.

[philnik777]

Do you have a reproducer?

[ldionne]

We can tell by inspection that we have a missing include, so IMO this change is fine. IIUC a reproducer is going to exist once we start being able to build libc++ with explicit modules.

[ldionne]

I agree, I don't think we can use __cstddef/size_t.h because it doesn't provide the right declaration (conceptually). In practice, of course, they're all the same type in the end.

This makes me wonder whether we should introduce an internal alias for decltype(sizeof(int)), something like __libcxx_size_t and then do namespace std { using size_t = ::__libcxx_size_t; }. However, until we have a reason to do that, I think including <stddef.h> here is fine.