[JJenkx]

What

strxfrm and wcsxfrm wrote count - 1 characters and a terminator when the transformed string did not fit. The libc.prx that titles ship does something different: it copies the first count characters and writes no terminator. When the result fits, it copies the whole string with its terminator. Both functions now do the same. The return value, the source's length, is unchanged.

call	titles' libc.prx	main
strxfrm(buf, "abcdef", 1)	a	\0
strxfrm(buf, "abcdef", 4)	abcd	abc\0
strxfrm(buf, "abcdef", 6)	abcdef	abcde\0
strxfrm(buf, "abcdef", 7)	abcdef\0	abcdef\0
wcsxfrm(buf, u"much too long", 4)	much	muc\0

All return 6 or 13 on both sides. C leaves the array's contents indeterminate when the result does not fit, so this changes only what a title can observe in that case.

Tested

Windows 11 Enterprise 10.0.22621, Ryzen 9 5950X, WinLibs GCC 15.2.0 UCRT r7, Ninja, Release, BUILD_TESTING=ON, on main e2a5118839.

• Comparison: a scratch loader runs the decrypted libc.prx of PPSA07923, PPSA13934 and PPSA34008 natively. Each title got 100,000 calls each to strxfrm and wcsxfrm: random strings of 0 to 19 non-null bytes or UTF-16 units, sizes 0 to 25, and a null destination with size 0. The rule above predicts the return value and all 32 bytes or units of the pre-filled destination in every call, on all three titles.
• guest_strings: the truncated strxfrm checks expect the measured characters: abcd for size 4, abcdefgh for size 8, and a for size 1, with the bytes after them untouched.
• guest_wide_strings: the wcsxfrm truncation checks expect much for size 4, w for size 1, wide for size 4, and four units of the surrogate case.
• guest_strings and guest_wide_strings pass in each of 3 runs on the exact head 7c7cc70f2e.
• Negative control: with main's Strings.cpp and these tests, both abort (exit 0xc0000409).
• python tools/check_conventions.py --base origin/main and git diff --check pass.
• Not tested: a console, Linux and macOS. feat(libc): add the C locale helpers _Getctype, _Getcoll, _Getcvt, _Getdateorder, _Strcoll and _Strxfrm #9466 adds the separate Dinkumware helper _Strxfrm and doesn't change these two functions.

Checklist

• Based on current main; no other open PR implements the same functions
• One topic per PR; follow-ups go in a new PR
• No comments in code except technical debt
• Unimplemented paths throw (NotImplemented_nid_no_patch); silent stubs are listed under Silent stubs in their library's technical debt file
• No notes, investigation .md files or images added to the repository (attach them to this PR)
• New third-party code is a submodule built from source: none
• Generic behaviour, not specific to one title
• Depends on: none
• Fixes: none
• Unblocks: none
• GPUs tested: n/a
• AI-assisted: yes

AI-assisted (Claude Code).

🤖 Generated with Claude Code

[MarcZX10]

Reviewed exact head 7c7cc70f2e443d2f20e6afe1209a96b3933f716a.

The implementation and added assertions match the reported title behavior: when the source length is below count, it copies the terminating unit; otherwise it writes exactly count bytes/UTF-16 units and leaves the following destination unit untouched. A zero count avoids writing, and the return remains the source length. The tests cover fit, truncation, count one, zero count/null destination, and UTF-16 surrogate data.

Local verification: Release build of guest_string_tests and guest_wide_string_tests succeeded; guest_wide_strings passed natively; guest_strings passed under QEMU Haswell because the host Xeon lacks AVX2 and the other required guest CPU features; conventions and git diff --check passed; the current-main merge preview is clean. Hosted Windows, Ubuntu register-context, Ubuntu/Windows relinker, ASan relinker, conventions, eligibility, intake, labels, and report checks passed. Linux product was skipped; macOS relinker/register-context/write-watch checks are still queued. I did not run on a PS5 console. The native title comparison described in the PR is author-provided evidence.

[github-actions]

Some comparisons could not be completed because a pull request conflicts with the base branch. Review these shared files manually before concluding that there is no overlap.

Pull request	Shared files not compared
PR 5021	core/libs/prx/libc/src/Strings.cpp
PR 11664	core/libs/tests/GuestStrings.cpp

[JJenkx]

Linux run at this PR's exact head 7c7cc70f2e (7c7cc70f2e443d2f20e6afe1209a96b3933f716a), not merged onto anything else; its base on main is e2a5118839.

Arch Linux, kernel 7.2.9, Ryzen 9 7950X, g++ (GCC) 16.2.1 20260810, glibc 2.44, x86-64. Release, BUILD_TESTING=ON.

Passed in each of 3 runs, exit code 0 each time, with no failed, skipped or not-run tests:

• guest_strings
• guest_wide_strings

These are the focused tests only; no title or console was used.

AI-assisted: yes (Claude Code).