Reland "Fix wcpncpy() return value; add test."

[enh-google]

Reverts #146752, which was a revert of my accidental push, so we can actually review and presubmit this time.

[llvmbot]

@llvm/pr-subscribers-libc

Author: None (enh-google)

Changes

Reverts llvm/llvm-project#146752, which was a revert of my accidental push, so we can actually review and presubmit this time.

---

Full diff: https://github.com/llvm/llvm-project/pull/146753.diff

2 Files Affected:

• (modified) libc/src/wchar/wcpncpy.cpp (+3-2)
• (modified) libc/test/src/wchar/wcpncpy_test.cpp (+9)

diff --git a/libc/src/wchar/wcpncpy.cpp b/libc/src/wchar/wcpncpy.cpp
index 9f451b73f07cb..0e729db7abf60 100644
--- a/libc/src/wchar/wcpncpy.cpp
+++ b/libc/src/wchar/wcpncpy.cpp
@@ -28,8 +28,9 @@ LLVM_LIBC_FUNCTION(wchar_t *, wcpncpy,
   for (i = 0; i < n && s2[i] != '\0'; ++i)
     s1[i] = s2[i];
   // When n>strlen(src), n-strlen(src) \0 are appended.
-  for (; i < n; ++i)
-    s1[i] = L'\0';
+  for (size_t j = i; j < n; ++j)
+    s1[j] = L'\0';
+  // ...but our result points to the first \0 (if any).
   return s1 + i;
 }
 
diff --git a/libc/test/src/wchar/wcpncpy_test.cpp b/libc/test/src/wchar/wcpncpy_test.cpp
index 98738e230e32d..5ce3e8ce7cd93 100644
--- a/libc/test/src/wchar/wcpncpy_test.cpp
+++ b/libc/test/src/wchar/wcpncpy_test.cpp
@@ -75,6 +75,15 @@ TEST(LlvmLibcWCPNCpyTest, CopyTwoWithNull) {
   ASSERT_EQ(dest + 2, res);
 }
 
+TEST(LlvmLibcWCPNCpyTest, CopyAndFill) {
+  wchar_t dest[] = {L'a', L'b', L'c'};
+  wchar_t *res = LIBC_NAMESPACE::wcpncpy(dest, L"x", 3);
+  ASSERT_TRUE(dest[0] == L'x');
+  ASSERT_TRUE(dest[1] == L'\0');
+  ASSERT_TRUE(dest[2] == L'\0');
+  ASSERT_EQ(dest + 1, res);
+}
+
 #if defined(LIBC_ADD_NULL_CHECKS) && !defined(LIBC_HAS_SANITIZER)
 TEST(LlvmLibcWCPNCpyTest, NullptrCrash) {
   // Passing in a nullptr should crash the program.

[enh-google]

i've also sent a fix to the man page upstream, since the man page has the same bug: https://lore.kernel.org/linux-man/CAJgzZoq0AvK8EDicLk7ZMVbWS8MmoqW0Nv4U9HCFUXnNw+yUGw@mail.gmail.com/T/#u

https://pubs.opengroup.org/onlinepubs/9799919799/functions/wcpncpy.html (and the fact that this failed an existing bionic unit test -- that also passes when run against glibc -- when i tried to switch Android over to the llvm-libc wcpncpy :-) ) is my proof that the new behavior is the expected behavior.

[sribee8]

The other tests need to be changed to match the new behavior of the function