Fix mbsrtowcs(3)'s handling of len parameter.

The len parameter is a _maximum_ length. The previous code was treating
it as an exact length, causing the following typical call to fail:

    mbsrtowcs(out, &in, sizeof(out), state); // sizeof(out) > strlen(in)

Change-Id: I48e474fd54ea5f122bc168a4d74bfe08704f28cc

1 files changed, 11 insertions, 0 deletions

diff --git a/tests/wchar_test.cpp b/tests/wchar_test.cpp
index a5f5f63a2..f052ce6f1 100644
--- a/tests/wchar_test.cpp
+++ b/tests/wchar_test.cpp
@@ -340,8 +340,19 @@ void test_mbsrtowcs(mbstate_t* ps) {
   ASSERT_EQ(static_cast<wchar_t>(0x00a2), out[1]);
   ASSERT_EQ(static_cast<wchar_t>(0x20ac), out[2]);
   ASSERT_EQ(static_cast<wchar_t>(0x24b62), out[3]);
+  // Check that valid has advanced to the next unread character.
   ASSERT_EQ('e', *valid);
 
+  wmemset(out, L'x', sizeof(out) / sizeof(wchar_t));
+  ASSERT_EQ(2U, mbsrtowcs(out, &valid, 4, ps));
+  ASSERT_EQ(L'e', out[0]);
+  ASSERT_EQ(L'f', out[1]);
+  ASSERT_EQ(L'\0', out[2]);
+  // Check that we didn't clobber the rest of out.
+  ASSERT_EQ(L'x', out[3]);
+  // Check that valid has advanced to the end of the string.
+  ASSERT_EQ(L'\0', *valid);
+
   const char* invalid = "A" "\xc2\x20" "ef";
   ASSERT_EQ(static_cast<size_t>(-1), mbsrtowcs(out, &invalid, 4, ps));
   EXPECT_EQ(EILSEQ, errno);