3 files changed, 86 insertions, 0 deletions

diff --git a/Tests/LibC/TestWchar.cpp b/Tests/LibC/TestWchar.cpp
index b96c432ab3..a158f33748 100644
--- a/Tests/LibC/TestWchar.cpp
+++ b/Tests/LibC/TestWchar.cpp
@@ -363,3 +363,54 @@ TEST_CASE(wcsrtombs)
     EXPECT_EQ(memcmp(buf, "\xf0\x9f\x90\x9e\xf0\x9f\x90\x9e", 9), 0);
     EXPECT_EQ(src, nullptr);
 }
+
+TEST_CASE(mbsrtowcs)
+{
+    mbstate_t state = {};
+    wchar_t buf[4];
+    const char good_chars[] = "\xf0\x9f\x90\x9e\xf0\x9f\x90\x9e";
+    const char bad_chars[] = "\xf0\x9f\x90\x9e\xf0\xff\x90\x9e";
+    const char* src;
+    size_t ret = 0;
+
+    // Convert normal and valid multibyte sequences.
+    src = good_chars;
+    ret = mbsrtowcs(buf, &src, 3, &state);
+    EXPECT_EQ(ret, 2ul);
+    EXPECT_EQ(buf[0], L'\U0001F41E');
+    EXPECT_EQ(buf[1], L'\U0001F41E');
+    EXPECT_EQ(buf[2], L'\0');
+    EXPECT_EQ(src, nullptr);
+    EXPECT_NE(mbsinit(&state), 0);
+
+    // Stop on invalid multibyte sequences.
+    src = bad_chars;
+    ret = mbsrtowcs(buf, &src, 3, &state);
+    EXPECT_EQ(ret, -1ul);
+    EXPECT_EQ(buf[0], L'\U0001F41E');
+    EXPECT_EQ(errno, EILSEQ);
+    EXPECT_EQ(src, bad_chars + 4);
+
+    // Valid sequence but not enough space.
+    src = good_chars;
+    ret = mbsrtowcs(buf, &src, 1, &state);
+    EXPECT_EQ(ret, 1ul);
+    EXPECT_EQ(buf[0], L'\U0001F41E');
+    EXPECT_EQ(src, good_chars + 4);
+
+    // Try a conversion with no destination and too short length.
+    src = good_chars;
+    ret = mbsrtowcs(nullptr, &src, 1, &state);
+    EXPECT_EQ(ret, 2ul);
+    EXPECT_EQ(src, nullptr);
+    EXPECT_NE(mbsinit(&state), 0);
+
+    // Try a conversion using the internal anonymous state.
+    src = good_chars;
+    ret = mbsrtowcs(buf, &src, 3, nullptr);
+    EXPECT_EQ(ret, 2ul);
+    EXPECT_EQ(buf[0], L'\U0001F41E');
+    EXPECT_EQ(buf[1], L'\U0001F41E');
+    EXPECT_EQ(buf[2], L'\0');
+    EXPECT_EQ(src, nullptr);
+}
diff --git a/Userland/Libraries/LibC/wchar.cpp b/Userland/Libraries/LibC/wchar.cpp
index 7bed4b5518..dd7ca983cf 100644
--- a/Userland/Libraries/LibC/wchar.cpp
+++ b/Userland/Libraries/LibC/wchar.cpp
@@ -505,4 +505,38 @@ size_t wcsrtombs(char* dest, const wchar_t** src, size_t len, mbstate_t* ps)
         written += ret;
     }
 }
+
+size_t mbsrtowcs(wchar_t* dst, const char** src, size_t len, mbstate_t* ps)
+{
+    static mbstate_t _anonymous_state = {};
+
+    if (ps == nullptr)
+        ps = &_anonymous_state;
+
+    size_t written = 0;
+    while (written < len || !dst) {
+        // Convert next multibyte to wchar.
+        size_t ret = mbrtowc(dst, *src, MB_LEN_MAX, ps);
+
+        // Multibyte sequence is invalid.
+        if (ret == -1ul) {
+            errno = EILSEQ;
+            return (size_t)-1;
+        }
+
+        // Null byte has been reached.
+        if (**src == '\0') {
+            *src = nullptr;
+            return written;
+        }
+
+        *src += ret;
+        written += 1;
+        if (dst)
+            dst += 1;
+    }
+
+    // If we are here, we have written `len` wchars, but not reached the null byte.
+    return written;
+}
 }
diff --git a/Userland/Libraries/LibC/wchar.h b/Userland/Libraries/LibC/wchar.h
index b7f2008415..543540e00a 100644
--- a/Userland/Libraries/LibC/wchar.h
+++ b/Userland/Libraries/LibC/wchar.h
@@ -57,5 +57,6 @@ long double wcstold(const wchar_t*, wchar_t**);
 int swprintf(wchar_t*, size_t, const wchar_t*, ...);
 int wcwidth(wchar_t);
 size_t wcsrtombs(char*, const wchar_t**, size_t, mbstate_t*);
+size_t mbsrtowcs(wchar_t*, const char**, size_t, mbstate_t*);
 
 __END_DECLS