diff --git a/libc/test/src/stdlib/CMakeLists.txt b/libc/test/src/stdlib/CMakeLists.txt
--- a/libc/test/src/stdlib/CMakeLists.txt
+++ b/libc/test/src/stdlib/CMakeLists.txt
@@ -88,6 +88,30 @@
     libc.src.errno.errno
 )
 
+add_libc_test(
+  strtoint32_test
+  SUITE
+    libc-stdlib-tests
+  SRCS
+    strtoint32_test.cpp
+  DEPENDS
+    libc.src.__support.str_to_integer
+    libc.src.errno.errno
+    .strtol_test_support
+)
+
+add_libc_test(
+  strtoint64_test
+  SUITE
+    libc-stdlib-tests
+  SRCS
+    strtoint64_test.cpp
+  DEPENDS
+    libc.src.__support.str_to_integer
+    libc.src.errno.errno
+    .strtol_test_support
+)
+
 add_libc_test(
   strtol_test
   SUITE
diff --git a/libc/test/src/stdlib/StrtolTest.h b/libc/test/src/stdlib/StrtolTest.h
--- a/libc/test/src/stdlib/StrtolTest.h
+++ b/libc/test/src/stdlib/StrtolTest.h
@@ -325,7 +325,8 @@
               ((is_signed_v<ReturnT> && sizeof(ReturnT) == 4)
                    ? T_MAX
                    : ReturnT(0xFFFFFFFF)));
-    ASSERT_EQ(libc_errno, 0);
+    ASSERT_EQ(libc_errno,
+              is_signed_v<ReturnT> && sizeof(ReturnT) == 4 ? ERANGE : 0);
     EXPECT_EQ(str_end - max_32_bit_value, ptrdiff_t(10));
 
     const char *negative_max_32_bit_value = "-0xFFFFFFFF";
@@ -334,7 +335,8 @@
               ((is_signed_v<ReturnT> && sizeof(ReturnT) == 4)
                    ? T_MIN
                    : -ReturnT(0xFFFFFFFF)));
-    ASSERT_EQ(libc_errno, 0);
+    ASSERT_EQ(libc_errno,
+              is_signed_v<ReturnT> && sizeof(ReturnT) == 4 ? ERANGE : 0);
     EXPECT_EQ(str_end - negative_max_32_bit_value, ptrdiff_t(11));
 
     // Max size for signed 32 bit numbers
@@ -357,30 +359,41 @@
     const char *max_64_bit_value = "0xFFFFFFFFFFFFFFFF";
     libc_errno = 0;
     ASSERT_EQ(func(max_64_bit_value, &str_end, 0),
-              (is_signed_v<ReturnT> ? T_MAX : ReturnT(0xFFFFFFFFFFFFFFFF)));
-    ASSERT_EQ(libc_errno, (is_signed_v<ReturnT> ? ERANGE : 0));
+              (is_signed_v<ReturnT> || sizeof(ReturnT) < 8
+                   ? T_MAX
+                   : ReturnT(0xFFFFFFFFFFFFFFFF)));
+    ASSERT_EQ(libc_errno,
+              (is_signed_v<ReturnT> || sizeof(ReturnT) < 8 ? ERANGE : 0));
     EXPECT_EQ(str_end - max_64_bit_value, ptrdiff_t(18));
 
+    // See the end of CleanBase10Decode for an explanation of how this large
+    // negative number can end up as T_MAX.
     const char *negative_max_64_bit_value = "-0xFFFFFFFFFFFFFFFF";
     libc_errno = 0;
-    ASSERT_EQ(func(negative_max_64_bit_value, &str_end, 0),
-              (is_signed_v<ReturnT> ? T_MIN : -ReturnT(0xFFFFFFFFFFFFFFFF)));
-    ASSERT_EQ(libc_errno, (is_signed_v<ReturnT> ? ERANGE : 0));
+    ASSERT_EQ(
+        func(negative_max_64_bit_value, &str_end, 0),
+        (is_signed_v<ReturnT>
+             ? T_MIN
+             : (sizeof(ReturnT) < 8 ? T_MAX : -ReturnT(0xFFFFFFFFFFFFFFFF))));
+    ASSERT_EQ(libc_errno,
+              (is_signed_v<ReturnT> || sizeof(ReturnT) < 8 ? ERANGE : 0));
     EXPECT_EQ(str_end - negative_max_64_bit_value, ptrdiff_t(19));
 
     // Max size for signed 64 bit numbers
 
     const char *max_63_bit_value = "0x7FFFFFFFFFFFFFFF";
     libc_errno = 0;
-    ASSERT_EQ(func(max_63_bit_value, &str_end, 0), ReturnT(0x7FFFFFFFFFFFFFFF));
-    ASSERT_EQ(libc_errno, 0);
+    ASSERT_EQ(func(max_63_bit_value, &str_end, 0),
+              (sizeof(ReturnT) < 8 ? T_MAX : ReturnT(0x7FFFFFFFFFFFFFFF)));
+    ASSERT_EQ(libc_errno, sizeof(ReturnT) < 8 ? ERANGE : 0);
     EXPECT_EQ(str_end - max_63_bit_value, ptrdiff_t(18));
 
     const char *negative_max_63_bit_value = "-0x7FFFFFFFFFFFFFFF";
     libc_errno = 0;
     ASSERT_EQ(func(negative_max_63_bit_value, &str_end, 0),
-              -ReturnT(0x7FFFFFFFFFFFFFFF));
-    ASSERT_EQ(libc_errno, 0);
+              (sizeof(ReturnT) >= 8 ? -ReturnT(0x7FFFFFFFFFFFFFFF)
+                                    : (is_signed_v<ReturnT> ? T_MIN : T_MAX)));
+    ASSERT_EQ(libc_errno, sizeof(ReturnT) < 8 ? ERANGE : 0);
     EXPECT_EQ(str_end - negative_max_63_bit_value, ptrdiff_t(19));
   }
 
diff --git a/libc/test/src/stdlib/atoi_test.cpp b/libc/test/src/stdlib/atoi_test.cpp
--- a/libc/test/src/stdlib/atoi_test.cpp
+++ b/libc/test/src/stdlib/atoi_test.cpp
@@ -1,4 +1,4 @@
-//===-- Unittests for atoi -----------------------------------------------===//
+//===-- Unittests for atoi ------------------------------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
diff --git a/libc/test/src/stdlib/atol_test.cpp b/libc/test/src/stdlib/atol_test.cpp
--- a/libc/test/src/stdlib/atol_test.cpp
+++ b/libc/test/src/stdlib/atol_test.cpp
@@ -1,4 +1,4 @@
-//===-- Unittests for atol -----------------------------------------------===//
+//===-- Unittests for atol ------------------------------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
diff --git a/libc/test/src/stdlib/strtod_test.cpp b/libc/test/src/stdlib/strtod_test.cpp
--- a/libc/test/src/stdlib/strtod_test.cpp
+++ b/libc/test/src/stdlib/strtod_test.cpp
@@ -1,4 +1,4 @@
-//===-- Unittests for strtod ---------------------------------------------===//
+//===-- Unittests for strtod ----------------------------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
diff --git a/libc/test/src/stdlib/strtof_test.cpp b/libc/test/src/stdlib/strtof_test.cpp
--- a/libc/test/src/stdlib/strtof_test.cpp
+++ b/libc/test/src/stdlib/strtof_test.cpp
@@ -1,4 +1,4 @@
-//===-- Unittests for strtof ---------------------------------------------===//
+//===-- Unittests for strtof ----------------------------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
diff --git a/libc/test/src/stdlib/strtoint32_test.cpp b/libc/test/src/stdlib/strtoint32_test.cpp
new file mode 100644
--- /dev/null
+++ b/libc/test/src/stdlib/strtoint32_test.cpp
@@ -0,0 +1,45 @@
+//===-- Unittests for strtoint32 ------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include <stdint.h>
+
+#include "src/__support/str_to_integer.h"
+#include "src/errno/libc_errno.h"
+
+#include "StrtolTest.h"
+#include "test/UnitTest/Test.h"
+
+namespace __llvm_libc {
+
+int32_t strtoint32(const char *__restrict str, char **__restrict str_end,
+                   int base) {
+  auto result = internal::strtointeger<int32_t>(str, base);
+  if (result.has_error())
+    libc_errno = result.error;
+
+  if (str_end != nullptr)
+    *str_end = const_cast<char *>(str + result.parsed_len);
+
+  return result;
+}
+
+uint32_t strtouint32(const char *__restrict str, char **__restrict str_end,
+                     int base) {
+  auto result = internal::strtointeger<uint32_t>(str, base);
+  if (result.has_error())
+    libc_errno = result.error;
+
+  if (str_end != nullptr)
+    *str_end = const_cast<char *>(str + result.parsed_len);
+
+  return result;
+}
+} // namespace __llvm_libc
+
+STRTOL_TEST(Strtoint32, __llvm_libc::strtoint32)
+STRTOL_TEST(Strtouint32, __llvm_libc::strtouint32)
diff --git a/libc/test/src/stdlib/strtoint64_test.cpp b/libc/test/src/stdlib/strtoint64_test.cpp
new file mode 100644
--- /dev/null
+++ b/libc/test/src/stdlib/strtoint64_test.cpp
@@ -0,0 +1,45 @@
+//===-- Unittests for strtoint64 ------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include <stdint.h>
+
+#include "src/__support/str_to_integer.h"
+#include "src/errno/libc_errno.h"
+
+#include "StrtolTest.h"
+#include "test/UnitTest/Test.h"
+
+namespace __llvm_libc {
+
+int64_t strtoint64(const char *__restrict str, char **__restrict str_end,
+                   int base) {
+  auto result = internal::strtointeger<int64_t>(str, base);
+  if (result.has_error())
+    libc_errno = result.error;
+
+  if (str_end != nullptr)
+    *str_end = const_cast<char *>(str + result.parsed_len);
+
+  return result;
+}
+
+uint64_t strtouint64(const char *__restrict str, char **__restrict str_end,
+                     int base) {
+  auto result = internal::strtointeger<uint64_t>(str, base);
+  if (result.has_error())
+    libc_errno = result.error;
+
+  if (str_end != nullptr)
+    *str_end = const_cast<char *>(str + result.parsed_len);
+
+  return result;
+}
+} // namespace __llvm_libc
+
+STRTOL_TEST(Strtoint64, __llvm_libc::strtoint64)
+STRTOL_TEST(Strtouint64, __llvm_libc::strtouint64)
diff --git a/libc/test/src/stdlib/strtol_test.cpp b/libc/test/src/stdlib/strtol_test.cpp
--- a/libc/test/src/stdlib/strtol_test.cpp
+++ b/libc/test/src/stdlib/strtol_test.cpp
@@ -1,4 +1,4 @@
-//===-- Unittests for strtol ---------------------------------------------===//
+//===-- Unittests for strtol ----------------------------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.