Add test for nullptr write error

Author: mleleszi

libc/src/stdio/printf_core/core_structs.h

@@ -144,14 +144,6 @@ template <typename T> LIBC_INLINE constexpr TypeDesc type_desc_from_type() {
 
 // This is the value to be returned by conversions when no error has occurred.
 constexpr int WRITE_OK = 0;
-// These are the printf return values for when an error has occurred. They are
-// all negative, and should be distinct.
-// constexpr int FILE_WRITE_ERROR = -1;
-// constexpr int FILE_STATUS_ERROR = -2;
-// constexpr int NULLPTR_WRITE_ERROR = -3;
-// constexpr int INT_CONVERSION_ERROR = -4;
-// constexpr int FIXED_POINT_CONVERSION_ERROR = -5;
-// constexpr int ALLOCATION_ERROR = -6;
 } // namespace printf_core
 } // namespace LIBC_NAMESPACE_DECL
 

libc/test/src/stdio/fprintf_test.cpp

@@ -112,4 +112,20 @@ TEST(LlvmLibcFPrintfTest, CharsWrittenOverflow) {
   ASSERT_ERRNO_EQ(EOVERFLOW);
 
   EXPECT_EQ(printf_test::fclose(file), 0);
-}
+}
+
+#ifndef LIBC_COPT_PRINTF_NO_NULLPTR_CHECKS
+TEST(LlvmLibcFPrintfTest, NullPtrCheck) {
+  const char *FILENAME = APPEND_LIBC_TEST("fprintf_nullptr.test");
+  auto FILE_PATH = libc_make_test_file_path(FILENAME);
+
+  ::FILE *file = printf_test::fopen(FILE_PATH, "w");
+  ASSERT_FALSE(file == nullptr);
+
+  int ret = LIBC_NAMESPACE::fprintf(file, "hello %n", (int *)nullptr);
+  EXPECT_LT(ret, 0);
+  ASSERT_ERRNO_EQ(EINVAL);
+
+  ASSERT_EQ(printf_test::fclose(file), 0);
+}
+#endif // LIBC_COPT_PRINTF_NO_NULLPTR_CHECKS