diff --git a/libc/src/stdio/scanf_core/reader.h b/libc/src/stdio/scanf_core/reader.h
@@ -9,44 +9,61 @@
 #ifndef LLVM_LIBC_SRC_STDIO_SCANF_CORE_READER_H
 #define LLVM_LIBC_SRC_STDIO_SCANF_CORE_READER_H
 
-#include "src/stdio/scanf_core/file_reader.h"
-#include "src/stdio/scanf_core/string_reader.h"
+#include "src/__support/macros/attributes.h" // For LIBC_INLINE
 #include <stddef.h>
 
 namespace __llvm_libc {
 namespace scanf_core {
 
-enum class ReaderType { String, File };
+using StreamGetc = int (*)(void *);
+using StreamUngetc = void (*)(int, void *);
 
-class Reader final {
-  union {
-    StringReader *string_reader;
-    FileReader *file_reader;
-  };
+// This is intended to be either a raw string or a buffer syncronized with the
+// file's internal buffer.
+struct ReadBuffer {
+  char *buffer;
+  size_t buff_len;
+  size_t buff_cur = 0;
+};
+
+class Reader {
+  ReadBuffer *rb;
 
-  const ReaderType reader_type;
+  void *input_stream = nullptr;
+
+  StreamGetc stream_getc = nullptr;
+  StreamUngetc stream_ungetc = nullptr;
 
   size_t cur_chars_read = 0;
 
 public:
-  Reader(StringReader *init_string_reader)
-      : string_reader(init_string_reader), reader_type(ReaderType::String) {}
+  // TODO: Set buff_len with a proper constant
+  Reader(ReadBuffer *string_buffer) : rb(string_buffer) {}
 
-  Reader(FileReader *init_file_reader)
-      : file_reader(init_file_reader), reader_type(ReaderType::File) {}
+  Reader(void *stream, StreamGetc stream_getc_in, StreamUngetc stream_ungetc_in,
+         ReadBuffer *stream_buffer = nullptr)
+      : rb(stream_buffer), input_stream(stream), stream_getc(stream_getc_in),
+        stream_ungetc(stream_ungetc_in) {}
 
   // This returns the next character from the input and advances it by one
   // character. When it hits the end of the string or file it returns '\0' to
   // signal to stop parsing.
-  char getc();
+  LIBC_INLINE char getc() {
+    ++cur_chars_read;
+    if (rb != nullptr) {
+      char output = rb->buffer[rb->buff_cur];
+      ++(rb->buff_cur);
+      return output;
+    }
+    // This should reset the buffer if applicable.
+    return static_cast<char>(stream_getc(input_stream));
+  }
 
   // This moves the input back by one character, placing c into the buffer if
   // this is a file reader, else c is ignored.
   void ungetc(char c);
 
   size_t chars_read() { return cur_chars_read; }
-
-  bool has_error();
 };
 
 } // namespace scanf_core

diff --git a/libc/src/stdio/scanf_core/scanf_main.cpp b/libc/src/stdio/scanf_core/scanf_main.cpp
@@ -38,11 +38,6 @@ int scanf_main(Reader *reader, const char *__restrict str,
     }
   }
 
-  if (conversions == 0 && reader->has_error()) {
-    // This is intended to be converted to EOF in the client call to avoid
-    // including stdio.h in this internal file.
-    return -1;
-  }
   return conversions;
 }
 

diff --git a/libc/src/stdio/scanf_core/string_reader.cpp b/libc/src/stdio/scanf_core/string_reader.cpp
diff --git a/libc/src/stdio/scanf_core/string_reader.h b/libc/src/stdio/scanf_core/string_reader.h
diff --git a/libc/src/stdio/scanf_core/vfscanf_internal.cpp b/libc/src/stdio/scanf_core/vfscanf_internal.cpp
diff --git a/libc/src/stdio/scanf_core/vfscanf_internal.h b/libc/src/stdio/scanf_core/vfscanf_internal.h
@@ -9,15 +9,63 @@
 #ifndef LLVM_LIBC_SRC_STDIO_SCANF_CORE_VFSCANF_INTERNAL_H
 #define LLVM_LIBC_SRC_STDIO_SCANF_CORE_VFSCANF_INTERNAL_H
 
+#include "src/__support/File/file.h"
 #include "src/__support/arg_list.h"
+#include "src/stdio/scanf_core/reader.h"
+#include "src/stdio/scanf_core/scanf_main.h"
 
 #include <stdio.h>
 
 namespace __llvm_libc {
+
+namespace internal {
+
+#ifndef LIBC_COPT_STDIO_USE_SYSTEM_FILE
+
+LIBC_INLINE int getc(void *f) {
+  unsigned char c;
+  auto result = reinterpret_cast<__llvm_libc::File *>(f)->read_unlocked(&c, 1);
+  size_t r = result.value;
+  if (result.has_error() || r != 1)
+    return '\0';
+
+  return c;
+}
+
+LIBC_INLINE void ungetc(int c, void *f) {
+  reinterpret_cast<__llvm_libc::File *>(f)->ungetc(c);
+}
+
+LIBC_INLINE int ferror_unlocked(FILE *f) {
+  return reinterpret_cast<__llvm_libc::File *>(f)->error_unlocked();
+}
+
+#else // defined(LIBC_COPT_STDIO_USE_SYSTEM_FILE)
+
+LIBC_INLINE int getc(void *f) { return ::getc(reinterpret_cast<::FILE *>(f)); }
+
+LIBC_INLINE void ungetc(int c, void *f) {
+  ::ungetc(c, reinterpret_cast<::FILE *>(f));
+}
+
+LIBC_INLINE int ferror_unlocked(::FILE *f) { return ::ferror_unlocked(f); }
+
+#endif // LIBC_COPT_STDIO_USE_SYSTEM_FILE
+
+} // namespace internal
+
 namespace scanf_core {
 
-int vfscanf_internal(::FILE *__restrict stream, const char *__restrict format,
-                     internal::ArgList &args);
+LIBC_INLINE int vfscanf_internal(::FILE *__restrict stream,
+                                 const char *__restrict format,
+                                 internal::ArgList &args) {
+  scanf_core::Reader reader(stream, &internal::getc, internal::ungetc);
+  int retval = scanf_core::scanf_main(&reader, format, args);
+  if (retval == 0 && internal::ferror_unlocked(stream))
+    return EOF;
+
+  return retval;
+}
 } // namespace scanf_core
 } // namespace __llvm_libc
 

diff --git a/libc/src/stdio/sscanf.cpp b/libc/src/stdio/sscanf.cpp
@@ -8,10 +8,10 @@
 
 #include "src/stdio/sscanf.h"
 
+#include "src/__support/CPP/limits.h"
 #include "src/__support/arg_list.h"
 #include "src/stdio/scanf_core/reader.h"
 #include "src/stdio/scanf_core/scanf_main.h"
-#include "src/stdio/scanf_core/string_reader.h"
 
 #include <stdarg.h>
 #include <stdio.h>
@@ -27,8 +27,9 @@ LLVM_LIBC_FUNCTION(int, sscanf,
                                  // and pointer semantics, as well as handling
                                  // destruction automatically.
   va_end(vlist);
-  scanf_core::StringReader string_reader(buffer);
-  scanf_core::Reader reader(&string_reader);
+  scanf_core::ReadBuffer rb{const_cast<char *>(buffer),
+                            cpp::numeric_limits<size_t>::max()};
+  scanf_core::Reader reader(&rb);
   int ret_val = scanf_core::scanf_main(&reader, format, args);
   // This is done to avoid including stdio.h in the internals. On most systems
   // EOF is -1, so this will be transformed into just "return ret_val".

diff --git a/libc/src/stdio/vprintf.cpp b/libc/src/stdio/vprintf.cpp
@@ -15,11 +15,11 @@
 #include <stdarg.h>
 #include <stdio.h>
 
-#ifndef LIBC_COPT_PRINTF_USE_SYSTEM_FILE
+#ifndef LIBC_COPT_STDIO_USE_SYSTEM_FILE
 #define PRINTF_STDOUT __llvm_libc::stdout
-#else // LIBC_COPT_PRINTF_USE_SYSTEM_FILE
+#else // LIBC_COPT_STDIO_USE_SYSTEM_FILE
 #define PRINTF_STDOUT ::stdout
-#endif // LIBC_COPT_PRINTF_USE_SYSTEM_FILE
+#endif // LIBC_COPT_STDIO_USE_SYSTEM_FILE
 
 namespace __llvm_libc {
 

diff --git a/libc/test/src/stdio/CMakeLists.txt b/libc/test/src/stdio/CMakeLists.txt
@@ -160,7 +160,7 @@ if(LLVM_LIBC_FULL_BUILD)
   set(hermetic_test_only HERMETIC_TEST_ONLY)
 else()
 # Else in overlay mode they use the system's FILE.
- set(fprintf_test_copts "-DLIBC_COPT_PRINTF_USE_SYSTEM_FILE")
+ set(use_system_file "-DLIBC_COPT_STDIO_USE_SYSTEM_FILE")
 endif()
 
 add_libc_unittest(
@@ -173,7 +173,7 @@ add_libc_unittest(
     libc.src.stdio.fprintf
     ${fprintf_test_deps}
   COMPILE_OPTIONS
-    ${fprintf_test_copts}
+    ${use_system_file}
 )
 
 add_libc_test(
@@ -218,7 +218,7 @@ add_libc_unittest(
     libc.src.stdio.vfprintf
     ${fprintf_test_deps}
   COMPILE_OPTIONS
-    ${fprintf_test_copts}
+    ${use_system_file}
 )
 
 add_libc_test(
@@ -232,6 +232,17 @@ add_libc_test(
     libc.src.stdio.vprintf
 )
 
+
+if(LLVM_LIBC_FULL_BUILD)
+  # In fullbuild mode, fscanf's tests use the internal FILE for other functions.
+  list(APPEND fscanf_test_deps
+       libc.src.stdio.fclose
+       libc.src.stdio.ferror
+       libc.src.stdio.fopen
+       libc.src.stdio.fwrite
+  )
+endif()
+
 add_libc_unittest(
   fscanf_test
   SUITE
@@ -240,11 +251,10 @@ add_libc_unittest(
     fscanf_test.cpp
   DEPENDS
     libc.src.stdio.fscanf
-    libc.src.stdio.fclose
-    libc.src.stdio.ferror
-    libc.src.stdio.fopen
-    libc.src.stdio.fwrite
+    ${fscanf_test_deps}
     libc.src.__support.CPP.string_view
+  COMPILE_OPTIONS
+    ${use_system_file}
 )
 
 add_libc_unittest(

diff --git a/libc/test/src/stdio/fprintf_test.cpp b/libc/test/src/stdio/fprintf_test.cpp
@@ -6,12 +6,12 @@
 //
 //===----------------------------------------------------------------------===//
 
-#ifndef LIBC_COPT_PRINTF_USE_SYSTEM_FILE
+#ifndef LIBC_COPT_STDIO_USE_SYSTEM_FILE
 #include "src/stdio/fclose.h"
 #include "src/stdio/ferror.h"
 #include "src/stdio/fopen.h"
 #include "src/stdio/fread.h"
-#endif // LIBC_COPT_PRINTF_USE_SYSTEM_FILE
+#endif // LIBC_COPT_STDIO_USE_SYSTEM_FILE
 
 #include "src/stdio/fprintf.h"
 
@@ -20,17 +20,17 @@
 #include <stdio.h>
 
 namespace printf_test {
-#ifndef LIBC_COPT_PRINTF_USE_SYSTEM_FILE
+#ifndef LIBC_COPT_STDIO_USE_SYSTEM_FILE
 using __llvm_libc::fclose;
 using __llvm_libc::ferror;
 using __llvm_libc::fopen;
 using __llvm_libc::fread;
-#else  // defined(LIBC_COPT_PRINTF_USE_SYSTEM_FILE)
+#else  // defined(LIBC_COPT_STDIO_USE_SYSTEM_FILE)
 using ::fclose;
 using ::ferror;
 using ::fopen;
 using ::fread;
-#endif // LIBC_COPT_PRINTF_USE_SYSTEM_FILE
+#endif // LIBC_COPT_STDIO_USE_SYSTEM_FILE
 } // namespace printf_test
 
 TEST(LlvmLibcFPrintfTest, WriteToFile) {

diff --git a/libc/test/src/stdio/fscanf_test.cpp b/libc/test/src/stdio/fscanf_test.cpp
@@ -7,47 +7,65 @@
 //===----------------------------------------------------------------------===//
 
 #include "src/__support/CPP/string_view.h"
+
+#ifndef LIBC_COPT_STDIO_USE_SYSTEM_FILE
 #include "src/stdio/fclose.h"
 #include "src/stdio/ferror.h"
 #include "src/stdio/fopen.h"
 #include "src/stdio/fwrite.h"
+#endif // LIBC_COPT_STDIO_USE_SYSTEM_FILE
 
 #include "src/stdio/fscanf.h"
 
 #include "test/UnitTest/Test.h"
 
 #include <stdio.h>
 
+namespace scanf_test {
+#ifndef LIBC_COPT_STDIO_USE_SYSTEM_FILE
+using __llvm_libc::fclose;
+using __llvm_libc::ferror;
+using __llvm_libc::fopen;
+using __llvm_libc::fwrite;
+#else  // defined(LIBC_COPT_STDIO_USE_SYSTEM_FILE)
+using ::fclose;
+using ::ferror;
+using ::fopen;
+using ::fwrite;
+#endif // LIBC_COPT_STDIO_USE_SYSTEM_FILE
+} // namespace scanf_test
+
 TEST(LlvmLibcFScanfTest, WriteToFile) {
-  constexpr char FILENAME[] = "testdata/fscanf_output.test";
-  ::FILE *file = __llvm_libc::fopen(FILENAME, "w");
+  const char *FILENAME = "fscanf_output.test";
+  auto FILE_PATH = libc_make_test_file_path(FILENAME);
+  ::FILE *file = scanf_test::fopen(FILE_PATH, "w");
   ASSERT_FALSE(file == nullptr);
 
   int read;
 
   constexpr char simple[] = "A simple string with no conversions.\n";
 
   ASSERT_EQ(sizeof(simple) - 1,
-            __llvm_libc::fwrite(simple, 1, sizeof(simple) - 1, file));
+            scanf_test::fwrite(simple, 1, sizeof(simple) - 1, file));
 
   constexpr char numbers[] = "1234567890\n";
 
   ASSERT_EQ(sizeof(numbers) - 1,
-            __llvm_libc::fwrite(numbers, 1, sizeof(numbers) - 1, file));
+            scanf_test::fwrite(numbers, 1, sizeof(numbers) - 1, file));
 
   constexpr char numbers_and_more[] = "1234 and more\n";
 
   ASSERT_EQ(sizeof(numbers_and_more) - 1,
-            __llvm_libc::fwrite(numbers_and_more, 1,
-                                sizeof(numbers_and_more) - 1, file));
+            scanf_test::fwrite(numbers_and_more, 1,
+                               sizeof(numbers_and_more) - 1, file));
 
   read =
       __llvm_libc::fscanf(file, "Reading from a write-only file should fail.");
   EXPECT_LT(read, 0);
 
-  ASSERT_EQ(0, __llvm_libc::fclose(file));
+  ASSERT_EQ(0, scanf_test::fclose(file));
 
-  file = __llvm_libc::fopen(FILENAME, "r");
+  file = scanf_test::fopen(FILE_PATH, "r");
   ASSERT_FALSE(file == nullptr);
 
   char data[50];
@@ -67,6 +85,6 @@ TEST(LlvmLibcFScanfTest, WriteToFile) {
   ASSERT_EQ(__llvm_libc::cpp::string_view(numbers_and_more),
             __llvm_libc::cpp::string_view(data, sizeof(numbers_and_more) - 1));
 
-  ASSERT_EQ(__llvm_libc::ferror(file), 0);
-  ASSERT_EQ(__llvm_libc::fclose(file), 0);
+  ASSERT_EQ(scanf_test::ferror(file), 0);
+  ASSERT_EQ(scanf_test::fclose(file), 0);
 }
diff --git a/libc/test/src/stdio/scanf_core/CMakeLists.txt b/libc/test/src/stdio/scanf_core/CMakeLists.txt
@@ -13,24 +13,23 @@ add_libc_unittest(
     libc.src.__support.arg_list
 )
 
-if(NOT (TARGET libc.src.__support.File.file))
-  # Not all platforms have a file implementation. If file is unvailable,
-  # then we must skip all the parts that need file.
-  return()
-endif()
-
 add_libc_unittest(
-  string_reader_test
+  reader_test
   SUITE
     libc_stdio_unittests
   SRCS
-    string_reader_test.cpp
+    reader_test.cpp
   DEPENDS
     libc.src.stdio.scanf_core.reader
-    libc.src.stdio.scanf_core.string_reader
     libc.src.__support.CPP.string_view
 )
 
+if(NOT (TARGET libc.src.__support.File.file))
+  # Not all platforms have a file implementation. If file is unvailable,
+  # then we must skip all the parts that need file.
+  return()
+endif()
+
 add_libc_unittest(
   converter_test
   SUITE

diff --git a/...c/stdio/scanf_core/string_reader_test.cpp → ...test/src/stdio/scanf_core/reader_test.cpp b/...c/stdio/scanf_core/string_reader_test.cpp → ...test/src/stdio/scanf_core/reader_test.cpp
@@ -8,20 +8,21 @@
 
 #include "src/__support/CPP/string_view.h"
 #include "src/stdio/scanf_core/reader.h"
-#include "src/stdio/scanf_core/string_reader.h"
 
 #include "test/UnitTest/Test.h"
 
 TEST(LlvmLibcScanfStringReaderTest, Constructor) {
   char str[10];
-  __llvm_libc::scanf_core::StringReader str_reader(str);
-  __llvm_libc::scanf_core::Reader reader(&str_reader);
+  // buff_len justneeds to be a big number. The specific value isn't important
+  // in the real world.
+  __llvm_libc::scanf_core::ReadBuffer rb{const_cast<char *>(str), 1000000};
+  __llvm_libc::scanf_core::Reader reader(&rb);
 }
 
 TEST(LlvmLibcScanfStringReaderTest, SimpleRead) {
   const char *str = "abc";
-  __llvm_libc::scanf_core::StringReader str_reader(str);
-  __llvm_libc::scanf_core::Reader reader(&str_reader);
+  __llvm_libc::scanf_core::ReadBuffer rb{const_cast<char *>(str), 1000000};
+  __llvm_libc::scanf_core::Reader reader(&rb);
 
   for (size_t i = 0; i < sizeof("abc"); ++i) {
     ASSERT_EQ(str[i], reader.getc());
@@ -30,8 +31,8 @@ TEST(LlvmLibcScanfStringReaderTest, SimpleRead) {
 
 TEST(LlvmLibcScanfStringReaderTest, ReadAndReverse) {
   const char *str = "abcDEF123";
-  __llvm_libc::scanf_core::StringReader str_reader(str);
-  __llvm_libc::scanf_core::Reader reader(&str_reader);
+  __llvm_libc::scanf_core::ReadBuffer rb{const_cast<char *>(str), 1000000};
+  __llvm_libc::scanf_core::Reader reader(&rb);
 
   for (size_t i = 0; i < 5; ++i) {
     ASSERT_EQ(str[i], reader.getc());

diff --git a/libc/test/src/stdio/vfprintf_test.cpp b/libc/test/src/stdio/vfprintf_test.cpp
@@ -10,12 +10,12 @@
 // because these functions are identical in every way except for how the varargs
 // are passed.
 
-#ifndef LIBC_COPT_PRINTF_USE_SYSTEM_FILE
+#ifndef LIBC_COPT_STDIO_USE_SYSTEM_FILE
 #include "src/stdio/fclose.h"
 #include "src/stdio/ferror.h"
 #include "src/stdio/fopen.h"
 #include "src/stdio/fread.h"
-#endif // LIBC_COPT_PRINTF_USE_SYSTEM_FILE
+#endif // LIBC_COPT_STDIO_USE_SYSTEM_FILE
 
 #include "src/stdio/vfprintf.h"
 
@@ -24,17 +24,17 @@
 #include <stdio.h>
 
 namespace printf_test {
-#ifndef LIBC_COPT_PRINTF_USE_SYSTEM_FILE
+#ifndef LIBC_COPT_STDIO_USE_SYSTEM_FILE
 using __llvm_libc::fclose;
 using __llvm_libc::ferror;
 using __llvm_libc::fopen;
 using __llvm_libc::fread;
-#else  // defined(LIBC_COPT_PRINTF_USE_SYSTEM_FILE)
+#else  // defined(LIBC_COPT_STDIO_USE_SYSTEM_FILE)
 using ::fclose;
 using ::ferror;
 using ::fopen;
 using ::fread;
-#endif // LIBC_COPT_PRINTF_USE_SYSTEM_FILE
+#endif // LIBC_COPT_STDIO_USE_SYSTEM_FILE
 } // namespace printf_test
 
 int call_vfprintf(::FILE *__restrict stream, const char *__restrict format,

diff --git a/utils/bazel/llvm-project-overlay/libc/BUILD.bazel b/utils/bazel/llvm-project-overlay/libc/BUILD.bazel
@@ -25,7 +25,7 @@ package(
 licenses(["notice"])
 
 PRINTF_COPTS = [
-    "LIBC_COPT_PRINTF_USE_SYSTEM_FILE",
+    "LIBC_COPT_STDIO_USE_SYSTEM_FILE",
     "LIBC_COPT_PRINTF_DISABLE_INDEX_MODE",
     "LIBC_COPT_PRINTF_DISABLE_WRITE_INT",
 ]