diff --git a/libc/config/linux/x86_64/entrypoints.txt b/libc/config/linux/x86_64/entrypoints.txt
--- a/libc/config/linux/x86_64/entrypoints.txt
+++ b/libc/config/linux/x86_64/entrypoints.txt
@@ -127,6 +127,9 @@
     libc.src.stdio.snprintf
     libc.src.stdio.fprintf
     libc.src.stdio.printf
+    libc.src.stdio.sscanf
+    libc.src.stdio.scanf
+    libc.src.stdio.fscanf
 
     # sys/mman.h entrypoints
     libc.src.sys.mman.madvise
@@ -433,9 +436,6 @@
     libc.src.stdio.getc_unlocked
     libc.src.stdio.getchar
     libc.src.stdio.getchar_unlocked
-    libc.src.stdio.sscanf
-    libc.src.stdio.scanf
-    libc.src.stdio.fscanf
     libc.src.stdio.putc
     libc.src.stdio.putchar
     libc.src.stdio.puts
diff --git a/libc/src/stdio/CMakeLists.txt b/libc/src/stdio/CMakeLists.txt
--- a/libc/src/stdio/CMakeLists.txt
+++ b/libc/src/stdio/CMakeLists.txt
@@ -460,6 +460,19 @@
     libc.src.stdio.scanf_core.scanf_main
 )
 
+list(APPEND scanf_deps 
+      libc.src.__support.arg_list 
+      libc.src.stdio.scanf_core.vfscanf_internal
+)
+if(LLVM_LIBC_FULL_BUILD)
+ list(APPEND scanf_deps  
+      libc.src.__support.File.file 
+      libc.src.__support.File.platform_file 
+  )
+else()
+ set(scanf_copts "-DLIBC_COPT_SCANF_USE_SYSTEM_FILE")
+endif()
+
 add_entrypoint_object(
   fscanf
   SRCS
@@ -467,8 +480,9 @@
   HDRS
     fscanf.h
   DEPENDS
-    libc.src.__support.arg_list
-    libc.src.stdio.scanf_core.vfscanf_internal
+    ${scanf_deps}
+  COMPILE_OPTIONS
+    ${scanf_copts}
 )
 
 add_entrypoint_object(
@@ -478,8 +492,9 @@
   HDRS
     scanf.h
   DEPENDS
-    libc.src.__support.arg_list
-    libc.src.stdio.scanf_core.vfscanf_internal
+    ${scanf_deps}
+  COMPILE_OPTIONS
+    ${scanf_copts}
 )
 
 add_entrypoint_object(
@@ -538,8 +553,7 @@
   HDRS
     fprintf.h
   DEPENDS
-    libc.src.__support.arg_list
-    libc.src.stdio.printf_core.vfprintf_internal
+    ${printf_deps}
   COMPILE_OPTIONS
     ${printf_copts}
 )
diff --git a/libc/src/stdio/fscanf.cpp b/libc/src/stdio/fscanf.cpp
--- a/libc/src/stdio/fscanf.cpp
+++ b/libc/src/stdio/fscanf.cpp
@@ -15,6 +15,12 @@
 #include <stdarg.h>
 #include <stdio.h>
 
+#ifndef LIBC_COPT_SCANF_USE_SYSTEM_FILE
+using FileT = __llvm_libc::File;
+#else  // defined(LIBC_COPT_SCANF_USE_SYSTEM_FILE)
+using FileT = ::FILE;
+#endif // LIBC_COPT_SCANF_USE_SYSTEM_FILE
+
 namespace __llvm_libc {
 
 LLVM_LIBC_FUNCTION(int, fscanf,
@@ -26,7 +32,8 @@
                                  // and pointer semantics, as well as handling
                                  // destruction automatically.
   va_end(vlist);
-  int ret_val = scanf_core::vfscanf_internal(stream, format, args);
+  int ret_val = scanf_core::vfscanf_internal(reinterpret_cast<FileT *>(stream),
+                                             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".
   return (ret_val == -1) ? EOF : ret_val;
diff --git a/libc/src/stdio/printf_core/CMakeLists.txt b/libc/src/stdio/printf_core/CMakeLists.txt
--- a/libc/src/stdio/printf_core/CMakeLists.txt
+++ b/libc/src/stdio/printf_core/CMakeLists.txt
@@ -139,8 +139,6 @@
   HDRS
     vfprintf_internal.h
   DEPENDS
-    libc.include.stdio
-    libc.src.__support.File.file
     libc.src.__support.arg_list
     libc.src.stdio.printf_core.printf_main
     libc.src.stdio.printf_core.file_writer
diff --git a/libc/src/stdio/printf_core/vfprintf_internal.h b/libc/src/stdio/printf_core/vfprintf_internal.h
--- a/libc/src/stdio/printf_core/vfprintf_internal.h
+++ b/libc/src/stdio/printf_core/vfprintf_internal.h
@@ -9,15 +9,12 @@
 #ifndef LLVM_LIBC_SRC_STDIO_PRINTF_CORE_VFPRINTF_INTERNAL_H
 #define LLVM_LIBC_SRC_STDIO_PRINTF_CORE_VFPRINTF_INTERNAL_H
 
-#include "src/__support/File/file.h"
 #include "src/__support/arg_list.h"
 #include "src/__support/macros/attributes.h" // For LIBC_INLINE
 #include "src/stdio/printf_core/file_writer.h"
 #include "src/stdio/printf_core/printf_main.h"
 #include "src/stdio/printf_core/writer.h"
 
-#include <stdio.h>
-
 namespace __llvm_libc {
 namespace printf_core {
 
diff --git a/libc/src/stdio/scanf.cpp b/libc/src/stdio/scanf.cpp
--- a/libc/src/stdio/scanf.cpp
+++ b/libc/src/stdio/scanf.cpp
@@ -8,13 +8,19 @@
 
 #include "src/stdio/scanf.h"
 
-#include "src/__support/File/file.h"
 #include "src/__support/arg_list.h"
 #include "src/stdio/scanf_core/vfscanf_internal.h"
 
 #include <stdarg.h>
 #include <stdio.h>
 
+#ifndef LIBC_COPT_SCANF_USE_SYSTEM_FILE
+#include "src/__support/File/file.h"
+#define SCANF_STDIN __llvm_libc::stdin
+#else // LIBC_COPT_SCANF_USE_SYSTEM_FILE
+#define SCANF_STDIN ::stdin
+#endif // LIBC_COPT_SCANF_USE_SYSTEM_FILE
+
 namespace __llvm_libc {
 
 LLVM_LIBC_FUNCTION(int, scanf, (const char *__restrict format, ...)) {
@@ -24,8 +30,7 @@
                                  // and pointer semantics, as well as handling
                                  // destruction automatically.
   va_end(vlist);
-  int ret_val = scanf_core::vfscanf_internal(
-      reinterpret_cast<::FILE *>(__llvm_libc::stdin), format, args);
+  int ret_val = scanf_core::vfscanf_internal(SCANF_STDIN, 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".
   return (ret_val == -1) ? EOF : ret_val;
diff --git a/libc/src/stdio/scanf_core/CMakeLists.txt b/libc/src/stdio/scanf_core/CMakeLists.txt
--- a/libc/src/stdio/scanf_core/CMakeLists.txt
+++ b/libc/src/stdio/scanf_core/CMakeLists.txt
@@ -24,9 +24,9 @@
     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.
+if(NOT (TARGET libc.src.__support.File.file) AND LLVM_LIBC_FULL_BUILD)
+  # Not all platforms have a file implementation. If file is unvailable, and a 
+  # full build is requested, then we must skip all file based printf sections.
   return()
 endif()
 
@@ -52,13 +52,12 @@
     string_reader.h
 )
 
-add_object_library(
+add_header_library(
   file_reader
-  SRCS
-    file_reader.cpp
   HDRS
     file_reader.h
   DEPENDS
+    libc.include.stdio
     libc.src.__support.File.file
 )
 
@@ -101,10 +100,8 @@
     libc.src.__support.str_to_float
 )
 
-add_object_library(
+add_header_library(
   vfscanf_internal
-  SRCS
-    vfscanf_internal.cpp
   HDRS
     vfscanf_internal.h
   DEPENDS
diff --git a/libc/src/stdio/scanf_core/converter.cpp b/libc/src/stdio/scanf_core/converter.cpp
--- a/libc/src/stdio/scanf_core/converter.cpp
+++ b/libc/src/stdio/scanf_core/converter.cpp
@@ -94,7 +94,9 @@
       }
     }
   }
-  reader->ungetc(cur_char);
+  if (!reader->ungetc(cur_char)) {
+    return UNGET_ERROR;
+  }
   return ret_val;
 }
 
diff --git a/libc/src/stdio/scanf_core/core_structs.h b/libc/src/stdio/scanf_core/core_structs.h
--- a/libc/src/stdio/scanf_core/core_structs.h
+++ b/libc/src/stdio/scanf_core/core_structs.h
@@ -85,6 +85,7 @@
   FILE_STATUS_ERROR = -2,
   MATCHING_FAILURE = -3,
   ALLOCATION_FAILURE = -4,
+  UNGET_ERROR = -5,
 };
 } // namespace scanf_core
 } // namespace __llvm_libc
diff --git a/libc/src/stdio/scanf_core/file_reader.h b/libc/src/stdio/scanf_core/file_reader.h
--- a/libc/src/stdio/scanf_core/file_reader.h
+++ b/libc/src/stdio/scanf_core/file_reader.h
@@ -17,22 +17,68 @@
 namespace __llvm_libc {
 namespace scanf_core {
 
-class FileReader {
-  __llvm_libc::File *file;
+template <typename file_t> class FileReader {
+  file_t *file;
 
 public:
-  FileReader(::FILE *init_file) {
-    file = reinterpret_cast<__llvm_libc::File *>(init_file);
-    file->lock();
-  }
+  FileReader(file_t *init_file);
 
-  ~FileReader() { file->unlock(); }
+  ~FileReader();
 
   char get_char();
-  void unget_char(char c);
-  bool has_error() { return file->error_unlocked(); }
+  bool unget_char(char c); // Returns true on success, false on failure.
+  bool has_error();
 };
 
+// The interface for using our internal file implementation.
+template <>
+LIBC_INLINE
+FileReader<__llvm_libc::File>::FileReader(__llvm_libc::File *init_file) {
+  file = init_file;
+  file->lock();
+}
+template <> LIBC_INLINE FileReader<__llvm_libc::File>::~FileReader() {
+  file->unlock();
+}
+
+template <> LIBC_INLINE char FileReader<__llvm_libc::File>::get_char() {
+  char tiny_buff = 0;
+  auto result = file->read_unlocked(&tiny_buff, 1);
+  if (result.value != 1 || result.has_error())
+    return 0;
+  return tiny_buff;
+}
+template <> LIBC_INLINE bool FileReader<__llvm_libc::File>::has_error() {
+  return file->error_unlocked();
+}
+template <> LIBC_INLINE bool FileReader<__llvm_libc::File>::unget_char(char c) {
+  int result = file->ungetc_unlocked(c);
+  return !(result == EOF || this->has_error());
+}
+
+// The interface for using the system's file implementation.
+template <> LIBC_INLINE FileReader<::FILE>::FileReader(::FILE *init_file) {
+  file = init_file;
+  ::flockfile(file);
+}
+template <> LIBC_INLINE FileReader<::FILE>::~FileReader() {
+  ::funlockfile(file);
+}
+
+template <> LIBC_INLINE char FileReader<::FILE>::get_char() {
+  int result = ::fgetc_unlocked(file);
+  if (result == EOF || ::ferror_unlocked(file))
+    return 0;
+  return static_cast<char>(result);
+}
+template <> LIBC_INLINE bool FileReader<::FILE>::has_error() {
+  return ::ferror(file) != 0;
+}
+template <> LIBC_INLINE bool FileReader<::FILE>::unget_char(char c) {
+  int result = ::ungetc(c, file); // There is no unlocked version of ungetc.
+  return !(result == EOF || this->has_error());
+}
+
 } // namespace scanf_core
 } // namespace __llvm_libc
 
diff --git a/libc/src/stdio/scanf_core/file_reader.cpp b/libc/src/stdio/scanf_core/file_reader.cpp
deleted file mode 100644
--- a/libc/src/stdio/scanf_core/file_reader.cpp
+++ /dev/null
@@ -1,27 +0,0 @@
-//===-- FILE Reader implementation for scanf --------------------*- C++ -*-===//
-//
-// 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 "src/stdio/scanf_core/file_reader.h"
-#include "src/__support/File/file.h"
-#include <stddef.h>
-
-namespace __llvm_libc {
-namespace scanf_core {
-
-char FileReader::get_char() {
-  char tiny_buff = 0;
-  auto result = file->read_unlocked(&tiny_buff, 1);
-  if (result.value != 1 || result.has_error())
-    return 0;
-  return tiny_buff;
-}
-
-void FileReader::unget_char(char c) { file->ungetc_unlocked(c); }
-
-} // namespace scanf_core
-} // namespace __llvm_libc
diff --git a/libc/src/stdio/scanf_core/float_converter.cpp b/libc/src/stdio/scanf_core/float_converter.cpp
--- a/libc/src/stdio/scanf_core/float_converter.cpp
+++ b/libc/src/stdio/scanf_core/float_converter.cpp
@@ -210,7 +210,9 @@
 
   // We always read one more character than will be used, so we have to put the
   // last one back.
-  reader->ungetc(cur_char);
+  if (!reader->ungetc(cur_char)) {
+    return UNGET_ERROR;
+  }
 
   // If we haven't actually found any digits, this is a matching failure (this
   // catches cases like "+.")
diff --git a/libc/src/stdio/scanf_core/int_converter.cpp b/libc/src/stdio/scanf_core/int_converter.cpp
--- a/libc/src/stdio/scanf_core/int_converter.cpp
+++ b/libc/src/stdio/scanf_core/int_converter.cpp
@@ -144,7 +144,9 @@
       } else {
         // If the first character isn't a valid digit, then there are no valid
         // digits at all. The number is 0.
-        reader->ungetc(cur_char);
+        if (!reader->ungetc(cur_char)) {
+          return UNGET_ERROR;
+        }
         write_int_with_length(0, to_conv);
         return MATCHING_FAILURE;
       }
@@ -195,7 +197,9 @@
 
   // We always read one more character than will be used, so we have to put the
   // last one back.
-  reader->ungetc(cur_char);
+  if (!reader->ungetc(cur_char)) {
+    return UNGET_ERROR;
+  }
 
   if (has_overflow) {
     write_int_with_length(MAX, to_conv);
diff --git a/libc/src/stdio/scanf_core/ptr_converter.cpp b/libc/src/stdio/scanf_core/ptr_converter.cpp
--- a/libc/src/stdio/scanf_core/ptr_converter.cpp
+++ b/libc/src/stdio/scanf_core/ptr_converter.cpp
@@ -34,7 +34,9 @@
     return MATCHING_FAILURE;
   }
 
-  reader->ungetc(cur_char);
+  if (!reader->ungetc(cur_char)) {
+    return UNGET_ERROR;
+  }
 
   // Else treat it as a hex int
   return convert_int(reader, to_conv);
diff --git a/libc/src/stdio/scanf_core/reader.h b/libc/src/stdio/scanf_core/reader.h
--- a/libc/src/stdio/scanf_core/reader.h
+++ b/libc/src/stdio/scanf_core/reader.h
@@ -16,12 +16,13 @@
 namespace __llvm_libc {
 namespace scanf_core {
 
-enum class ReaderType { String, File };
+enum class ReaderType { String, SystemFile, InternalFile };
 
 class Reader final {
   union {
     StringReader *string_reader;
-    FileReader *file_reader;
+    FileReader<::FILE> *system_file_reader;
+    FileReader<__llvm_libc::File> *internal_file_reader;
   };
 
   const ReaderType reader_type;
@@ -32,8 +33,13 @@
   Reader(StringReader *init_string_reader)
       : string_reader(init_string_reader), reader_type(ReaderType::String) {}
 
-  Reader(FileReader *init_file_reader)
-      : file_reader(init_file_reader), reader_type(ReaderType::File) {}
+  Reader(FileReader<__llvm_libc::File> *init_file_reader)
+      : internal_file_reader(init_file_reader),
+        reader_type(ReaderType::InternalFile) {}
+
+  Reader(FileReader<::FILE> *init_file_reader)
+      : system_file_reader(init_file_reader),
+        reader_type(ReaderType::SystemFile) {}
 
   // 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
@@ -41,8 +47,8 @@
   char getc();
 
   // 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);
+  // this is a file reader, else c is ignored. It returns true on success.
+  bool ungetc(char c);
 
   size_t chars_read() { return cur_chars_read; }
 
diff --git a/libc/src/stdio/scanf_core/reader.cpp b/libc/src/stdio/scanf_core/reader.cpp
--- a/libc/src/stdio/scanf_core/reader.cpp
+++ b/libc/src/stdio/scanf_core/reader.cpp
@@ -14,30 +14,41 @@
 
 char Reader::getc() {
   ++cur_chars_read;
-  if (reader_type == ReaderType::String) {
+  switch (reader_type) {
+  case ReaderType::String:
     return string_reader->get_char();
-  } else {
-    return file_reader->get_char();
+  case ReaderType::InternalFile:
+    return internal_file_reader->get_char();
+  case ReaderType::SystemFile:
+    return system_file_reader->get_char();
   }
 }
 
-void Reader::ungetc(char c) {
+bool Reader::ungetc(char c) {
   --cur_chars_read;
-  if (reader_type == ReaderType::String) {
+  switch (reader_type) {
+  case ReaderType::String:
     // The string reader ignores the char c passed to unget since it doesn't
     // need to place anything back into a buffer, and modifying the source
     // string would be dangerous.
     return string_reader->unget_char();
-  } else {
-    return file_reader->unget_char(c);
+  case ReaderType::InternalFile:
+    return internal_file_reader->unget_char(c);
+  case ReaderType::SystemFile:
+    return system_file_reader->unget_char(c);
   }
 }
 
 bool Reader::has_error() {
-  if (reader_type == ReaderType::File) {
-    return file_reader->has_error();
+  switch (reader_type) {
+  case ReaderType::String:
+    // The string reader can't error since it's just reading from a char*.
+    return false;
+  case ReaderType::InternalFile:
+    return internal_file_reader->has_error();
+  case ReaderType::SystemFile:
+    return system_file_reader->has_error();
   }
-  return false;
 }
 
 } // namespace scanf_core
diff --git a/libc/src/stdio/scanf_core/string_converter.cpp b/libc/src/stdio/scanf_core/string_converter.cpp
--- a/libc/src/stdio/scanf_core/string_converter.cpp
+++ b/libc/src/stdio/scanf_core/string_converter.cpp
@@ -56,7 +56,9 @@
 
   // We always read one more character than will be used, so we have to put the
   // last one back.
-  reader->ungetc(cur_char);
+  if (!reader->ungetc(cur_char)) {
+    return UNGET_ERROR;
+  }
 
   // If this is %s or %[]
   if (to_conv.conv_name != 'c' && (to_conv.flags & NO_WRITE) == 0) {
diff --git a/libc/src/stdio/scanf_core/string_reader.h b/libc/src/stdio/scanf_core/string_reader.h
--- a/libc/src/stdio/scanf_core/string_reader.h
+++ b/libc/src/stdio/scanf_core/string_reader.h
@@ -24,7 +24,7 @@
   ~StringReader() {}
 
   char get_char();
-  void unget_char();
+  bool unget_char();
 };
 
 } // namespace scanf_core
diff --git a/libc/src/stdio/scanf_core/string_reader.cpp b/libc/src/stdio/scanf_core/string_reader.cpp
--- a/libc/src/stdio/scanf_core/string_reader.cpp
+++ b/libc/src/stdio/scanf_core/string_reader.cpp
@@ -18,7 +18,10 @@
   return cur_char;
 }
 
-void StringReader::unget_char() { --cur_index; }
+bool StringReader::unget_char() {
+  --cur_index;
+  return true;
+}
 
 } // namespace scanf_core
 } // namespace __llvm_libc
diff --git a/libc/src/stdio/scanf_core/vfscanf_internal.h b/libc/src/stdio/scanf_core/vfscanf_internal.h
--- a/libc/src/stdio/scanf_core/vfscanf_internal.h
+++ b/libc/src/stdio/scanf_core/vfscanf_internal.h
@@ -10,14 +10,25 @@
 #define LLVM_LIBC_SRC_STDIO_SCANF_CORE_VFSCANF_INTERNAL_H
 
 #include "src/__support/arg_list.h"
+#include "src/__support/macros/attributes.h" // For LIBC_INLINE
+#include "src/stdio/scanf_core/file_reader.h"
+#include "src/stdio/scanf_core/reader.h"
+#include "src/stdio/scanf_core/scanf_main.h"
 
 #include <stdio.h>
 
 namespace __llvm_libc {
 namespace scanf_core {
 
-int vfscanf_internal(::FILE *__restrict stream, const char *__restrict format,
-                     internal::ArgList &args);
+template <typename file_t>
+LIBC_INLINE int vfscanf_internal(file_t *__restrict stream,
+                                 const char *__restrict format,
+                                 internal::ArgList &args) {
+  FileReader<file_t> file_reader(stream);
+  scanf_core::Reader reader(&file_reader);
+  return scanf_core::scanf_main(&reader, format, args);
+}
+
 } // namespace scanf_core
 } // namespace __llvm_libc
 
diff --git a/libc/src/stdio/scanf_core/vfscanf_internal.cpp b/libc/src/stdio/scanf_core/vfscanf_internal.cpp
deleted file mode 100644
--- a/libc/src/stdio/scanf_core/vfscanf_internal.cpp
+++ /dev/null
@@ -1,29 +0,0 @@
-//===-- Internal implementation of vfscanf ---------------------*- C++ -*-===//
-//
-// 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 "src/stdio/scanf_core/vfscanf_internal.h"
-
-#include "src/__support/arg_list.h"
-#include "src/stdio/scanf_core/file_reader.h"
-#include "src/stdio/scanf_core/reader.h"
-#include "src/stdio/scanf_core/scanf_main.h"
-
-#include <stdio.h>
-
-namespace __llvm_libc {
-namespace scanf_core {
-
-int vfscanf_internal(::FILE *__restrict stream, const char *__restrict format,
-                     internal::ArgList &args) {
-  FileReader file_reader(stream);
-  scanf_core::Reader reader(&file_reader);
-  return scanf_core::scanf_main(&reader, format, args);
-}
-
-} // namespace scanf_core
-} // namespace __llvm_libc
diff --git a/libc/test/src/stdio/CMakeLists.txt b/libc/test/src/stdio/CMakeLists.txt
--- a/libc/test/src/stdio/CMakeLists.txt
+++ b/libc/test/src/stdio/CMakeLists.txt
@@ -134,23 +134,22 @@
     libc.src.stdio.snprintf
 )
 
-# In fullbuild mode, fprintf's tests use the internal FILE for other functions.
+list(APPEND fprintf_test_deps
+  libc.src.stdio.fprintf
+)
 if(LLVM_LIBC_FULL_BUILD)
-add_libc_unittest(
-  fprintf_test
-  SUITE
-    libc_stdio_unittests
-  SRCS
-    fprintf_test.cpp
-  DEPENDS
-    libc.src.stdio.fprintf
+# In fullbuild mode, fprintf's tests use the internal FILE for other functions.
+  list(APPEND fprintf_test_deps
     libc.src.stdio.fclose
     libc.src.stdio.ferror
     libc.src.stdio.fopen
     libc.src.stdio.fread
-)
+  )
 else()
 # Else in overlay mode they use the system's FILE.
+ set(fprintf_test_copts "-DLIBC_COPT_PRINTF_USE_SYSTEM_FILE")
+endif()
+
 add_libc_unittest(
   fprintf_test
   SUITE
@@ -158,11 +157,10 @@
   SRCS
     fprintf_test.cpp
   DEPENDS
-    libc.src.stdio.fprintf
+    ${fprintf_test_deps}
   COMPILE_OPTIONS
-    -DLIBC_COPT_PRINTF_USE_SYSTEM_FILE
+    ${fprintf_test_copts}
 )
-endif()
 
 add_libc_unittest(
   printf_test
@@ -174,6 +172,23 @@
     libc.src.stdio.printf
 )
 
+list(APPEND fscanf_test_deps
+  libc.src.stdio.fscanf
+  libc.src.__support.CPP.string_view
+)
+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
+  )
+else()
+# Else in overlay mode they use the system's FILE.
+ set(fscanf_test_copts "-DLIBC_COPT_SCANF_USE_SYSTEM_FILE")
+endif()
+
 add_libc_unittest(
   fscanf_test
   SUITE
@@ -181,12 +196,9 @@
   SRCS
     fscanf_test.cpp
   DEPENDS
-    libc.src.stdio.fscanf
-    libc.src.stdio.fclose
-    libc.src.stdio.ferror
-    libc.src.stdio.fopen
-    libc.src.stdio.fwrite
-    libc.src.__support.CPP.string_view
+    ${fscanf_test_deps}
+  COMPILE_OPTIONS
+    ${fscanf_test_copts}
 )
 
 add_libc_unittest(
diff --git a/libc/test/src/stdio/fscanf_test.cpp b/libc/test/src/stdio/fscanf_test.cpp
--- a/libc/test/src/stdio/fscanf_test.cpp
+++ b/libc/test/src/stdio/fscanf_test.cpp
@@ -6,21 +6,41 @@
 //
 //===----------------------------------------------------------------------===//
 
-#include "src/__support/CPP/string_view.h"
+#ifndef LIBC_COPT_SCANF_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_SCANF_USE_SYSTEM_FILE
 
 #include "src/stdio/fscanf.h"
 
+#include "src/__support/CPP/string_view.h"
+
 #include "test/UnitTest/Test.h"
 
 #include <stdio.h>
 
+namespace scanf_test {
+#ifndef LIBC_COPT_SCANF_USE_SYSTEM_FILE
+using __llvm_libc::fclose;
+using __llvm_libc::ferror;
+using __llvm_libc::fopen;
+using __llvm_libc::fwrite;
+#else  // defined(LIBC_COPT_SCANF_USE_SYSTEM_FILE)
+using ::fclose;
+using ::ferror;
+using ::fopen;
+using ::fwrite;
+#endif // LIBC_COPT_SCANF_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;
@@ -28,26 +48,26 @@
   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(FILENAME, "r");
   ASSERT_FALSE(file == nullptr);
 
   char data[50];
@@ -67,6 +87,6 @@
   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);
 }