diff --git a/compiler-rt/lib/fuzzer/FuzzerIO.h b/compiler-rt/lib/fuzzer/FuzzerIO.h
--- a/compiler-rt/lib/fuzzer/FuzzerIO.h
+++ b/compiler-rt/lib/fuzzer/FuzzerIO.h
@@ -54,6 +54,10 @@
 
 void CloseStdout();
 
+// For testing.
+FILE *GetOutputFile();
+void SetOutputFile(FILE *NewOutputFile);
+
 void Printf(const char *Fmt, ...);
 void VPrintf(bool Verbose, const char *Fmt, ...);
 
diff --git a/compiler-rt/lib/fuzzer/FuzzerIO.cpp b/compiler-rt/lib/fuzzer/FuzzerIO.cpp
--- a/compiler-rt/lib/fuzzer/FuzzerIO.cpp
+++ b/compiler-rt/lib/fuzzer/FuzzerIO.cpp
@@ -23,6 +23,14 @@
 
 static FILE *OutputFile = stderr;
 
+FILE *GetOutputFile() {
+  return OutputFile;
+}
+
+void SetOutputFile(FILE *NewOutputFile) {
+  OutputFile = NewOutputFile;
+}
+
 long GetEpoch(const std::string &Path) {
   struct stat St;
   if (stat(Path.c_str(), &St))
diff --git a/compiler-rt/lib/fuzzer/FuzzerUtil.cpp b/compiler-rt/lib/fuzzer/FuzzerUtil.cpp
--- a/compiler-rt/lib/fuzzer/FuzzerUtil.cpp
+++ b/compiler-rt/lib/fuzzer/FuzzerUtil.cpp
@@ -43,7 +43,7 @@
   else if (Byte >= 32 && Byte < 127)
     Printf("%c", Byte);
   else
-    Printf("\\x%02x", Byte);
+    Printf("\\%03o", Byte);
 }
 
 void PrintASCII(const uint8_t *Data, size_t Size, const char *PrintAfter) {
diff --git a/compiler-rt/lib/fuzzer/tests/FuzzerUnittest.cpp b/compiler-rt/lib/fuzzer/tests/FuzzerUnittest.cpp
--- a/compiler-rt/lib/fuzzer/tests/FuzzerUnittest.cpp
+++ b/compiler-rt/lib/fuzzer/tests/FuzzerUnittest.cpp
@@ -591,6 +591,42 @@
   EXPECT_EQ("YWJjeHl6", Base64({'a', 'b', 'c', 'x', 'y', 'z'}));
 }
 
+#ifdef __GLIBC__
+class PrintfCapture {
+ public:
+  PrintfCapture() {
+    OldOutputFile = GetOutputFile();
+    SetOutputFile(open_memstream(&Buffer, &Size));
+  }
+  ~PrintfCapture() {
+    fclose(GetOutputFile());
+    SetOutputFile(OldOutputFile);
+    free(Buffer);
+  }
+  std::string str() { return std::string(Buffer, Size); }
+
+ private:
+  char *Buffer;
+  size_t Size;
+  FILE *OldOutputFile;
+};
+
+TEST(FuzzerUtil, PrintASCII) {
+  auto f = [](const char *Str, const char *PrintAfter = "") {
+    PrintfCapture Capture;
+    PrintASCII(reinterpret_cast<const uint8_t*>(Str), strlen(Str), PrintAfter);
+    return Capture.str();
+  };
+  EXPECT_EQ("hello", f("hello"));
+  EXPECT_EQ("c:\\\\", f("c:\\"));
+  EXPECT_EQ("\\\"hi\\\"", f("\"hi\""));
+  EXPECT_EQ("\\011a", f("\ta"));
+  EXPECT_EQ("\\0111", f("\t1"));
+  EXPECT_EQ("hello\\012", f("hello\n"));
+  EXPECT_EQ("hello\n", f("hello", "\n"));
+}
+#endif
+
 TEST(Corpus, Distribution) {
   DataFlowTrace DFT;
   Random Rand(0);