diff --git a/dom/media/AudioStream.cpp b/dom/media/AudioStream.cpp
index 24bd97dbaf75c..266379086a0c1 100644
--- a/dom/media/AudioStream.cpp
+++ b/dom/media/AudioStream.cpp
@@ -13,6 +13,7 @@
 #include "mozilla/Monitor.h"
 #include "mozilla/Mutex.h"
 #include "mozilla/Sprintf.h"
+#include "mozilla/Unused.h"
 #include <algorithm>
 #include "mozilla/Telemetry.h"
 #include "CubebUtils.h"
@@ -277,7 +278,7 @@ OpenDumpFile(uint32_t aChannels, uint32_t aRate)
   SetUint16LE(header + CHANNEL_OFFSET, aChannels);
   SetUint32LE(header + SAMPLE_RATE_OFFSET, aRate);
   SetUint16LE(header + BLOCK_ALIGN_OFFSET, aChannels * 2);
-  fwrite(header, sizeof(header), 1, f);
+  Unused << fwrite(header, sizeof(header), 1, f);
 
   return f;
 }
@@ -285,7 +286,7 @@ OpenDumpFile(uint32_t aChannels, uint32_t aRate)
 template <typename T>
 typename EnableIf<IsSame<T, int16_t>::value, void>::Type
 WriteDumpFileHelper(T* aInput, size_t aSamples, FILE* aFile) {
-  fwrite(aInput, sizeof(T), aSamples, aFile);
+  Unused << fwrite(aInput, sizeof(T), aSamples, aFile);
 }
 
 template <typename T>
@@ -297,7 +298,7 @@ WriteDumpFileHelper(T* aInput, size_t aSamples, FILE* aFile) {
   for (uint32_t i = 0; i < aSamples; ++i) {
     SetUint16LE(output + i*2, int16_t(aInput[i]*32767.0f));
   }
-  fwrite(output, 2, aSamples, aFile);
+  Unused << fwrite(output, 2, aSamples, aFile);
   fflush(aFile);
 }
 
diff --git a/dom/media/mediasource/ResourceQueue.cpp b/dom/media/mediasource/ResourceQueue.cpp
index 6e2621d9e0de5..a3481745de30c 100644
--- a/dom/media/mediasource/ResourceQueue.cpp
+++ b/dom/media/mediasource/ResourceQueue.cpp
@@ -11,6 +11,7 @@
 #include "mozilla/IntegerPrintfMacros.h"
 #include "mozilla/Logging.h"
 #include "mozilla/Sprintf.h"
+#include "mozilla/Unused.h"
 
 extern mozilla::LogModule* GetSourceBufferResourceLog();
 
@@ -171,7 +172,7 @@ ResourceQueue::Dump(const char* aPath)
     if (!fp) {
       return;
     }
-    fwrite(item->mData->Elements(), item->mData->Length(), 1, fp);
+    Unused << fwrite(item->mData->Elements(), item->mData->Length(), 1, fp);
     fclose(fp);
   }
 }
diff --git a/gfx/thebes/gfxUtils.cpp b/gfx/thebes/gfxUtils.cpp
index 10d113d110a34..d346ac7ef9168 100644
--- a/gfx/thebes/gfxUtils.cpp
+++ b/gfx/thebes/gfxUtils.cpp
@@ -25,6 +25,7 @@
 #include "mozilla/Maybe.h"
 #include "mozilla/RefPtr.h"
 #include "mozilla/UniquePtrExtensions.h"
+#include "mozilla/Unused.h"
 #include "mozilla/Vector.h"
 #include "nsComponentManagerUtils.h"
 #include "nsIClipboardHelper.h"
@@ -1048,7 +1049,7 @@ EncodeSourceSurfaceInternal(SourceSurface* aSurface,
 
   if (aBinaryOrData == gfxUtils::eBinaryEncode) {
     if (aFile) {
-      fwrite(imgData.begin(), 1, imgSize, aFile);
+      Unused << fwrite(imgData.begin(), 1, imgSize, aFile);
     }
     return NS_OK;
   }
diff --git a/media/webrtc/signaling/gtest/mediaconduit_unittests.cpp b/media/webrtc/signaling/gtest/mediaconduit_unittests.cpp
old mode 100644
new mode 100755
index 32e162df7d45c..6ad1b0616d353
--- a/media/webrtc/signaling/gtest/mediaconduit_unittests.cpp
+++ b/media/webrtc/signaling/gtest/mediaconduit_unittests.cpp
@@ -13,6 +13,7 @@ using namespace std;
 #include <MediaConduitInterface.h>
 #include <VideoConduit.h>
 #include "mozilla/UniquePtr.h"
+#include "mozilla/Unused.h"
 #include "nss.h"
 #include "runnable_utils.h"
 #include "signaling/src/common/EncodingConstraints.h"
@@ -224,7 +225,7 @@ void AudioSendAndReceive::GenerateAndReadSamples()
    //Create input file with the music
    WriteWaveHeader(PLAYOUT_SAMPLE_FREQUENCY, 1, inFile);
    GenerateMusic(inbuf, SAMPLES);
-   fwrite(inbuf,1,SAMPLES*sizeof(inbuf[0])*CHANNELS,inFile);
+   Unused << fwrite(inbuf,1,SAMPLES*sizeof(inbuf[0])*CHANNELS,inFile);
    FinishWaveHeader(inFile);
    fclose(inFile);
 
diff --git a/security/pkix/test/lib/pkixtestutil.cpp b/security/pkix/test/lib/pkixtestutil.cpp
index ddc8cd0826304..8f365b329b2e2 100644
--- a/security/pkix/test/lib/pkixtestutil.cpp
+++ b/security/pkix/test/lib/pkixtestutil.cpp
@@ -34,6 +34,8 @@
 #include "pkixder.h"
 #include "pkixutil.h"
 
+#include "mozilla/Unused.h"
+
 using namespace std;
 
 namespace mozilla { namespace pkix { namespace test {
@@ -509,7 +511,7 @@ MaybeLogOutput(const ByteString& result, const char* suffix)
     ++counter;
     ScopedFILE file(OpenFile(logPath, filename, "wb"));
     if (file) {
-      (void) fwrite(result.data(), result.length(), 1, file.get());
+      Unused << fwrite(result.data(), result.length(), 1, file.get());
     }
   }
 }
diff --git a/testing/gtest/gtest/src/gtest.cc b/testing/gtest/gtest/src/gtest.cc
old mode 100644
new mode 100755
index d882ab2e36a12..41b10d220c9f0
--- a/testing/gtest/gtest/src/gtest.cc
+++ b/testing/gtest/gtest/src/gtest.cc
@@ -35,6 +35,8 @@
 #include "gtest/internal/custom/gtest.h"
 #include "gtest/gtest-spi.h"
 
+#include "mozilla/Unused.h"
+
 #include <ctype.h>
 #include <math.h>
 #include <stdarg.h>
@@ -3866,7 +3868,7 @@ class ScopedPrematureExitFile {
       // errors are ignored as there's nothing better we can do and we
       // don't want to fail the test because of this.
       FILE* pfile = posix::FOpen(premature_exit_filepath, "w");
-      fwrite("0", 1, 1, pfile);
+      Unused << fwrite("0", 1, 1, pfile);
       fclose(pfile);
     }
   }
diff --git a/xpcom/build/LateWriteChecks.cpp b/xpcom/build/LateWriteChecks.cpp
index 1afb0135f1e4c..2d58e6b76c7fc 100755
--- a/xpcom/build/LateWriteChecks.cpp
+++ b/xpcom/build/LateWriteChecks.cpp
@@ -13,6 +13,7 @@
 #include "mozilla/Scoped.h"
 #include "mozilla/StaticPtr.h"
 #include "mozilla/Telemetry.h"
+#include "mozilla/Unused.h"
 #include "nsAppDirectoryServiceDefs.h"
 #include "nsDirectoryServiceUtils.h"
 #include "nsPrintfCString.h"
@@ -59,7 +60,7 @@ class SHA1Stream
     str.AppendPrintf(aFormat, list);
     va_end(list);
     mSHA1.update(str.get(), str.Length());
-    fwrite(str.get(), 1, str.Length(), mFile);
+    Unused << fwrite(str.get(), 1, str.Length(), mFile);
   }
   void Finish(SHA1Sum::Hash& aHash)
   {