Restore behavior to FileWrapper::Read.

- Returning the number of bytes read was mistakenly removed in r1175 in
  an overzealous attempt to unify the interface.
- Now both Read and WriteText return the number of bytes/characters
  processed. Write unfortunately cannot be easily changed due to the
  inheritance from OutStream.
- Improve the interface comments.

TBR=henrika@webrtc.org
BUG=issue196, issue198
TEST=voe_auto_test passes at last...

Review URL: http://webrtc-codereview.appspot.com/326001

git-svn-id: http://webrtc.googlecode.com/svn/trunk@1188 4adac7df-926f-26a2-2b94-8c16560cd09d

src/system_wrappers/interface/file_wrapper.h

@@ -53,12 +53,14 @@ public:
     virtual int FileName(char* fileNameUTF8,
                          size_t size) const = 0;
 
-    // Write text to the opened file. The written text can contain plain text
-    // and text with type specifiers in the same way as sprintf works.
+    // Write |format| to the opened file. Supply arguments to this function as
+    // you would printf. Returns the number of characters written or -1 on
+    // error.
     virtual int WriteText(const char* format, ...) = 0;
 
     // Inherited from Instream.
-    // Reads |len| bytes from file to |buf|.
+    // Reads |len| bytes from file to |buf|. Returns the number of bytes read
+    // or -1 on error.
     virtual int Read(void* buf, int len) = 0;
 
     // Inherited from OutStream.

src/system_wrappers/source/file_impl.cc

@@ -188,6 +188,10 @@ int FileWrapperImpl::OpenFile(const char *fileNameUTF8, bool readOnly,
 
 int FileWrapperImpl::Read(void *buf, int len)
 {
+    if (len < 0)
+    {
+        return -1;
+    }
     if (_id != NULL)
     {
         int res = static_cast<int>(fread(buf, 1, len, _id));
@@ -198,7 +202,7 @@ int FileWrapperImpl::Read(void *buf, int len)
                 CloseFile();
             }
         }
-        return 0;
+        return res;
     }
     return -1;
 }
@@ -219,9 +223,9 @@ int FileWrapperImpl::WriteText(const char* format, ...)
     int num_chars = vfprintf(_id, format, args);
     va_end(args);
 
-    if (num_chars > 0)
+    if (num_chars >= 0)
     {
-        return 0;
+        return num_chars;
     }
     else
     {