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trivial_example: Check the return value of fread().

Silence a gcc warning by checking the return value of the fread()
call instead of the feof() guard. This prevents an infinite loop
in the case of a read error. Otherwise, when end-of-file is reached
fread() will certainly return a smaller number of elements read
than requested, so both cases are handled now.

Add a comment to clarify that we're dropping a partial frame on
purpose to keep the code simple.

Also add more braces around conditional bodies for less error-prone
style.

Signed-off-by: Mark Harris <mark.hsj@gmail.com>

--- a/doc/trivial_example.c

+++ b/doc/trivial_example.c

@@ -113,14 +113,25 @@

       int i;

       unsigned char pcm_bytes[MAX_FRAME_SIZE*CHANNELS*2];

       int frame_size;

+      size_t samples;

       /* Read a 16 bits/sample audio frame. */

-      fread(pcm_bytes, sizeof(short)*CHANNELS, FRAME_SIZE, fin);

-      if (feof(fin))

+      samples = fread(pcm_bytes, sizeof(short)*CHANNELS, FRAME_SIZE, fin);

+

+      /* For simplicity, only read whole frames. In a real application,

+       * we'd pad the final partial frame with zeroes, record the exact

+       * duration, and trim the decoded audio to match.

+       */

+      if (samples != FRAME_SIZE)

+      {

          break;

+      }

+

       /* Convert from little-endian ordering. */

       for (i=0;i<CHANNELS*FRAME_SIZE;i++)

+      {

          in[i]=pcm_bytes[2*i+1]<<8|pcm_bytes[2*i];

+      }

       /* Encode the frame. */

       nbBytes = opus_encode(encoder, in, FRAME_SIZE, cbits, MAX_PACKET_SIZE);