Hi,

in one of our internal applications we are using SoX (the 14.4.2+git20190427 version from Ubuntu) to convert from a variety of audio formats to the WAV file format. We observed that the tests for the conversion occasionally failed and over the last days I found time to dig deeper into this.

We are using sox_open_memstream_write() to write to a dynamically allocated in-memory stream. In our tests sometimes the size of the resulting WAV buffer would have the expected size, sometimes it would be 44 bytes, the size of the WAV header. Valgrind told me that the behavior of is_seekable() in formats.c depends on uninitialized memory. In your git repository I found a fix for this:

commit bb38934e11035c8fab141f70dabda3afdd17da36
Author: Mans Rullgard <mans@mansr.com>
Date:   Tue Aug 4 17:19:49 2020 +0100

    format: improve is_seekable() test
    
    Streams opened with fmemopen() do not have an underlying file descriptor,
    so the fstat() will fail, and a random result is returned.
    
    A simpler method that works regardless of file type is to call fseek()
    and check if it reports success.
    
    Suggested by Stefan Sauer <ensonic@google.com>.


Now with this fix applied valgrind was happy, however now our conversion from MP3 to WAV would always result in only 44 bytes, as read from the buffer_size_ptr location passed to sox_open_memstream_write(). It turns out that with above change the undefined behavior is fixed for streams created with open_memstream() and is_seekable() will now reliably returns sox_true for such streams. This allows the WAV writer code to do an fseek() to the start of the stream followed by a write of the WAV header with correct length information. However such a seek followed by a write causes the dynamically allocated memory stream to be truncated. Thus after calling sox_close() the size reported for the stream will be 44 bytes, that's not what we want. Unfortunately we can not simply fix this by reporting the full buffer size as the buffer will actually have been truncated, and a trailing null byte is appended after the WAV header. It looks like we can indeed not seek and fix data in a dynamically allocated stream. Thus I am attaching a patch that changes the code in formats.c to set ft->seekable to false for streams opened with open_memstream(). With this change applied on top of the improvement for the is_seekable() test, our tests pass reliably and valgrind seems happy as well.

I am attaching the patch here, please consider it for inclusion. I am also attaching a simple test application that writes to a stream, seeks to the front and performs another write. The output of this program illustrates that the buffer is truncated:

  buf = `hello', size = 5
  buf = `hello, world', size = 12
  buf = `heyho', size = 5


Regards,
Sven