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bug#30066: 'get-bytevector-some' returns only 1 byte from unbuffered por

From: Ludovic Courtès
Subject: bug#30066: 'get-bytevector-some' returns only 1 byte from unbuffered ports
Date: Thu, 11 Jan 2018 22:02:29 +0100
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/25.3 (gnu/linux)


Mark H Weaver <address@hidden> skribis:

> address@hidden (Ludovic Courtès) writes:


>> +  if (SCM_UNBUFFEREDP (port) && (avail < max_buffer_size))
>> +    {
>> +      /* PORT is unbuffered.  Read as much as possible from PORT.  */
>> +      size_t read;
>> +
>> +      bv = scm_c_make_bytevector (max_buffer_size);
>> +      scm_port_buffer_take (buf, (scm_t_uint8 *) SCM_BYTEVECTOR_CONTENTS 
>> (bv),
>> +                            avail, cur, avail);
>> +
>> +      read = scm_i_read_bytes (port, bv, avail,
>> +                               SCM_BYTEVECTOR_LENGTH (bv) - avail);
> Here's the R6RS specification for 'get-bytevector-some':
>   "Reads from BINARY-INPUT-PORT, blocking as necessary, until bytes are
>    available from BINARY-INPUT-PORT or until an end of file is reached.
>    If bytes become available, 'get-bytevector-some' returns a freshly
>    allocated bytevector containing the initial available bytes (at least
>    one), and it updates BINARY-INPUT-PORT to point just past these
>    bytes.  If no input bytes are seen before an end of file is reached,
>    the end-of-file object is returned."
> By my reading of this, we should block only if necessary to ensure that
> we return at least one byte (or EOF).  In other words, if we can return
> at least one byte (or EOF), then we must not block, which means that we
> must not initiate another 'read'.

Indeed.  So perhaps the condition above should be changed to:

  if (SCM_UNBUFFEREDP (port) && (avail == 0))


> Out of curiosity, is there a reason why you're using an unbuffered port
> in your use case?

It’s to implement redirect à la socat:



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