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Problem with printf-like logging framework and inline hexdumps.

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Hi,

For one of my projects, I created a simple logging framework. I have an
(opaque) context object which contains a buffer:

typedef struct dc_context_t {
      char msg[4096];
      /* More stuff here. */
} dc_context_t;

and a printf-like function to log a message. The logging function uses
the vsnprintf() function to write a string representation of the
arguments to the context buffer. The resulting string is then passed to
a user-defined callback function.

int
dc_context_log (dc_context_t *context, const char *format, ...)
{
      va_list ap;

      va_start (ap, format);
      vsnprintf (context->msg, sizeof (context->msg), format, ap);
      va_end (ap);

      callback (context->msg);
}

This works perfect for all printf-style logging. For example:

     dc_context_log (context, "%s (%d)", "An error message", 3);

However, I would like to log some binary data too. Let's say I want to
log a message with an inline hexdump, something like this:

      "DEBUG: data=<DATA>, size=<SIZE>"

Since there is no printf format string available for hexdumps, I could
write a little dc_context_hexdump() helper function to convert the
binary data into a hex string, and then produce the log message in 3
steps:

      dc_context_log (context, "%s: data=", "DEBUG");
      dc_context_hexdump (context, hexdata, size);
      dc_context_log (context, "", size);

This would work fine, except that I need the output in only one piece.
There are several reasons for that. I simplified the code a bit for this
post, but in reality the dc_context_log() function also receives extra
info, like the __FILE__ and __LINE__ macros. When printing the output, I
only want to see the file and line info appear only once, and not three
times. Also in a multithreaded environment, another thread may call the
logging function between each of the three calls, and thus interleaving
the output.

How would you workaround this?

A possible approach might be to convert the binary data to string
first, and then pass that to the dc_context_log call. Something like
this:

void
somefunction (dc_context_t *context, unsigned char *data, unsigned int
size)
{
      /* Do something useful here. */

      /* Log the binary data. */
      hexstr = hexdump (data, size);
      dc_context_log (context, "%s: data=%s, size=%u",
          "DEBUG", hexstr, size);
}

But this hexdump function isn't thread-safe (because it would have to
return a pointer to a static buffer). And even if I would make a
re-entrant hexdump_r() version, it's not ideal because then I would have
to allocate a buffer inside the somefunction(). I don't want to do that
because the dc_context_log is actually called through a macro, which can
be configured at compile time to omit the actual call (e.g. in
production builds). But then the memory allocation would still be there,
and I don't want to pollute application code with #ifdef's. An temporary
buffer on the stack won't work either, because the size isn't fixed.

Suggestions are welcome!

Jef

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Thread

Problem with printf-like logging framework and inline hexdumps. Jef Driesen <jefdriesen@hotmail.com.invalid> - 2012-09-13 18:11 +0200
  Re: Problem with printf-like logging framework and inline hexdumps. "Heinrich Wolf" <invalid@invalid.invalid> - 2012-09-13 18:55 +0200
    Re: Problem with printf-like logging framework and inline hexdumps. Jorgen Grahn <grahn+nntp@snipabacken.se> - 2012-09-20 17:19 +0000
  Re: Problem with printf-like logging framework and inline hexdumps. Barry Schwarz <schwarzb@dqel.com> - 2012-09-13 11:26 -0700
    Re: Problem with printf-like logging framework and inline hexdumps. Jef Driesen <jefdriesen@hotmail.com.invalid> - 2012-09-14 00:06 +0200
  Re: Problem with printf-like logging framework and inline hexdumps. Ben Bacarisse <ben.usenet@bsb.me.uk> - 2012-09-13 21:24 +0100
    Re: Problem with printf-like logging framework and inline hexdumps. Jef Driesen <jefdriesen@hotmail.com.invalid> - 2012-09-13 23:47 +0200
      Re: Problem with printf-like logging framework and inline hexdumps. Ben Bacarisse <ben.usenet@bsb.me.uk> - 2012-09-13 23:54 +0100
        Re: Problem with printf-like logging framework and inline hexdumps. Jef Driesen <jefdriesen@hotmail.com.invalid> - 2012-09-14 18:31 +0200
          Re: Problem with printf-like logging framework and inline hexdumps. Ben Bacarisse <ben.usenet@bsb.me.uk> - 2012-09-14 22:02 +0100
            Re: Problem with printf-like logging framework and inline hexdumps. Ben Bacarisse <ben.usenet@bsb.me.uk> - 2012-09-14 22:49 +0100
              Re: Problem with printf-like logging framework and inline hexdumps. Jef Driesen <jefdriesen@hotmail.com.invalid> - 2012-09-17 07:09 +0200
              Re: Problem with printf-like logging framework and inline hexdumps. Kaz Kylheku <kaz@kylheku.com> - 2012-09-17 06:39 +0000
                Re: Problem with printf-like logging framework and inline hexdumps. Ben Bacarisse <ben.usenet@bsb.me.uk> - 2012-09-17 13:30 +0100
      Re: Problem with printf-like logging framework and inline hexdumps. Nick Keighley <nick_keighley_nospam@hotmail.com> - 2012-09-14 03:29 -0700
        Re: Problem with printf-like logging framework and inline hexdumps. Ben Bacarisse <ben.usenet@bsb.me.uk> - 2012-09-14 12:01 +0100
  Re: Problem with printf-like logging framework and inline hexdumps. Jef Driesen <jefdriesen@hotmail.com.invalid> - 2012-09-14 22:50 +0200

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