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Re: Problem with printf-like logging framework and inline hexdumps.

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On 13-09-12 18:11, Jef Driesen wrote:
> 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!

An alternative solution that I have been thinking about, is to
introduce a number of helper functions:

      dc_context_printf
      dc_context_hexdump
      dc_context_flush

The first two functions would just write to the msg buffer as before,
but now with an extra variable to track the current writing position,
such that successive calls will append their data. The third function is
there to generating the output and reset the writing position.

Thread-safety could be achieved by providing two more locking functions
(dc_context_lock and dc_context_unlock) called before and after the
printing functions.

The only downside, is that I would need several function calls to
generate a single log message. For my example in the original post,
there would be at least 6 calls:

      dc_context_lock (context);
      dc_context_printf (context, "%s: data=", "DEBUG");
      dc_context_hexdump (context, data, size);
      dc_context_printf (context, "size=%d", size);
      dc_context_flush (context);
      dc_context_unlock (context);

Although it works, it's also getting pretty verbose, and all these function 
calls will have some runtime overhead too. (The original dc_context_log function 
does only call the vsnprintf function if the loglevel is high enough.)

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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