[GRASS5] Messages and GUI

Radim Blazek blazek at itc.it
Sat Nov 13 05:22:57 EST 2004

Glynn Clements wrote:
>>Cannot it happen that messages printed by a module and
>>a process created by the module (e.g. dbmi driver)
>>will be mixed together? For example:
>>module message 1
>>driver message 1
>>module message 2
>>module message 3
>>driver message 2
> Only if the actual message is split across multiple write() calls.
> Actually, that's quite possible with the current implementation. As
> stderr is unbuffered, each fprintf() call will result in a separate
> write.
> For the GUI case, you don't need to do the formatting (the GUI can do
> that if the text widget can't do it automatically). You can just do
> e.g.:
> 	fprintf(stderr, "%s\n%s\GRASS_INFO_END\n", prefix, msg);
> This should result in a single write(); so long as the total message
> is less than PIPE_BUF bytes, the data won't be interleaved with that
> from another process. On Linux, PIPE_BUF is 4Kb, and the functions
> which call print_error() are already limited by a 2000 byte buffer. 
> POSIX requires that PIPE_BUF is at least 512 bytes, which should be
> sufficient for most error messages.
> An alternative would be to prefix each line with a message-ID composed
> from the PID and a monotonic counter which increments once per
> message. All lines beginning with the same ID would be part of a
> single message.

SQL statements printed as part of error message can be quite long,
for example for v.to.db option=query, so I have used the second
possibility + GRASS_INFO_END is printed at the end because
somebody could want to open a message window on error.


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