Re: Logging and Threads

[Marian Beermann <public@xxxxxxxxx>]

I'm not sure what you mean by blocking here, but I'm pretty sure it's 
not an issue either way.

These are all blocking file I/O, though normally the page cache will 
absorb any writes. Note that fflush(3) just flushes the FILE* buffers to 
the file descriptor. It doesn't fsync(2) (which may block for I/O for 
some time).

There's also a lock in FILE* which is taken by the buffered I/O 
functions. That's what makes these thread-safe. Naturally, any 
in-progress fprintf which has taken the lock on a given FILE will 
mutually exclude any other fprintf from starting progress on the same FILE.

Cheers, Marian

On 3/4/25 21:25, g4-lisz@xxxxxxxxxxxx wrote:
> Hi there!
>
> I never thought about this... until now: I'm using libssh in a 
> multithreaded application and I'm logging from within the threads, 
> using the lib's logging functions.
> I'm quite sure that this isn't an issue as long as the output goes to 
> stderr.
>
> But I added a callback (ssh_set_log_callback) which writes the 
> messages to a file handle, like:
> logging_cb( ... )
> {
> fprintf(log_fd, "[%s, %d] %s - %s\n", date, verbosity, (char 
> *)userdata, buffer);
> fflush(log_fd);
> }
>
> fprintf() is thread-safe. But can it block? In other words: Could one 
> thread temporarily block another because of the logging?
>
> So far I couldn't observe any blocking. But to be honest, I'm not sure 
> why it works fine.
> Maybe the OS layer write buffer is simply big enough to "take it all"?
>
> Cheers
> Till