IIRC, stderr doesn't buffer, stdout does. Both must block eventually once they hit the maximum buffer size the kernel and/or libc impose. Buffering != blocking. -Adam
Robert Keizer robert@keizer.ca wrote:
On 12-10-10 10:33 AM, Gilbert E. Detillieux wrote:
On 2012-10-10 09:50, Robert Keizer wrote:
I have a production service that prints some debug to stdout. I could make it print to stderr, but it really doesn't matter in this case.
Are you sure it doesn't matter? Read on...
I want to have some hope of if something goes wrong figuring out what did. Log files aren't really an option so I ran it in screen. Yes yes I could have made it network log to somewhere else, but again, not really an viable option in this case. Works great as the program dies and the output stops, leaving the last debug messages in the buffer of screen.
One of the classic C/UNIX lessons about stdout vs stderr is that the C library buffers stdout by default but not stderr, exactly for the reason that you want error messages to be flushed out right away, leaving nothing in the buffer. This is particularly important if the program dies in a way that prevents the usual exit processing, such as flushing buffers and closing file I/O.
So, issues with "screen" aside, you probably should either send your debug output to stderr, or set your stdout to unbuffered mode while debugging (or both).
Interesting!
Thought I'd test out stdout vs stderr just to see if this was in fact the case. It's not - stderr and stdout have the same effect in this case.
Below is a script the listens for incoming data on a given port ( example is 1337 ). When data is received it writes to either stdout or stderr and then sends data on to another port ( in this case 1338 ). This script also fills up the screen buffer by a simple loop ( tested filling with both stderr and stdout ).
Using telnet to feed the script data, and nc ( nc -lk 1338 ) to listen on port 1338 for the output, one can test to confirm.
net = require "net" server = net.createServer ( c ) -> c.setEncoding "utf8" c.on "data", ( data ) -> _c = net.createConnection 1338, ( ) -> process.stderr.write "Data: " + data + "\n" _c.write data _c.end( )
server.listen 1337, "127.0.0.1" loops = ( ) -> process.stderr.write "Loopies: " + Date( ) + "\n" setTimeout loops, 100 loops( )
Screenshot is attached - it shows this script in the top left running in screen, with copy mode enabled. Right of it is the telnet session feeding it, and on the bottom left is the nc or "output". Note that the last two inputs did not make it to the output. Ergo, unless I'm totally an idiot in my implementation: This is blocking on stderr as well.
Thoughts? Because now I'm officially annoyed that this blocks on stderr as well!
Rob
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Adam Thompson