Possible pipe(2) bug

["Bert Belder" <H dot Belder at student dot TUDelft dot NL>]

I think the pipe(2) implementation may have a bug. 

The pipe function creates a pipe and returns two file descriptors (which are just numbers), one for reading and one for writing. However sometimes (but rarely) two subsequent calls to the pipe function returns the same file descriptor twice. That is normal when a file descriptor 'number' is re-used after being closed first, but otherwise it is wrong. To demonstrate the problem I wrote a simple program (attached as test.c) that does the following:

1. Create three pipes, so we get 6 file descriptors
2. (dump the FD's we got)
3. Close 3 of the 6 file descriptors
4. (Dump the FD's that were closed)
5. Repeat steps 1-4 a 100 times 

The first three rounds give sound results:
? ??0 opened: pipe1=(3, 4), pipe2=(5, 6), pipe3=(7, 8)
? ??0 closed: pipe1[0]=3, pipe2[1]=6, pipe3[1]=8
? ??1 opened: pipe1=(3, 6), pipe2=(8, 9), pipe3=(10, 11)
? ??1 closed: pipe1[0]=3, pipe2[1]=9, pipe3[1]=11
? ??2 opened: pipe1=(3, 9), pipe2=(11, 12), pipe3=(13, 14)
? ??2 closed: pipe1[0]=3, pipe2[1]=12, pipe3[1]=14
??? ...
As can be seen the FD's that were closed are re-used next round. Perfectly fine.

But then suddenly at round 94...
??? 93 opened: pipe1=(3, 282), pipe2=(284, 285), pipe3=(286, 287)
??? 93 closed: pipe1[0]=3, pipe2[1]=285, pipe3[1]=287
??? 94 opened: pipe1=(3, 285), pipe2=(287, 288), pipe3=(287, 289) ?<- !!!!!!!!!!!!
File descriptor 287 is issued twice! 

I think this is a bug. It actually causes problems under certain circumstances when running a Cygwin build of nodejs (http://nodejs.org).
Or am I just doing something stupid?

Thanks for your time
- Bert

Attachment: cygcheck.out
Description: cygcheck.out

Attachment: test.c
Description: test.c

Attachment: test.out
Description: test.out

--
Problem reports:       http://cygwin.com/problems.html
FAQ:                   http://cygwin.com/faq/
Documentation:         http://cygwin.com/docs.html
Unsubscribe info:      http://cygwin.com/ml/#unsubscribe-simple