CODEWITHSUNDEEP
solarisfcntl
Brian Vandenberg
Brian Vandenberg

Reputation: 4121

solaris: O_NDELAY set on stdin: when process exits the shell exits

TLDR: In Solaris, if O_NDELAY is set on stdin by a child process, bash exits. Why?

The following code causes interactive bash (v4.3.33) or tcsh (6.19.00) shells to exit after the process finishes running:

#include <fcntl.h>

int main() {
  fcntl( 0, F_SETFL, O_NDELAY );

//int x = fcntl( 0, F_GETFL );
//fcntl( 0, F_SETFL, ~(x ^ (~O_NDELAY)) );

  return 0;
}

The versions of ksh, csh and zsh we have aren't affected by this problem.

To investigate I ran bash & csh under truss (similar to strace on Linux) like this:

$ truss -eaf -o bash.txt -u'*' -{v,r,w}all bash --noprofile --norc
$ truss -eaf -o csh.txt -u'*' -{v,r,w}all csh -f

After csh finishes running the process it does the following:

fcntl( 0, F_GETFL ) = FWRITE|FNDELAY
fcntl( 0, F_SETFL, FWRITE) = 0

... which gave me an idea. I changed the program to the commented out code above so it would toggle the state of O_NDELAY. If I run it twice in a row bash doesn't exit.

Upvotes: 0

Views: 197

Answers (1)

Brian Vandenberg
Brian Vandenberg

Reputation: 4121

This answer got me started on the right path. The man page for read (in Solaris) says:

When attempting to read a file associated with a terminal that has no data currently available:

* If O_NDELAY is set, read() returns 0
* If O_NONBLOCK is set, read() returns -1 and sets errno to EAGAIN

... so when bash tries to read stdin it returns 0 causing it to assume EOF was hit.

This page indicates O_NDELAY shouldn't be used anymore, instead recommending O_NONBLOCK. I've found similar statements regarding O_NDELAY / FIONBIO for various flavors of UNIX.

As an aside, in Linux O_NDELAY == FNDELAY == O_NONBLOCK, so it's not terribly surprising I was unable to reproduce this problem in that environment.

Unfortunately, the tool that's doing this isn't one I have the source code for, though from my experimenting I've found ways to work around the problem.

If nothing else I can make a simple program that removes O_NDELAY as above then wrap execution of this tool in a shell script that always runs the "fixer" program after the other one.

Upvotes: 0

Related Questions