popen thread safe?

Printable View

03-31-2011
Drogin

popen thread safe?

Is popen safe to use with pthreads?

I've tried to google it, but I found no straight answer.
And in forums, some people claim it's not because it forks, while others say it's considered threadsafe(without giving a link to a trustable site), and other argues its safe because it forks and then immediately execs.

So what I hope is, does anyone here know for sure if it's safe to use popen with pthreads?
03-31-2011
brewbuck

Quote:

Originally Posted by Drogin

Is popen safe to use with pthreads?

I've tried to google it, but I found no straight answer.
And in forums, some people claim it's not because it forks, while others say it's considered threadsafe(without giving a link to a trustable site), and other argues its safe because it forks and then immediately execs.

So what I hope is, does anyone here know for sure if it's safe to use popen with pthreads?

You're right, there is no clear answer out there. Amazing.

However, I really doubt there's anything inherently problematic about using popen() in a program that has multiple threads. It may not be properly re-entrant, in which case you should serialize calls to popen()/pclose() by protecting it with a mutex.

Yet another line item in my growing list of why the POSIX API really isn't as great as I used to think it was.
03-31-2011
Yarin

You probably won't have any issues on Linux, but this definitely doesn't hold true to all *nixes, and so isn't portable at all.
I've run pthreads from forks in Linux with no ill effects, but the same code completely broke on OBSD.

I agree, POSIX has a couple retarded shortcomings. But what OS' API doesn't?
04-01-2011
Codeplug

It's not in the list of "async-signal-safe" functions (though execl and fork are):
http://pubs.opengroup.org/onlinepubs...ag_15_04_03_03
"Async-signal-safe" implies "thread-safe".

It's not in the list of "functions that synchronize memory" (though fork is):
http://pubs.opengroup.org/onlinepubs...html#tag_04_11
Functions that synchronize imply "thread-safe".

But here is what we really need:
http://pubs.opengroup.org/onlinepubs...l#tag_15_09_01

Quote:

All functions defined by this volume of POSIX.1-2008 shall be thread-safe, except that the following functions need not be thread-safe.

popen is not in that list.

So you can call any Posix function in an application that uses threads. You just have to be aware that the functions in that list may not be "thread-safe" - like strtok. But if only one thread is using those functions, then there's nothing to worry about.

gg