Subj : Re: How to deal with errno in a multipthread program
To   : comp.programming.threads
From : Eric Sosman
Date : Fri Apr 15 2005 12:49 pm



icoming wrote:
> Is it enough to deal with it as follow:
> int err=errno;
> func();  //this function may modify the errno
> errno=err;
> I read W.Richard Stevens's UNIX Network Programming, it uses this way,
> but I do not think it's enough, but do not know there is better one.

    This is appropriate when you want to preserve the
errno value within a single thread, preventing func()
from changing it (or rather, repairing whatever changes
func() may have made).  It really has nothing to do
with threads.

    However, you do not need to worry.  The thread
implementation itself arranges to have a unique `errno'
for each thread: Threads T1 and T2 can simultaneously
store different values in errno, and this works because
the single name `errno' refers to a different memory
location for each thread.  Different implementations
achieve this rather magical effect in different ways,
but the outcome is that "it just works."

    In a threaded program there is no such thing as "the
stack" or "the program counter;" there are multiples,
one for each thread.  Similarly with "the errno variable;"
each thread has its own.

-- 
Eric.Sosman@sun.com

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