Question about gcc -pthread option

[Paul Joselow <pjoselow at optonline dot net>]

I work at a company that uses pthreads.  I have some questions about what
this option actually does during a compile.  There are cases where even
though a program uses pthreads, the option was omitted from the compile and
build and the program builds successfully.  So the questions involve what
things will and will not work when the pthread option is used or omitted.
This is a question I would like to answer across several architectures,
specifically Alpha, Intel Linux, and Solaris.

I have been shown a case where common variables (e.g. errno) that should be
local to each thread end up being shared across all threads if the pthread
option is omitted.  What are the problems with always using the pthread
option on all compiles and links regardless of whether a program uses
pthreads or not?