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MT-Level of Libraries

Libraries are classified into four categories which define
the level of the libraries' ability to support threads.

The MT-Level category of the libraries in this section are
shown on each man page under MT-Level. Pages containing rou-
tines that are of multiple or differing MT-Levels show this
under the NOTES section.

Safe Safe is simply an attribute of code that can be
called from a multithreaded application. It is a
generic term used to differentiate between code
that is unsafe.

Unsafe An unsafe library contains global and static
data that is not protected. It is not safe to
use unless the application arranges for only one
thread at time to execute within the library.
Unsafe libraries may contain routines that are
safe; however, most of the library's routines
are unsafe to call.

MT-Safe An MT-Safe library is fully prepared for mul-
tithreaded access. It protects its global and
static data with locks, and can provide a rea-
sonable amount of concurrency. Note that a
library can be safe to use, but not MT-Safe. For
example, surrounding an entire library with a
monitor makes the library safe, but it supports
no concurrency so it is not considered MT-Safe.
An MT-Safe library must permit a reasonable
amount of concurrency. (This definition's pur-
pose is to give precision to what is meant when
a library is described as safe. The definition
of a "safe" library does not specfiy if the
library supports concurrency. The MT-Safe
definition makes it clear that the library is
safe, and supports some concurrency. This clar-
ifies the safe definition, which can mean any-
thing from being single threaded to being any
degree of multithreaded.)

Async-Safe Async-Safe refers to particular library routines
that can be safely called from a signal handler.
A thread that is executing an Async-Safe routine
will not deadlock with itself if interrupted by
a signal. Signals are only a problem for MT-Safe
routines that acquire locks.

Signals are disabled when locks are acquired in
Async-Safe routines. This prevents a signal
handler that might acquire the same lock from
being called.

MT-Safe with exceptions
See the NOTES sections of these pages for a
description of the exceptions.

Safe with exceptions
See the NOTES sections of these pages for a
description of the exceptions.