[fpc-devel] Threading support and C library under Linux/Unix

[Michael Schnell]

On 06/22/2010 12:40 PM, Graeme Geldenhuys wrote:
> Just another question. Windows gets its threading support from the Win API.
> Similar for OS/2. Doesn't the Linux kernel have a similar threading API, or
> do they just rely on the C library?
>   
The "Linux Way" is more like doing separate executable than doing
threads, as Linux always provided high-performance inter process
communication (e.g. pipes, usable with "select()" etc.) and process
starting ( "fork()" ).

Thus older Linux Kernels did not have special threading support.
Threading could be done using "clone()" (instead of "fork()" ) to create
processes that share some resources (e.g. memory). The kernel did not
really know about the processes forming a multi-threaded application so,
to provide some managing capabilities the thread library created an
additional "manager thread" to do housekeeping (e.g. kill the other
threads if the main thread dies). This is called "Linux Threads".

Modern Kernels Kernels do provide "Native Posix Threads (NPTL) ",
managing the specifics of multi-threaded processes in the Kernel itself
(providing better Posix compatibility). Moreover the "FUTEX" API is
provided to allow for thread synchronization to be even faster than
process synchronization.

The "PThread library" user-API did not change. Just using the
appropriate version of glibc lets you use "Linux Threads" or "Native
Posix Threads" as appropriate, and FUTEX is automatically used for Posix
semaphores, if available in the arch used.

Of course you can avoid using the library, do the stuff in Pascal and
directly access the Kernel API, but this might be dangerous in case the
API might be modified some time in the future or if it's different with
different archs (e.g. some archs don't provide FUTEX).

-Michael