Grand Central Dispatch (GCD) on FreeBSD

This is a status and quick-start page on using Apple's Grand Central Dispatch (GCD) technology on the FreeBSD operating system. GCD support on FreeBSD was added by RobertWatson and StaceySon; the port is maintained by StanislavSedov.

Status

GCD support is considered production-worthy on FreeBSD 8.1 and later, and requires recent versions of the llvm-devel and compiler-rt ports. The kernel extensions required to support libdispatch first appeared in FreeBSD 8.1.

Branch

Date

Minimum SVN revision

7-STABLE

unsupported

unsupported

8-STABLE

2009-11-01

r198732

9-CURRENT

2009-09-18

r197293

Experimental patches exist to add pthread workqueue support to the FreeBSD 9.x kernel, but are not present in shipped kernels (including 9-CURRENT).

Quick-start

  1. Install FreeBSD 8.1 (or later).
  2. Install the devel/libdispatch port, which will install llvm-devel and compiler-rt as dependencies.

libdispatch works fine with gcc as shipped with FreeBSD for the function-centric interface dispatch APIs, but to use C Blocks you will need to build with clang -fblocks. Also, llvm-gcc4 (derived from Apple's gcc) and will work but is unsupported upstream.

As of version 10.0, FreeBSD ships with clang and compiler_rt in the base system so GCD is available out of the box. On Tier-2 platforms, where clang is not yet shipped by default, support for blocks has been added to GCC in base.

To build applications with libdispatch, add -I/usr/local/include to compiler command lines, and -L/usr/local/lib -ldispatch to linker command lines.

A simple GCD program

This program schedules deferred execution of a C block that prints a message and exits:

#include <dispatch/dispatch.h>

#include <err.h>
#include <stdio.h>
#include <stdlib.h>

int
main(int argc, char *argv[])
{
        dispatch_queue_t q;
        dispatch_time_t t;

        q = dispatch_get_main_queue();
        t = dispatch_time(DISPATCH_TIME_NOW, 5LL * NSEC_PER_SEC);

        // Print a message and exit after 5 seconds.
        dispatch_after(t, q, ^{
                printf("block_dispatch\n");
                exit(0);
            });

        dispatch_main();
        return (0);
}

Compile this with:

clang -Wall -Werror -fblocks -L/usr/local/lib -I/usr/local/include -o test test.c -ldispatch

The same program, without C Blocks

Here's a similar program, performing the same work but using deferred functions rather than C Blocks:

#include <dispatch/dispatch.h>

#include <err.h>
#include <stdio.h>
#include <stdlib.h>

void
deferred_code(__unused void *arg)
{

        printf("block_dispatch\n");
        exit(0);
}

int
main(int argc, char *argv[])
{
        dispatch_queue_t q;
        dispatch_time_t t;

        q = dispatch_get_main_queue();
        t = dispatch_time(DISPATCH_TIME_NOW, 5LL * NSEC_PER_SEC);

        dispatch_after_f(t, q, NULL, deferred_code);

        dispatch_main();
        return (0);
}

Compile this with:

gcc -Wall -Werror -I/usr/local/include -L/usr/local/lib -o test2 test2.c -ldispatch

Experimental Patches

Useful references

Demo with GCD in action

libdispatch ships with informative man pages describing not just the API, but also how to structure programs around GCD.

April-September 2009 FreeBSD status report - Grand Central Dispatch port - status on the FreeBSD port of GCD

Mac OS X Forge - open source libdispatch home - libdispatch subversion repository and mailing lists

Grand Central Dispatch: Technology Brief - a white paper from Apple introducing GCD.

Mac OS X Reference Library: Concurrency Programming Guide - a guide to concurrent programming on Mac OS X, much of which also applies to FreeBSD with the GCD port installed.

Mac Dev Center: Grand Central Dispatch Reference - reference material on GCD from Apple.

Mac OS Reference Library: Dispatch_Samples - sample code using GCD from Apple.

20090918-devsummit-gcd-public.pdf - 2008 DevSummit Presentation

GCD (last edited 2014-01-29 21:24:30 by PedroGiffuni)