Grand Central Dispatch (GCD)

GCD is a concurrent programming framework first shipped with Mac OS X Snow Leopard. This package is an open source bundling of libdispatch, the core user space library implementing GCD. At the time of writing, support for the BSD kqueue API, and specifically extensions introduced in Mac OS X Snow Leopard and FreeBSD 9-CURRENT, are required to use libdispatch. Linux is supported, but requires specific packages to be installed (see Linux section at the end of the file). Other systems are currently unsupported.

Configuring and installing libdispatch (general comments)

GCD is built using autoconf, automake, and libtool, and has a number of compile-time configuration options that should be reviewed before starting. An uncustomized install of the C-API to libdispatch requires:

make install

libdispatch can be optionally built to include a Swift API. This requires a Swift toolchain to compile the Swift code in libdispatch and can be done in two possible scenarios.

If you are building your own Swift toolchain from source, then you should build libdispatch simply by giving additional arguments to swift/utils/build-script:

./swift/utils/build-script --libdispatch -- --install-libdispatch

To build libdispatch using a pre-built Swift toolchain and install libdispatch into that toolchain (to allow that toolchain to compile Swift code containing “import Dispatch”) requires:

./configure --with-swift-toolchain=<PATH_TO_SWIFT_TOOLCHAIN> --prefix=<PATH_TO_SWIFT_TOOLCHAIN>
make install

Note that once libdispatch is installed into a Swift toolchain, that toolchain cannot be used to compile libdispatch again (you must ‘make uninstall’ libdispatch from the toolchain before using it to rebuild libdispatch).

You can also use the build-toolchain script to create a toolchain that includes libdispatch on Linux:

  1. Add libdispatch and install-libdispatch lines to ./swift/utils/build-presets.ini under [preset: buildbot_linux] section, as following:

    [preset: buildbot_linux]
  2. Run:

    ./swift/utils/build-toolchain local.swift

Note that adding libdispatch in build-presets.ini is for Linux only as Swift on macOS platforms uses the system installed libdispatch, so its not required.

Building and installing on OS X

The following configure options may be of general interest:


Specify the path to Apple's libpthread package, so that appropriate headers can be found and used.


Specify the path to Apple's libplatform package, so that appropriate headers can be found and used.


Specify the path to Apple's XNU package, so that appropriate headers can be found and used.


On systems where -fblocks is supported, specify an additional library path in which libBlocksRuntime can be found. This is not required on OS X, where the Blocks runtime is included in libSystem, but is required on FreeBSD.

The following options are likely to only be useful when building libdispatch on OS X as a replacement for /usr/lib/system/libdispatch.dylib:


Do not tag libdispatch's init routine as __constructor, in which case it must be run manually before libdispatch routines can be called. This is the default when building on OS X. For /usr/lib/system/libdispatch.dylib the init routine is called automatically during process start.


Use a non-portable allocation scheme for pthread per-thread data (TSD) keys when building libdispatch for /usr/lib/system on OS X. This should not be used on other OS's, or on OS X when building a stand-alone library.

Typical configuration commands

The following command lines create the configuration required to build libdispatch for /usr/lib/system on OS X El Capitan:

clangpath=$(dirname `xcrun --find clang`)
sudo mkdir -p "$clangpath/../local/lib/clang/enable_objc_gc"
LIBTOOLIZE=glibtoolize sh
cflags='-arch x86_64 -arch i386 -g -Os'
./configure CFLAGS="$cflags" OBJCFLAGS="$cflags" CXXFLAGS="$cflags" \
	--prefix=/usr --libdir=/usr/lib/system --disable-static \
	--enable-apple-tsd-optimizations \
	--with-apple-libpthread-source=/path/to/10.11.0/libpthread-137.1.1 \
	--with-apple-libplatform-source=/path/to/10.11.0/libplatform-73.1.1 \
	--with-apple-xnu-source=/path/to/10.11.0/xnu-3247.1.106 \
make check

Building and installing for FreeBSD

Typical configuration line for FreeBSD 8.x and 9.x to build libdispatch with clang and blocks support:

./configure CC=clang --with-blocks-runtime=/usr/local/lib
make check

Building and installing for Linux

Note that libdispatch development and testing is done only on Ubuntu; currently supported versions are 14.04, 15.10 and 16.04.

  1. The first thing to do is install required packages:

    sudo apt-get install autoconf libtool pkg-config clang systemtap-sdt-dev libbsd-dev linux-libc-dev

    Note: compiling libdispatch requires clang 3.8 or better and the gold linker. If the default clang on your Ubuntu version is too old, see to install a newer version. On older Ubuntu releases, you may need to install binutils-gold to get the gold linker.

  2. Build (as in the general instructions above)

    make install