simple_example.html   [plain text]

<TITLE>Using the Garbage Collector: A simple example</title>
<H1>Using the Garbage Collector: A simple example</h1>
The following consists of step-by-step instructions for building and
using the collector.  We'll assume a Linux/gcc platform and
a single-threaded application.  <FONT COLOR=green>The green
text contains information about other platforms or scenarios.
It can be skipped, especially on first reading</font>.
<H2>Building the collector</h2>
If you haven't already so, unpack the collector and enter
the newly created directory with
tar xvfz gc<version>.tar.gz
cd gc<version>
You can configure, build, and install the collector in a private
directory, say /home/xyz/gc, with the following commands:
./configure --prefix=/home/xyz/gc --disable-threads
make check
make install
Here the "<TT>make check</tt>" command is optional, but highly recommended.
It runs a basic correctness test which usually takes well under a minute.
<FONT COLOR=green>
<H3>Other platforms</h3>
On non-Unix, non-Linux platforms, the collector is usually built by copying
the appropriate makefile (see the platform-specific README in doc/
in the distribution) to the file "Makefile" (overwriting the copy of that was originally there), and then typing "make"
(or "nmake" or ...).  This builds the library in the source tree.  You may
want to move it and the files in the include directory to a more convenient
If you use a makefile that does not require running a configure script,
you should first look at the makefile, and adjust any options that are
documented there.
If your platform provides a "make" utility, that is generally preferred
to platform- and compiler- dependent "project" files.  (At least that is the
strong preference of the would-be maintainer of those project files.)
If you need thread support, configure the collector with
--enable-threads=posix --enable-thread-local-alloc --enable-parallel-mark
instead of
If your target is a real old-fashioned uniprocessor (no "hyperthreading",
etc.) you will want to omit <TT>--enable-parallel-mark</tt>.
You will need to include the C++ support, which unfortunately tends to
be among the least portable parts of the collector, since it seems
to rely on some corner cases of the language.  On Linux, it
suffices to add <TT>--enable-cplusplus</tt> to the configure options.
<H2>Writing the program</h2>
You will need a
#include "gc.h"
at the beginning of every file that allocates memory through the
garbage collector.  Call <TT>GC_MALLOC</tt> wherever you would
have call <TT>malloc</tt>.  This initializes memory to zero like
<TT>calloc</tt>; there is no need to explicitly clear the
If you know that an object will not contain pointers to the
garbage-collected heap, and you don't need it to be initialized,
call <TT>GC_MALLOC_ATOMIC</tt> instead.
A function <TT>GC_FREE</tt> is provided but need not be called.
For very small objects, your program will probably perform better if
you do not call it, and let the collector do its job.
A <TT>GC_REALLOC</tt> function behaves like the C library <TT>realloc</tt>.
It allocates uninitialized pointer-free memory if the original
object was allocated that way.
The following program <TT>loop.c</tt> is a trivial example:
#include "gc.h"
#include &lt;assert.h&gt;
#include &lt;stdio.h&gt;

int main()
  int i;

  GC_INIT();	/* Optional on Linux/X86; see below.  */
  for (i = 0; i < 10000000; ++i)
     int **p = (int **) GC_MALLOC(sizeof(int *));
     int *q = (int *) GC_MALLOC_ATOMIC(sizeof(int));
     assert(*p == 0);
     *p = (int *) GC_REALLOC(q, 2 * sizeof(int));
     if (i % 100000 == 0)
       printf("Heap size = %d\n", GC_get_heap_size());
  return 0;
<FONT COLOR=green>
<H3>Interaction with the system malloc</h3>
It is usually best not to mix garbage-collected allocation with the system
<TT>malloc-free</tt>.  If you do, you need to be careful not to store
pointers to the garbage-collected heap in memory allocated with the system
<H3>Other Platforms</h3>
On some other platforms it is necessary to call <TT>GC_INIT()</tt> from the main program,
which is presumed to be part of the main executable, not a dynamic library.
This can never hurt, and is thus generally good practice.

For a multithreaded program some more rules apply:
Files that either allocate through the GC <I>or make thread-related calls</i>
should first define the macro <TT>GC_THREADS</tt>, and then
include <TT>"gc.h"</tt>.  On some platforms this will redefine some
threads primitives, e.g. to let the collector keep track of thread creation.
To take advantage of fast thread-local allocation, use the following instead
of including <TT>gc.h</tt>:
#include "gc_local_alloc.h"
This will cause GC_MALLOC and GC_MALLOC_ATOMIC to keep per-thread allocation
caches, and greatly reduce the number of lock acquisitions during allocation.

In the case of C++, you need to be especially careful not to store pointers
to the garbage-collected heap in areas that are not traced by the collector.
The collector includes some <A HREF="gcinterface.html">alternate interfaces</a>
to make that easier.

Additional debug checks can be performed by defining <TT>GC_DEBUG</tt> before
including <TT>gc.h</tt>.  Additional options are available if the collector
is also built with <TT>--enable-full_debug</tt> and all allocations are
performed with <TT>GC_DEBUG</tt> defined.

<H3>What if I can't rewrite/recompile my program?</h3>
You may be able to build the collector with <TT>--enable-redirect-malloc</tt>
and set the <TT>LD_PRELOAD</tt> environment variable to point to the resulting
library, thus replacing the standard <TT>malloc</tt> with its garbage-collected
counterpart.  This is rather platform dependent.  See the
<A HREF="leak.html">leak detection documentation</a> for some more details.


<H2>Compiling and linking</h2>

The above application <TT>loop.c</tt> test program can be compiled and linked

cc -I/home/xyz/gc/include loop.c /home/xyz/gc/lib/libgc.a -o loop

The <TT>-I</tt> option directs the compiler to the right include
directory.  In this case, we list the static library
directly on the compile line; the dynamic library could have been
used instead, provided we arranged for the dynamic loader to find
it, e.g. by setting <TT>LD_LIBRARY_PATH</tt>.

<FONT COLOR=green>


On pthread platforms, you will of course also have to link with
and compile with any thread-safety options required by your compiler.
On some platforms, you may also need to link with <TT>-ldl</tt>
or <TT>-lrt</tt>.
Looking at threadlibs.c in the GC build directory
should give you the appropriate
list if a plain <TT>-lpthread</tt> doesn't work.


<H2>Running the executable</h2>

The executable can of course be run normally, e.g. by typing


The operation of the collector is affected by a number of environment variables.
For example, setting <TT>GC_PRINT_STATS</tt> produces some
GC statistics on stdout.
See <TT>README.environment</tt> in the distribution for details.