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Using the Garbage Collector as Leak Detector

The garbage collector may be used as a leak detector. In this case, the primary function of the collector is to report objects that were allocated (typically with GC_MALLOC), not deallocated (normally with GC_FREE), but are no longer accessible. Since the object is no longer accessible, there is normally no way to deallocate the object at a later time; thus it can safely be assumed that the object has been "leaked".

This is substantially different from counting leak detectors, which simply verify that all allocated objects are eventually deallocated. A garbage-collector based leak detector can provide somewhat more precise information when an object was leaked. More importantly, it does not report objects that are never deallocated because they are part of "permanent" data structures. Thus it does not require all objects to be deallocated at process exit time, a potentially useless activity that often triggers large amounts of paging.

The garbage collector provides leak detection support. This includes the following features:

  1. Leak detection mode can be initiated at run-time by GC_set_find_leak(1) call at program startup instead of building the collector with FIND_LEAK macro defined.
  2. Leaked objects should be reported and then correctly garbage collected.

To use the collector as a leak detector, do the following steps:

  1. Activate the leak detection mode as described above.
  2. Change the program so that all allocation and deallocation goes through the garbage collector.
  3. Arrange to call GC_gcollect (or CHECK_LEAKS()) at appropriate points to check for leaks. (This happens implicitly but probably not with a sufficient frequency for long running programs.)

The second step can usually be accomplished with the -DREDIRECT_MALLOC=GC_malloc option when the collector is built, or by defining malloc, calloc, realloc, free (as well as posix_memalign, reallocarray, strdup, strndup, wcsdup, BSD memalign, GNU valloc, GNU pvalloc) to call the corresponding garbage collector function. But this, by itself, will not yield very informative diagnostics, since the collector does not keep track of the information about how objects were allocated. The error reports will include only object addresses.

For more precise error reports, as much of the program as possible should use the all uppercase variants of these functions, after defining GC_DEBUG, and then including gc.h. In this environment GC_MALLOC is a macro which causes at least the file name and line number at the allocation point to be saved as part of the object. Leak reports will then also include this information.

Many collector features (e.g. finalization and disappearing links) are less useful in this context, and are not fully supported. Their use will usually generate additional bogus leak reports, since the collector itself drops some associated objects.

The same is generally true of thread support. However, the correct leak reports should be generated with LinuxThreads, at least.

On a few platforms (currently Linux/i686, Linux/x86_64 and SPARC), GC_MALLOC also causes some more information about its call stack to be saved in the object. Such information is reproduced in the error reports in very non-symbolic form, but it can be very useful with the aid of a debugger.

An Example

The leak_detector.h file is located in the include/gc subdirectory of the distribution.

Assume the collector has been built with -DFIND_LEAK or GC_set_find_leak(1) exists as the first statement in main.

The program to be tested for leaks could look like tests/leak.c file of the distribution.

On Linux/x86_64 the output to the stderr stream would be like:

Found 1 leaked objects:
0x7f5229ccbe70 (tests/leak.c:50, sz= 6, NORMAL)
        Call chain at allocation:
                bdwgc/.libs/libgc.so.1(+0x173f3) [0x7f5229eff3f3]
                bdwgc/.libs/leaktest(+0x12f1) [0x55bcdfc5a2f1]
                /lib/x86_64-linux-gnu/libc.so.6(__libc_start_main+0xea) [0x7f5229d1fd0a]
                bdwgc/.libs/leaktest(+0x135a) [0x55bcdfc5a35a]

On Solaris/SPARC host the output would be like:

Found 1 leaked objects:
0xef621fc8 (tests/leak.c:50, sz= 6, NORMAL)
        Call chain at allocation:
                args: 4 (0x4), 200656 (0x30FD0)
                ##PC##= 0x14ADC
                args: 1 (0x1), -268436012 (0xEFFFFDD4)
                ##PC##= 0x14A64

On some operating systems the output would contain only information about the immediate caller:

Found 1 leaked objects:
0x10040fe0 (tests/leak.c:50, sz= 6, NORMAL)
        Caller at allocation:
                ##PC##= 0x10004910

On most other operating systems the output would look like:

Found 1 leaked objects:
0x806dff0 (tests/leak.c:50, sz= 6, NORMAL)

In the first 3 cases some information is given about how malloc was called when the leaked object was allocated. For Solaris, the first line specifies the arguments to GC_debug_malloc (the actual allocation routine), the second one specifies the program counter inside main, the third line specifies the arguments to main, and, finally, the program counter inside the caller to main (i.e. in the C startup code).

In many cases, a debugger is needed to interpret the additional information. On systems supporting the adb debugger, the tools/callprocs.sh script can be used to replace program counter values with symbolic names. The collector tries to generate symbolic names for call stacks if it knows how to do so on the platform. This is true on Linux/i686 and Linux/x86_64, but not on most other platforms.

Simplified leak detection under Linux

It should be possible to run the collector in the leak detection mode on a program a.out under Linux/i686 and Linux/x86_64 as follows:

  1. If possible, ensure that a.out is a single-threaded executable. On some platforms this does not work at all for the multi-threaded programs.
  2. If possible, ensure that the addr2line program is installed in /usr/bin. (It comes with most Linux distributions.)
  3. If possible, compile your program, which we'll call a.out, with full debug information. This will improve the quality of the leak reports. With this approach, it is no longer necessary to call GC_ routines explicitly, though that can also improve the quality of the leak reports.
  4. Build the collector and install it in directory foo as follows (it may be safe to omit the --disable-threads option on Linux, but the combination of thread support and malloc replacement is not yet rock solid):
  • configure --prefix=_foo_ --enable-gc-debug --enable-redirect-malloc --disable-threads
  • make
  • make install
  1. Set environment variables as follows (the last two are optional, just to confirm the collector is running, and to facilitate debugging from another console window if something goes wrong, respectively):
  • LD_PRELOAD=_foo_/lib/libgc.so
  • GC_FIND_LEAK
  • GC_PRINT_STATS
  • GC_LOOP_ON_ABORT
  1. Simply run a.out as you normally would. Note that if you run anything else (e.g. your editor) with those environment variables set, it will also be leak tested. This may or may not be useful and/or embarrassing. It can generate mountains of leak reports if the application was not designed to avoid leaks, e.g. because it's always short-lived.

This has not yet been thoroughly tested on large applications, but it's known to do the right thing on at least some small ones.