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<TITLE>Using the Garbage Collector as Leak Detector</title>
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<H1>Using the Garbage Collector as Leak Detector</h1>
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The garbage collector may be used as a leak detector.
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In this case, the primary function of the collector is to report
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objects that were allocated (typically with <TT>GC_MALLOC</tt>),
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not deallocated (normally with <TT>GC_FREE</tt>), but are
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no longer accessible. Since the object is no longer accessible,
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there in normally no way to deallocate the object at a later time;
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thus it can safely be assumed that the object has been "leaked".
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This is substantially different from counting leak detectors,
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which simply verify that all allocated objects are eventually
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deallocated. A garbage-collector based leak detector can provide
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somewhat more precise information when an object was leaked.
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More importantly, it does not report objects that are never
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deallocated because they are part of "permanent" data structures.
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Thus it does not require all objects to be deallocated at process
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exit time, a potentially useless activity that often triggers
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large amounts of paging.
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All non-ancient versions of the garbage collector provide
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leak detection support. Version 5.3 adds the following
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<LI> Leak detection mode can be initiated at run-time by
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setting GC_find_leak instead of building the collector with FIND_LEAK
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defined. This variable should be set to a nonzero value
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<LI> Leaked objects should be reported and then correctly garbage collected.
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Prior versions either reported leaks or functioned as a garbage collector.
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For the rest of this description we will give instructions that work
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with any reasonable version of the collector.
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To use the collector as a leak detector, follow the following steps:
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<LI> Build the collector with -DFIND_LEAK. Otherwise use default
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<LI> Change the program so that all allocation and deallocation goes
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through the garbage collector.
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<LI> Arrange to call <TT>GC_gcollect</tt> at appropriate points to check
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(For sufficiently long running programs, this will happen implicitly,
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but probably not with sufficient frequency.)
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The second step can usually be accomplished with the
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<TT>-DREDIRECT_MALLOC=GC_malloc</tt> option when the collector is built,
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or by defining <TT>malloc</tt>, <TT>calloc</tt>,
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<TT>realloc</tt> and <TT>free</tt>
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to call the corresponding garbage collector functions.
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But this, by itself, will not yield very informative diagnostics,
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since the collector does not keep track of information about
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how objects were allocated. The error reports will include
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only object addresses.
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For more precise error reports, as much of the program as possible
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should use the all uppercase variants of these functions, after
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defining <TT>GC_DEBUG</tt>, and then including <TT>gc.h</tt>.
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In this environment <TT>GC_MALLOC</tt> is a macro which causes
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at least the file name and line number at the allocation point to
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be saved as part of the object. Leak reports will then also include
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Many collector features (<I>e.g</i> stubborn objects, finalization,
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and disappearing links) are less useful in this context, and are not
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fully supported. Their use will usually generate additional bogus
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leak reports, since the collector itself drops some associated objects.
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The same is generally true of thread support. However, as of 6.0alpha4,
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correct leak reports should be generated with linuxthreads.
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On a few platforms (currently Solaris/SPARC, Irix, and, with -DSAVE_CALL_CHAIN,
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Linux/X86), <TT>GC_MALLOC</tt>
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also causes some more information about its call stack to be saved
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in the object. Such information is reproduced in the error
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reports in very non-symbolic form, but it can be very useful with the
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The following header file <TT>leak_detector.h</tt> is included in the
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"include" subdirectory of the distribution:
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#define malloc(n) GC_MALLOC(n)
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#define calloc(m,n) GC_MALLOC((m)*(n))
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#define free(p) GC_FREE(p)
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#define realloc(p,n) GC_REALLOC((p),(n))
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#define CHECK_LEAKS() GC_gcollect()
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Assume the collector has been built with -DFIND_LEAK. (For very
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new versions of the collector, we could instead add the statement
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<TT>GC_find_leak = 1</tt> as the first statement in <TT>main</tt>.
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The program to be tested for leaks can then look like:
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#include "leak_detector.h"
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/* GC_find_leak = 1; for new collector versions not */
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/* compiled with -DFIND_LEAK. */
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for (i = 0; i < 10; ++i) {
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p[i] = malloc(sizeof(int)+i);
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for (i = 1; i < 10; ++i) {
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for (i = 0; i < 9; ++i) {
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p[i] = malloc(sizeof(int)+i);
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On an Intel X86 Linux system this produces on the stderr stream:
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Leaked composite object at 0x806dff0 (leak_test.c:8, sz=4)
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(On most unmentioned operating systems, the output is similar to this.
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If the collector had been built on Linux/X86 with -DSAVE_CALL_CHAIN,
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the output would be closer to the Solaris example. For this to work,
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the program should not be compiled with -fomit_frame_pointer.)
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Leaked composite object at 0x10040fe0 (leak_test.c:8, sz=4)
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Caller at allocation:
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and on Solaris the error report is
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Leaked composite object at 0xef621fc8 (leak_test.c:8, sz=4)
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Call chain at allocation:
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args: 4 (0x4), 200656 (0x30FD0)
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args: 1 (0x1), -268436012 (0xEFFFFDD4)
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In the latter two cases some additional information is given about
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how malloc was called when the leaked object was allocated. For
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Solaris, the first line specifies the arguments to <TT>GC_debug_malloc</tt>
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(the actual allocation routine), The second the program counter inside
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main, the third the arguments to <TT>main</tt>, and finally the program
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counter inside the caller to main (i.e. in the C startup code).
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In the Irix case, only the address inside the caller to main is given.
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In many cases, a debugger is needed to interpret the additional information.
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On systems supporting the "adb" debugger, the <TT>callprocs</tt> script
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can be used to replace program counter values with symbolic names.
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As of version 6.1, the collector tries to generate symbolic names for
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call stacks if it knows how to do so on the platform. This is true on
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Linux/X86, but not on most other platforms.
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<H2>Simplified leak detection under Linux</h2>
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Since version 6.1, it should be possible to run the collector in leak
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detection mode on a program a.out under Linux/X86 as follows:
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<LI> Ensure that a.out is a single-threaded executable. This doesn't yet work
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for multithreaded programs.
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<LI> If possible, ensure that the addr2line program is installed in
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/usr/bin. (It comes with RedHat Linux.)
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<LI> If possible, compile a.out with full debug information.
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This will improve the quality of the leak reports. With this approach, it is
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no longer necessary to call GC_ routines explicitly, though that can also
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improve the quality of the leak reports.
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<LI> Build the collector and install it in directory <I>foo</i> as follows:
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<LI> configure --prefix=<I>foo</i> --enable-full-debug --enable-redirect-malloc
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<LI> Set environment variables as follows:
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<LI> LD_PRELOAD=<I>foo</i>/lib/libgc.so
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<LI> You may also want to set GC_PRINT_STATS (to confirm that the collector
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is running) and/or GC_LOOP_ON_ABORT (to facilitate debugging from another
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window if something goes wrong).
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<LI> Simply run a.out as you normally would. Note that if you run anything
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else (<I>e.g.</i> your editor) with those environment variables set,
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it will also be leak tested. This may or may not be useful and/or
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embarrassing. It can generate
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mountains of leak reports if the application wasn't designed to avoid leaks,
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<I>e.g.</i> because it's always short-lived.
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This has not yet been thropughly tested on large applications, but it's known
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to do the right thing on at least some small ones.
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