mirror of
https://gcc.gnu.org/git/gcc.git
synced 2024-11-27 13:54:19 +08:00
198 lines
8.1 KiB
HTML
198 lines
8.1 KiB
HTML
<HTML>
|
|
<HEAD>
|
|
<TITLE>Using the Garbage Collector as Leak Detector</title>
|
|
</head>
|
|
<BODY>
|
|
<H1>Using the Garbage Collector as Leak Detector</h1>
|
|
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 <TT>GC_MALLOC</tt>),
|
|
not deallocated (normally with <TT>GC_FREE</tt>), but are
|
|
no longer accessible. Since the object is no longer accessible,
|
|
there in normally no way to deallocate the object at a later time;
|
|
thus it can safely be assumed that the object has been "leaked".
|
|
<P>
|
|
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.
|
|
<P>
|
|
All non-ancient versions of the garbage collector provide
|
|
leak detection support. Version 5.3 adds the following
|
|
features:
|
|
<OL>
|
|
<LI> Leak detection mode can be initiated at run-time by
|
|
setting GC_find_leak instead of building the collector with FIND_LEAK
|
|
defined. This variable should be set to a nonzero value
|
|
at program startup.
|
|
<LI> Leaked objects should be reported and then correctly garbage collected.
|
|
Prior versions either reported leaks or functioned as a garbage collector.
|
|
</ol>
|
|
For the rest of this description we will give instructions that work
|
|
with any reasonable version of the collector.
|
|
<P>
|
|
To use the collector as a leak detector, follow the following steps:
|
|
<OL>
|
|
<LI> Build the collector with -DFIND_LEAK. Otherwise use default
|
|
build options.
|
|
<LI> Change the program so that all allocation and deallocation goes
|
|
through the garbage collector.
|
|
<LI> Arrange to call <TT>GC_gcollect</tt> at appropriate points to check
|
|
for leaks.
|
|
(For sufficiently long running programs, this will happen implicitly,
|
|
but probably not with sufficient frequency.)
|
|
</ol>
|
|
The second step can usually be accomplished with the
|
|
<TT>-DREDIRECT_MALLOC=GC_malloc</tt> option when the collector is built,
|
|
or by defining <TT>malloc</tt>, <TT>calloc</tt>,
|
|
<TT>realloc</tt> and <TT>free</tt>
|
|
to call the corresponding garbage collector functions.
|
|
But this, by itself, will not yield very informative diagnostics,
|
|
since the collector does not keep track of information about
|
|
how objects were allocated. The error reports will include
|
|
only object addresses.
|
|
<P>
|
|
For more precise error reports, as much of the program as possible
|
|
should use the all uppercase variants of these functions, after
|
|
defining <TT>GC_DEBUG</tt>, and then including <TT>gc.h</tt>.
|
|
In this environment <TT>GC_MALLOC</tt> 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.
|
|
<P>
|
|
Many collector features (<I>e.g</i> stubborn objects, 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.
|
|
<P>
|
|
The same is generally true of thread support. However, as of 6.0alpha4,
|
|
correct leak reports should be generated with linuxthreads.
|
|
<P>
|
|
On a few platforms (currently Solaris/SPARC, Irix, and, with -DSAVE_CALL_CHAIN,
|
|
Linux/X86), <TT>GC_MALLOC</tt>
|
|
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.
|
|
<H2>An Example</h2>
|
|
The following header file <TT>leak_detector.h</tt> is included in the
|
|
"include" subdirectory of the distribution:
|
|
<PRE>
|
|
#define GC_DEBUG
|
|
#include "gc.h"
|
|
#define malloc(n) GC_MALLOC(n)
|
|
#define calloc(m,n) GC_MALLOC((m)*(n))
|
|
#define free(p) GC_FREE(p)
|
|
#define realloc(p,n) GC_REALLOC((p),(n))
|
|
#define CHECK_LEAKS() GC_gcollect()
|
|
</pre>
|
|
<P>
|
|
Assume the collector has been built with -DFIND_LEAK. (For very
|
|
new versions of the collector, we could instead add the statement
|
|
<TT>GC_find_leak = 1</tt> as the first statement in <TT>main</tt>.
|
|
<P>
|
|
The program to be tested for leaks can then look like:
|
|
<PRE>
|
|
#include "leak_detector.h"
|
|
|
|
main() {
|
|
int *p[10];
|
|
int i;
|
|
/* GC_find_leak = 1; for new collector versions not */
|
|
/* compiled with -DFIND_LEAK. */
|
|
for (i = 0; i < 10; ++i) {
|
|
p[i] = malloc(sizeof(int)+i);
|
|
}
|
|
for (i = 1; i < 10; ++i) {
|
|
free(p[i]);
|
|
}
|
|
for (i = 0; i < 9; ++i) {
|
|
p[i] = malloc(sizeof(int)+i);
|
|
}
|
|
CHECK_LEAKS();
|
|
}
|
|
</pre>
|
|
<P>
|
|
On an Intel X86 Linux system this produces on the stderr stream:
|
|
<PRE>
|
|
Leaked composite object at 0x806dff0 (leak_test.c:8, sz=4)
|
|
</pre>
|
|
(On most unmentioned operating systems, the output is similar to this.
|
|
If the collector had been built on Linux/X86 with -DSAVE_CALL_CHAIN,
|
|
the output would be closer to the Solaris example. For this to work,
|
|
the program should not be compiled with -fomit_frame_pointer.)
|
|
<P>
|
|
On Irix it reports
|
|
<PRE>
|
|
Leaked composite object at 0x10040fe0 (leak_test.c:8, sz=4)
|
|
Caller at allocation:
|
|
##PC##= 0x10004910
|
|
</pre>
|
|
and on Solaris the error report is
|
|
<PRE>
|
|
Leaked composite object at 0xef621fc8 (leak_test.c:8, sz=4)
|
|
Call chain at allocation:
|
|
args: 4 (0x4), 200656 (0x30FD0)
|
|
##PC##= 0x14ADC
|
|
args: 1 (0x1), -268436012 (0xEFFFFDD4)
|
|
##PC##= 0x14A64
|
|
</pre>
|
|
In the latter two cases some additional information is given about
|
|
how malloc was called when the leaked object was allocated. For
|
|
Solaris, the first line specifies the arguments to <TT>GC_debug_malloc</tt>
|
|
(the actual allocation routine), The second the program counter inside
|
|
main, the third the arguments to <TT>main</tt>, and finally the program
|
|
counter inside the caller to main (i.e. in the C startup code).
|
|
<P>
|
|
In the Irix case, only the address inside the caller to main is given.
|
|
<P>
|
|
In many cases, a debugger is needed to interpret the additional information.
|
|
On systems supporting the "adb" debugger, the <TT>callprocs</tt> script
|
|
can be used to replace program counter values with symbolic names.
|
|
As of version 6.1, 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/X86, but not on most other platforms.
|
|
<H2>Simplified leak detection under Linux</h2>
|
|
Since version 6.1, it should be possible to run the collector in leak
|
|
detection mode on a program a.out under Linux/X86 as follows:
|
|
<OL>
|
|
<LI> Ensure that a.out is a single-threaded executable. This doesn't yet work
|
|
for multithreaded programs.
|
|
<LI> If possible, ensure that the addr2line program is installed in
|
|
/usr/bin. (It comes with RedHat Linux.)
|
|
<LI> If possible, compile 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.
|
|
<LI> Build the collector and install it in directory <I>foo</i> as follows:
|
|
<UL>
|
|
<LI> configure --prefix=<I>foo</i> --enable-full-debug --enable-redirect-malloc
|
|
--disable-threads
|
|
<LI> make
|
|
<LI> make install
|
|
</ul>
|
|
<LI> Set environment variables as follows:
|
|
<UL>
|
|
<LI> LD_PRELOAD=<I>foo</i>/lib/libgc.so
|
|
<LI> GC_FIND_LEAK
|
|
<LI> You may also want to set GC_PRINT_STATS (to confirm that the collector
|
|
is running) and/or GC_LOOP_ON_ABORT (to facilitate debugging from another
|
|
window if something goes wrong).
|
|
</ul
|
|
<LI> Simply run a.out as you normally would. Note that if you run anything
|
|
else (<I>e.g.</i> 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 wasn't designed to avoid leaks,
|
|
<I>e.g.</i> because it's always short-lived.
|
|
</ol>
|
|
This has not yet been thropughly tested on large applications, but it's known
|
|
to do the right thing on at least some small ones.
|
|
</body>
|
|
</html>
|