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1 | <HTML> |
2 | <HEAD> | |
3 | <TITLE>Using the Garbage Collector as Leak Detector</title> | |
4 | </head> | |
5 | <BODY> | |
6 | <H1>Using the Garbage Collector as Leak Detector</h1> | |
7 | The garbage collector may be used as a leak detector. | |
8 | In this case, the primary function of the collector is to report | |
9 | objects that were allocated (typically with <TT>GC_MALLOC</tt>), | |
10 | not deallocated (normally with <TT>GC_FREE</tt>), but are | |
11 | no longer accessible. Since the object is no longer accessible, | |
12 | there in normally no way to deallocate the object at a later time; | |
13 | thus it can safely be assumed that the object has been "leaked". | |
14 | <P> | |
15 | This is substantially different from counting leak detectors, | |
16 | which simply verify that all allocated objects are eventually | |
17 | deallocated. A garbage-collector based leak detector can provide | |
18 | somewhat more precise information when an object was leaked. | |
19 | More importantly, it does not report objects that are never | |
20 | deallocated because they are part of "permanent" data structures. | |
21 | Thus it does not require all objects to be deallocated at process | |
22 | exit time, a potentially useless activity that often triggers | |
23 | large amounts of paging. | |
24 | <P> | |
25 | All non-ancient versions of the garbage collector provide | |
26 | leak detection support. Version 5.3 adds the following | |
27 | features: | |
28 | <OL> | |
29 | <LI> Leak detection mode can be initiated at run-time by | |
30 | setting GC_find_leak instead of building the collector with FIND_LEAK | |
31 | defined. This variable should be set to a nonzero value | |
32 | at program startup. | |
33 | <LI> Leaked objects should be reported and then correctly garbage collected. | |
34 | Prior versions either reported leaks or functioned as a garbage collector. | |
35 | </ol> | |
36 | For the rest of this description we will give instructions that work | |
37 | with any reasonable version of the collector. | |
38 | <P> | |
39 | To use the collector as a leak detector, follow the following steps: | |
40 | <OL> | |
41 | <LI> Build the collector with -DFIND_LEAK. Otherwise use default | |
42 | build options. | |
43 | <LI> Change the program so that all allocation and deallocation goes | |
44 | through the garbage collector. | |
45 | <LI> Arrange to call <TT>GC_gcollect</tt> at appropriate points to check | |
46 | for leaks. | |
47 | (For sufficiently long running programs, this will happen implicitly, | |
48 | but probably not with sufficient frequency.) | |
49 | </ol> | |
50 | The second step can usually be accomplished with the | |
51 | <TT>-DREDIRECT_MALLOC=GC_malloc</tt> option when the collector is built, | |
52 | or by defining <TT>malloc</tt>, <TT>calloc</tt>, | |
53 | <TT>realloc</tt> and <TT>free</tt> | |
54 | to call the corresponding garbage collector functions. | |
55 | But this, by itself, will not yield very informative diagnostics, | |
56 | since the collector does not keep track of information about | |
57 | how objects were allocated. The error reports will include | |
58 | only object addresses. | |
59 | <P> | |
60 | For more precise error reports, as much of the program as possible | |
61 | should use the all uppercase variants of these functions, after | |
62 | defining <TT>GC_DEBUG</tt>, and then including <TT>gc.h</tt>. | |
63 | In this environment <TT>GC_MALLOC</tt> is a macro which causes | |
64 | at least the file name and line number at the allocation point to | |
65 | be saved as part of the object. Leak reports will then also include | |
66 | this information. | |
67 | <P> | |
68 | Many collector features (<I>e.g</i> stubborn objects, finalization, | |
69 | and disappearing links) are less useful in this context, and are not | |
70 | fully supported. Their use will usually generate additional bogus | |
71 | leak reports, since the collector itself drops some associated objects. | |
72 | <P> | |
73 | The same is generally true of thread support. However, as of 6.0alpha4, | |
74 | correct leak reports should be generated with linuxthreads. | |
75 | <P> | |
76 | On a few platforms (currently Solaris/SPARC, Irix, and, with -DSAVE_CALL_CHAIN, | |
77 | Linux/X86), <TT>GC_MALLOC</tt> | |
78 | also causes some more information about its call stack to be saved | |
79 | in the object. Such information is reproduced in the error | |
80 | reports in very non-symbolic form, but it can be very useful with the | |
81 | aid of a debugger. | |
82 | <H2>An Example</h2> | |
83 | The following header file <TT>leak_detector.h</tt> is included in the | |
84 | "include" subdirectory of the distribution: | |
85 | <PRE> | |
86 | #define GC_DEBUG | |
87 | #include "gc.h" | |
88 | #define malloc(n) GC_MALLOC(n) | |
89 | #define calloc(m,n) GC_MALLOC((m)*(n)) | |
90 | #define free(p) GC_FREE(p) | |
91 | #define realloc(p,n) GC_REALLOC((p),(n)) | |
92 | #define CHECK_LEAKS() GC_gcollect() | |
93 | </pre> | |
94 | <P> | |
95 | Assume the collector has been built with -DFIND_LEAK. (For very | |
96 | new versions of the collector, we could instead add the statement | |
97 | <TT>GC_find_leak = 1</tt> as the first statement in <TT>main</tt>. | |
98 | <P> | |
99 | The program to be tested for leaks can then look like: | |
100 | <PRE> | |
101 | #include "leak_detector.h" | |
102 | ||
103 | main() { | |
104 | int *p[10]; | |
105 | int i; | |
106 | /* GC_find_leak = 1; for new collector versions not */ | |
107 | /* compiled with -DFIND_LEAK. */ | |
108 | for (i = 0; i < 10; ++i) { | |
109 | p[i] = malloc(sizeof(int)+i); | |
110 | } | |
111 | for (i = 1; i < 10; ++i) { | |
112 | free(p[i]); | |
113 | } | |
114 | for (i = 0; i < 9; ++i) { | |
115 | p[i] = malloc(sizeof(int)+i); | |
116 | } | |
117 | CHECK_LEAKS(); | |
118 | } | |
119 | </pre> | |
120 | <P> | |
121 | On an Intel X86 Linux system this produces on the stderr stream: | |
122 | <PRE> | |
123 | Leaked composite object at 0x806dff0 (leak_test.c:8, sz=4) | |
124 | </pre> | |
125 | (On most unmentioned operating systems, the output is similar to this. | |
126 | If the collector had been built on Linux/X86 with -DSAVE_CALL_CHAIN, | |
127 | the output would be closer to the Solaris example. For this to work, | |
128 | the program should not be compiled with -fomit_frame_pointer.) | |
129 | <P> | |
130 | On Irix it reports | |
131 | <PRE> | |
132 | Leaked composite object at 0x10040fe0 (leak_test.c:8, sz=4) | |
133 | Caller at allocation: | |
134 | ##PC##= 0x10004910 | |
135 | </pre> | |
136 | and on Solaris the error report is | |
137 | <PRE> | |
138 | Leaked composite object at 0xef621fc8 (leak_test.c:8, sz=4) | |
139 | Call chain at allocation: | |
140 | args: 4 (0x4), 200656 (0x30FD0) | |
141 | ##PC##= 0x14ADC | |
142 | args: 1 (0x1), -268436012 (0xEFFFFDD4) | |
143 | ##PC##= 0x14A64 | |
144 | </pre> | |
145 | In the latter two cases some additional information is given about | |
146 | how malloc was called when the leaked object was allocated. For | |
147 | Solaris, the first line specifies the arguments to <TT>GC_debug_malloc</tt> | |
148 | (the actual allocation routine), The second the program counter inside | |
149 | main, the third the arguments to <TT>main</tt>, and finally the program | |
150 | counter inside the caller to main (i.e. in the C startup code). | |
151 | <P> | |
152 | In the Irix case, only the address inside the caller to main is given. | |
153 | <P> | |
154 | In many cases, a debugger is needed to interpret the additional information. | |
155 | On systems supporting the "adb" debugger, the <TT>callprocs</tt> script | |
156 | can be used to replace program counter values with symbolic names. | |
157 | As of version 6.1, the collector tries to generate symbolic names for | |
158 | call stacks if it knows how to do so on the platform. This is true on | |
159 | Linux/X86, but not on most other platforms. | |
160 | <H2>Simplified leak detection under Linux</h2> | |
161 | Since version 6.1, it should be possible to run the collector in leak | |
162 | detection mode on a program a.out under Linux/X86 as follows: | |
163 | <OL> | |
164 | <LI> <I>Ensure that a.out is a single-threaded executable, or you are using | |
165 | a very recent (7.0alpha7+) collector version on Linux.</i> | |
166 | On most platforms this does not | |
167 | work at all for multithreaded programs. | |
168 | <LI> If possible, ensure that the addr2line program is installed in | |
169 | /usr/bin. (It comes with RedHat Linux.) | |
170 | <LI> If possible, compile a.out with full debug information. | |
171 | This will improve the quality of the leak reports. With this approach, it is | |
172 | no longer necessary to call GC_ routines explicitly, though that can also | |
173 | improve the quality of the leak reports. | |
174 | <LI> Build the collector and install it in directory <I>foo</i> as follows: | |
175 | <UL> | |
176 | <LI> <TT>configure --prefix=<I>foo</i> --enable-full-debug --enable-redirect-malloc | |
177 | --disable-threads</tt> | |
178 | <LI> <TT>make</tt> | |
179 | <LI> <TT>make install</tt> | |
180 | </ul> | |
181 | With a very recent collector on Linux, it may be safe to omit the <TT>--disable-threads</tt>. | |
182 | <LI> Set environment variables as follows: | |
183 | <UL> | |
184 | <LI> LD_PRELOAD=<I>foo</i>/lib/libgc.so | |
185 | <LI> GC_FIND_LEAK | |
186 | <LI> You may also want to set GC_PRINT_STATS (to confirm that the collector | |
187 | is running) and/or GC_LOOP_ON_ABORT (to facilitate debugging from another | |
188 | window if something goes wrong). | |
189 | </ul | |
190 | <LI> Simply run a.out as you normally would. Note that if you run anything | |
191 | else (<I>e.g.</i> your editor) with those environment variables set, | |
192 | it will also be leak tested. This may or may not be useful and/or | |
193 | embarrassing. It can generate | |
194 | mountains of leak reports if the application wasn't designed to avoid leaks, | |
195 | <I>e.g.</i> because it's always short-lived. | |
196 | </ol> | |
197 | This has not yet been thoroughly tested on large applications, but it's known | |
198 | to do the right thing on at least some small ones. | |
199 | </body> | |
200 | </html> |