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.
All non-ancient versions of the garbage collector provide leak detection support. Version 5.3 adds the following features:
To use the collector as a leak detector, follow the following steps:
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 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.
The same is generally true of thread support. However, as of 6.0alpha4, correct leak reports should be generated with linuxthreads.
On a few platforms (currently Solaris/SPARC, Irix, and, with -DSAVE_CALL_CHAIN, Linux/X86), 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.
#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()
Assume the collector has been built with -DFIND_LEAK. (For newer versions of the collector, we could instead add the statement GC_find_leak = 1 as the first statement in main().
The program to be tested for leaks can then look like:
#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();
}
On an Intel X86 Linux system this produces on the stderr stream:
Leaked composite object at 0x806dff0 (leak_test.c:8, sz=4)(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.)
On Irix it reports
Leaked composite object at 0x10040fe0 (leak_test.c:8, sz=4)
Caller at allocation:
##PC##= 0x10004910
and on Solaris the error report is
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
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 GC_debug_malloc
(the actual allocation routine), The second the program counter inside
main, the third the arguments to main, and finally the program
counter inside the caller to main (i.e. in the C startup code).
In the Irix case, only the address inside the caller to main is given.
In many cases, a debugger is needed to interpret the additional information. On systems supporting the "adb" debugger, the callprocs 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.