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 in 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.

All non-ancient versions of the garbage collector provide leak detection support. Version 5.0 alpha 3 adds the following features:

  1. 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.
  2. Leaked objects should be reported and then correctly garbage collected. Prior versions either reported leaks or functioned as a garbage collector.
For the rest of this description we will give instructions that work with any reasonable version of the collector.

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

  1. Build the collector with -DFIND_LEAK. Otherwise use default build options.
  2. Change the program so that all allocation and deallocation goes through the garbage collector.
  3. Arrange to call GC_gcollect at appropriate points to check for leaks. (For sufficiently long running programs, this will happen implicitly, but probably not with sufficient frequency.)
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 and free 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.

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.

On a few platforms (currently Solaris and Irix), 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

Assume we have the following header file leak_detector.h: 
#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(n)
#define CHECK_LEAKS() GC_gcollect()

Assume the collector has been built with -DFIND_LEAK. (For very new 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 collect 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 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 all 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.