# 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 5.3 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.

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.

## An Example

The following header file `leak_detector.h` is included in the "include" subdirectory of the distribution:

    #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 `tools/callprocs.sh` 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.

## Simplified leak detection under Linux

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:

1. Ensure that a.out is a single-threaded executable, or you are using a very recent (7.0alpha7+) collector version on Linux. On most platforms this does not work at all for the multi-threaded programs.
2. If possible, ensure that the `addr2line` program is installed in `/usr/bin`. (It comes with most Linux distributions.)
3. If possible, compile your program, which we'll call `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.
4. Build the collector and install it in directory foo as follows:
   - `configure --prefix=foo --enable-gc-debug --enable-redirect-malloc --disable-threads`
   - `make`
   - `make install`
   With a very recent collector on Linux, it may sometimes be safe to omit the `--disable-threads`. But the combination of thread support and `malloc` replacement is not yet rock solid.
5. Set environment variables as follows:
   - `LD_PRELOAD=`foo`/lib/libgc.so`
   - `GC_FIND_LEAK`
   - 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).
6. Simply run `a.out` as you normally would. Note that if you run anything else (e.g. 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, e.g. because it's always short-lived.

This has not yet been thoroughly tested on large applications, but it's known to do the right thing on at least some small ones.