25. leakFinder

src/leakfinder.c contains some routines useful for debugging and finding memory leaks. It is not enabled by default. To enable it, use

configure --enable-leakfinder ...

The module has three public functions: leakAdd, leakFree, and leakTouch Note, these are actually macros that insert __FILE__ and __LINE__ arguments to the real functions.

leakAdd should be called when a pointer is first created. Usually this follows immediately after a call to malloc or some other memory allocation function. For example:

    ...
    void *p;
    p = malloc(100);
    leakAdd(p);
    ...

leakFree is the opposite. Call it just before releasing the pointer memory, such as a call to free. For example:

    ...
    leakFree(foo);
    free(foo);
    return;

The definition of a leak is memory that was allocated but never freed. Thus, to find a leak we need to track the pointer between the time it got allocated and the time when it should have been freed. Use leakTouch to accomplish this. You can sprinkle leakTouch calls throughout the code where the pointer is used. For example:

void
myfunc(void *ptr)
{
    ...
    leakTouch(ptr);
    ...
}

For each pointer tracked, the module remembers the filename, line number, and time of last access. You can view this data with the cache manager by selecting the leaks option. You can also do it from the command line:

% client mgr:leaks | less

The way to identify possible leaks is to look at the time of last access. Pointers that haven't been accessed for a long time are candidates for leaks. The filename and line numbers tell you where that pointer was last accessed. If there is a leak, then the bug occurs somewhere after that point of the code.