Debugging Memory Allocation in APR
The allocation mechanism's within APR have a number of debugging modes
that can be used to assist in finding memory problems. This document
describes the modes available and gives instructions on activating
them.
Debugging support: Define this to enable code which
helps detect re-use of free()d memory and other such
nonsense.
The theory is simple. The FILL_BYTE (0xa5)
is written over all malloc'd memory as we receive it, and
is written over everything that we free up during a
clear_pool. We check that blocks on the free list always
have the FILL_BYTE in them, and we check during
palloc() that the bytes still have FILL_BYTE
in them. If you ever see garbage URLs or whatnot containing lots
of 0xa5s then you know something used data that's been
freed or uninitialized.
If defined all allocations will be done with
malloc() and free()d appropriately at the
end.
This is intended to be used with something like Electric
Fence or Purify to help detect memory problems. Note that if
you're using efence then you should also add in ALLOC_DEBUG.
But don't add in ALLOC_DEBUG if you're using Purify because
ALLOC_DEBUG would hide all the uninitialized read errors
that Purify can diagnose.
This is intended to detect cases where the wrong pool is
used when assigning data to an object in another pool.
In particular, it causes the table_{set,add,merge}n
routines to check that their arguments are safe for the
apr_table_t they're being placed in. It currently only works
with the unix multiprocess model, but could be extended to others.
Provide diagnostic information about make_table() calls
which are possibly too small.
This requires a recent gcc which supports
__builtin_return_address(). The error_log output will be a
message such as:
table_push: apr_table_t created by 0x804d874 hit limit of 10
Use l *0x804d874 to find the
source that corresponds to. It indicates that a apr_table_t
allocated by a call at that address has possibly too small an
initial apr_table_t size guess.
Provide some statistics on the cost of allocations.
This requires a bit of an understanding of how alloc.c works.
Not all the options outlined above can be activated at the
same time. the following table gives more information.
| ALLOC DEBUG |
- | No | Yes | Yes | Yes |
|---|
| ALLOC USE MALLOC |
No | - | No | No | No |
|---|
| POOL DEBUG |
Yes | No | - | Yes | Yes |
|---|
| MAKE TABLE PROFILE |
Yes | No | Yes | - | Yes |
|---|
| ALLOC STATS |
Yes | No | Yes | Yes | - |
|---|
Additionally the debugging options are not suitable for
multi-threaded versions of the server. When trying to debug
with these options the server should be started in single
process mode.
The various options for debugging memory are now enabled in
the apr_general.h header file in APR. The various options are
enabled by uncommenting the define for the option you wish to
use. The section of the code currently looks like this
(contained in srclib/apr/include/apr_pools.h)
/*
#define ALLOC_DEBUG
#define POOL_DEBUG
#define ALLOC_USE_MALLOC
#define MAKE_TABLE_PROFILE
#define ALLOC_STATS
*/
typedef struct ap_pool_t {
union block_hdr *first;
union block_hdr *last;
struct cleanup *cleanups;
struct process_chain *subprocesses;
struct ap_pool_t *sub_pools;
struct ap_pool_t *sub_next;
struct ap_pool_t *sub_prev;
struct ap_pool_t *parent;
char *free_first_avail;
#ifdef ALLOC_USE_MALLOC
void *allocation_list;
#endif
#ifdef POOL_DEBUG
struct ap_pool_t *joined;
#endif
int (*apr_abort)(int retcode);
struct datastruct *prog_data;
} ap_pool_t;
To enable allocation debugging simply move the #define
ALLOC_DEBUG above the start of the comments block and rebuild
the server.
Note
In order to use the various options the server must
be rebuilt after editing the header file.
