Debugging Memory Allocation in APR

Available Languages: en

- -

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.

Available debugging options
Allowable Combinations
Activating Debugging Options

Available debugging options

Allocation Debugging - ALLOC_DEBUG

- - -

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.

- - -

Malloc Support - ALLOC_USE_MALLOC

- - -

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.

- - -

Pool Debugging - POOL_DEBUG

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.

- - -

Table Debugging - MAKE_TABLE_PROFILE

- - -

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.

- - -

Allocation Statistics - ALLOC_STATS

- - -

Provide some statistics on the cost of allocations.

- -

This requires a bit of an understanding of how alloc.c works.

- -

Allowable Combinations

- -

Not all the options outlined above can be activated at the - same time. the following table gives more information.

- - - - - - - - - - - - - - - - -

-	ALLOC DEBUG	ALLOC USE MALLOC	POOL DEBUG	MAKE TABLE PROFILE	ALLOC STATS
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.

Activating Debugging Options

- -

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.