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

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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

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Provide some statistics on the cost of allocations.

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

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Allowable Combinations

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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.