The NSAllocDebugZone class

The NSDefaultZone uses system malloc, calloc, realloc and free directly, without any checks.

The NSAllocDebugZone is used to help in tracking down memory allocation bugs. It provides the following features:

• check double deallocation for pointers
• check deallocation of non-existing pointers
• use a block of memory past its margins
• list blocks allocated since a moment (marked by a special call)
• each allocated pointer has a serial number
• stop when alloc-ing a pointer with a certain serial number
• do SYSTEM_MALLOC_CHECK and internal checks every count operation
• be able to control things from gdb
• be able to control things from environment variables

The NSAllocDebugZone is controlled by the following environment variables:

• ALLOCDEBUG: must be set to something to use the alloc debug zone
• ALLOCDEBUG_STOP: stop in debugger SIGINT when alloc-ing pointer with given serial number (this serial number is reported when and error with that pointer occurs). Undefined or 0 means no stop.
• ALLOCDEBUG_COUNT: number of passes inside allocation/deallocation functions to SYSTEM_MALLOC_CHECK and internal check. Undefined or 0 means no checks.
• ALLOCDEBUG_UPPER and ALLOCDEBUG_LOWER: number of bytes to alloc at top/bottom of object block. These bytes are set to a given value (0x88) and checked at free and internal check to guard against using memory past the limit. Undefined or zero means no margin. Note that this size must be multiples of the machine address alignment (4, 8, 16 are recommended values).

The NSAllocDebugZone provides these functions to be used from the GNU debugger.

• debuggerStopMark(unsigned mark): overrides ALLOCDEBUG_STOP
• debuggerCheckTime(unsigned count): overrides ALLOCDEBUG_COUNT
• debuggerDescription(id obj): performs printf("%s\n", [obj description])
• debuggerPerform(id obj, char* sel): performs [obj sel]
• debuggerPerformWith(id obj, char* sel, id arg): performs
  [obj sel:(id)atoi(arg)]

The program instantiates two zones: one NSDefaultZone and one NSAllocDebugZone, and uses one or the other depending on the ALLOCDEBUG environment variable. If this variable exists in your environment then the NSAllocDebugZone instance will be used.

Below are some setting you might find useful when working with the NSAllocDebugZone class.

# must be set to something to use alloc debug zone
ALLOCDEBUG=YES
export ALLOCDEBUG=YES

# stop in debugger (SIGINT) when alloc-ing pointer number *
# nil or 0 means no stop
ALLOCDEBUG_STOP=0
export ALLOCDEBUG_STOP

# stop in debugger (SIGINT) when alloc-ing pointer number *
# nil or 0 means no stop
ALLOCDEBUG_MARK=0
export ALLOCDEBUG_MARK

# number of passes inside allocation/deallocation functions
# to SYSTEM_MALLOC_CHECK and internal check
# nil or 0 means no checks
ALLOCDEBUG_COUNT=16
export ALLOCDEBUG_COUNT

# number of bytes to alloc at top/bottom of object block
# these bytes are set to a given value (0x88) and checked
# at free and internal check to guard against using memory
# past the limit.
ALLOCDEBUG_UPPER=8
ALLOCDEBUG_LOWER=8
export ALLOCDEBUG_UPPER ALLOCDEBUG_LOWER

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