WATCHMALLOC(3MALLOC) Memory Allocation Library Functions WATCHMALLOC(3MALLOC)


watchmalloc - debugging memory allocator


#include <stdlib.h>

void *malloc(size_t size);

void free(void *ptr);

void *realloc(void *ptr, size_t size);

void *memalign(size_t alignment, size_t size);

void *valloc(size_t size);

void *calloc(size_t nelem, size_t elsize);

#include <malloc.h>

int mallopt(int cmd, int value);

struct mallinfo mallinfo(void);


The collection of malloc() functions in this shared object are an
optional replacement for the standard versions of the same functions in
the system C library. See malloc(3C). They provide a more strict
interface than the standard versions and enable enforcement of the
interface through the watchpoint facility of /proc. See proc(5).

Any dynamically linked application can be run with these functions in
place of the standard functions if the following string is present in the
environment (see

The individual function interfaces are identical to the standard ones as
described in malloc(3C). However, laxities provided in the standard
versions are not permitted when the watchpoint facility is enabled (see

o Memory may not be freed more than once.

o A pointer to freed memory may not be used in a call to

o A call to malloc() immediately following a call to free() will
not return the same space.

o Any reference to memory that has been freed yields undefined

To enforce these restrictions partially, without great loss in speed as
compared to the watchpoint facility described below, a freed block of
memory is overwritten with the pattern 0xdeadbeef before returning from
free(). The malloc() function returns with the allocated memory filled
with the pattern 0xbaddcafe as a precaution against applications
incorrectly expecting to receive back unmodified memory from the last
free(). The calloc() function always returns with the memory zero-filled.

Entry points for mallopt() and mallinfo() are provided as empty routines,
and are present only because some malloc() implementations provide them.


The watchpoint facility of /proc can be applied by a process to itself.
The functions in use this feature if the following
string is present in the environment:


This causes every block of freed memory to be covered with WA_WRITE
watched areas. If the application attempts to write any part of freed
memory, it will trigger a watchpoint trap, resulting in a SIGTRAP signal,
which normally produces an application core dump.

A header is maintained before each block of allocated memory. Each header
is covered with a watched area, thereby providing a red zone before and
after each block of allocated memory (the header for the subsequent
memory block serves as the trailing red zone for its preceding memory
block). Writing just before or just after a memory block returned by
malloc() will trigger a watchpoint trap.

Watchpoints incur a large performance penalty. Requesting
MALLOC_DEBUG=WATCH can cause the application to run 10 to 100 times
slower, depending on the use made of allocated memory.

Further options are enabled by specifying a comma-separated string of


Enables WA_WRITE watched areas as described above.

Enables both WA_READ and WA_WRITE watched areas. An attempt
either to read or write freed memory or the red zones will
trigger a watchpoint trap. This incurs even more overhead and
can cause the application to run up to 1000 times slower.

The process will stop showing a FLTWATCH machine fault if it
triggers a watchpoint trap, rather than dumping core with a
SIGTRAP signal. This allows a debugger to be attached to the
live process at the point where it underwent the watchpoint
trap. Also, the various /proc tools described in proc(1) can be
used to examine the stopped process.

One of WATCH or RW must be specified, else the watchpoint facility is not
engaged. RW overrides WATCH. Unrecognized options are silently ignored.


Sizes of memory blocks allocated by malloc() are rounded up to the worst-
case alignment size, 8 bytes for 32-bit processes and 16 bytes for 64-bit
processes. Accessing the extra space allocated for a memory block is
technically a memory violation but is in fact innocuous. Such accesses
are not detected by the watchpoint facility of watchmalloc.

Interposition of fails innocuously if the target
application is statically linked with respect to its malloc() functions.


See attributes(7) for descriptions of the following attributes:

|MT-Level | MT-Safe |


proc(1), calloc(3C), free(3C), malloc(3C), memalign(3C), realloc(3C),
valloc(3C), libmapmalloc(3LIB), bsdmalloc(3MALLOC), malloc(3MALLOC),
mapmalloc(3MALLOC), proc(5), attributes(7)

January 10, 2007 WATCHMALLOC(3MALLOC)