Tribblix: manual page: watchmalloc.3malloc

WATCHMALLOC(3MALLOC) Memory Allocation Library Functions

NAME

watchmalloc - debugging memory allocator

SYNOPSIS

#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);

DESCRIPTION

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 ld.so.1(1)):

LD_PRELOAD=watchmalloc.so.1

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 WATCHPOINTS below):

o Memory may not be freed more than once.

o A pointer to freed memory may not be used in a call to
realloc().

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 results.

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.

WATCHPOINTS

The watchpoint facility of /proc can be applied by a process to
itself. The functions in watchmalloc.so.1 use this feature if the
following string is present in the environment:

MALLOC_DEBUG=WATCH

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
options:

MALLOC_DEBUG=WATCH,RW,STOP

WATCH
Enables WA_WRITE watched areas as described above.

RW
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.

STOP
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.

LIMITATIONS

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 watchmalloc.so.1 fails innocuously if the target
application is statically linked with respect to its malloc()
functions.