Memory Allocation Library Functions watchmalloc(3MALOC)
NAME
watchmalloc - debugging memory allocator
SYNOPSIS
#include
void *malloc(sizet size);
void free(void *ptr);
void *realloc(void *ptr, sizet size);
void *memalign(sizet alignment, sizet size);
void *valloc(sizet size);
void *calloc(sizet nelem, sizet elsize);
#include
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 ver-
sions and enable enforcement of the interface through the
watchpoint facility of /proc. See proc(4).
Any dynamically linked application can be run with these
functions in place of the standard functions if the follow-
ing string is present in the environment (see ld.so.1(1)):
LDPRELOAD=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
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Memory Allocation Library Functions watchmalloc(3MALOC)
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 pat-
tern 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 environ-
ment:
MALOCDEBUG=WATCH
This causes every block of freed memory to be covered with
WAWRITE 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 allo-
cated 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
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Memory Allocation Library Functions watchmalloc(3MALOC)
returned by malloc() will trigger a watchpoint trap.
Watchpoints incur a large performance penalty. Requesting
MALOCDEBUG=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:
MALOCDEBUG=WATCH,RW,STOP
WATCH Enables WAWRITE watched areas as described above.
RW Enables both WAREAD and WAWRITE 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 appli-
cation 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.
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Memory Allocation Library Functions watchmalloc(3MALOC)
ATRIBUTES
See attributes(5) for descriptions of the following attri-
butes:
ATRIBUTE TYPE ATRIBUTE VALUE
MT-Level MT-Safe
SEE ALSO
proc(1), bsdmalloc(3MALOC), calloc(3C), free(3C),
malloc(3C), malloc(3MALOC), mapmalloc(3MALOC),
memalign(3C), realloc(3C), valloc(3C), libmapmalloc(3LIB),
proc(4), attributes(5)
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