NAME
TCL_MEM_DEBUG - Compile-time flag to enable Tcl memory
debugging.
DESCRIPTION
When Tcl is compiled with TCL_MEM_DEBUG defined, a power
ful set of memory debugging aids are included in the com
piled binary. This includes C and Tcl functions which can
aid with debugging memory leaks, memory allocation over
runs, and other memory related errors.
ENABLING MEMORY DEBUGGING
To enable memory debugging, Tcl should be recompiled from
scratch with TCL_MEM_DEBUG defined. This will also com
pile in a non-stub version of Tcl_InitMemory to add the
memory command to Tcl.
TCL_MEM_DEBUG must be either left defined for all modules
or undefined for all modules that are going to be linked
together. If they are not, link errors will occur, with
either TclDbCkfree and Tcl_DbCkalloc or Tcl_Ckalloc and
Tcl_Ckfree being undefined.
Once memory debugging support has been compiled into Tcl,
the C functions Tcl_ValidateAllMemory, and Tcl_DumpActive
Memory, and the Tcl memory command can be used to validate
and examine memory usage.
GUARD ZONES
When memory debugging is enabled, whenever a call to
ckalloc is made, slightly more memory than requested is
allocated so the memory debugging code can keep track of
the allocated memory, and eight-byte ``guard zones'' are
placed in front of and behind the space that will be
returned to the caller. (The size of the guard zone is
defined by the C #define GUARD_SIZE in base
line/src/ckalloc.c -- it can be extended if you suspect
large overwrite problems, at some cost in performance.) A
known pattern is written into the guard zones and, on a
call to ckfree, the guard zones of the space being freed
are checked to see if either zone has been modified in any
way. If one has been, the guard bytes and their new con
tents are identified, and a ``low guard failed'' or ``high
guard failed'' message is issued. The ``guard failed''
message includes the address of the memory packet and the
file name and line number of the code that called ckfree.
This allows you to detect the common sorts of one-off
problems, where not enough space was allocated to contain
the data written, for example.
Normally, Tcl compiled with memory debugging enabled will
make it easy to isolate a corruption problem. Turning on
memory validation with the memory command can help isolate
difficult problems. If you suspect (or know) that corrup
tion is occurring before the Tcl interpreter comes up far
enough for you to issue commands, you can set MEM_VALIDATE
define, recompile tclCkalloc.c and rebuild Tcl. This will
enable memory validation from the first call to ckalloc,
again, at a large performance impact.
If you are desperate and validating memory on every call
to ckalloc and ckfree isn't enough, you can explicitly
call Tcl_ValidateAllMemory directly at any point. It
takes a char * and an int which are normally the filename
and line number of the caller, but they can actually be
anything you want. Remember to remove the calls after you
find the problem.
SEE ALSO
memory, Tcl_ValidateAllMemory, Tcl_DumpActiveMemory
KEYWORDS
memory, debug
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