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ActiveTcl User Guide |
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TCL_MEM_DEBUG - Compile-time flag to enable Tcl memory debugging.
When Tcl is compiled with TCL_MEM_DEBUG defined, a powerful
set of memory debugging aids are included in the compiled binary.
This includes C and Tcl functions which can aid with debugging
memory leaks, memory allocation overruns, and other memory related
errors.
To enable memory debugging, Tcl should be recompiled from scratch
with TCL_MEM_DEBUG defined. This will also compile 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_DumpActiveMemory, and
the Tcl memory command
can be used to validate and examine memory usage.
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 sizes of the guard zones are defined by the C
#define LOW_GUARD_SIZE and #define HIGH_GUARD_SIZE in
the file generic/tclCkalloc.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
contents 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 corruption 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.
ckalloc, memory, Tcl_ValidateAllMemory,
Tcl_DumpActiveMemory
memory, debug
Copyright © 1992-1999 Karl Lehenbauer and Mark Diekhans.
Copyright © 2000 by Scriptics Corporation.
Copyright © 1995-1997 Roger E. Critchlow Jr.