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 is 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
To enable memory debugging, Tcl should be recompiled from scratch with
TCL_MEM_DEBUG defined (e.g. by passing the
--enable-symbols=mem flag to the configure script when
building). 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 Tcl_DbCkfree and Tcl_DbCkalloc or
Tcl_Alloc and Tcl_Free 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
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
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”
“high guard failed”
message is issued. The
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
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 is not 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
Copyright © 1995-1997 Roger E. Critchlow Jr.
Copyright © 1992-1999 Karl Lehenbauer and Mark Diekhans.
Copyright © 2000 by Scriptics Corporation.