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TCL_MEM_DEBUG(3) Tcl Library Procedures TCL_MEM_DEBUG(3)
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NAME
TCL_MEM_DEBUG - Compile-time flag to enable Tcl memory debugging
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DESCRIPTION
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 errors.
ENABLING MEMORY DEBUGGING
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 build-
ing). 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_Ckalloc and Tcl_Ckfree being undefined.
Once memory debugging support has been compiled into Tcl, the C functions Tcl_Vali-
dateAllMemory, and Tcl_DumpActiveMemory, and the Tcl memory command can be used to vali-
date 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 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.
DEBUGGING DIFFICULT MEMORY CORRUPTION PROBLEMS
Normally, Tcl compiled with memory debugging enabled will make it easy to isolate a cor-
ruption 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 prob-
lem.
SEE ALSO
ckalloc, memory, Tcl_ValidateAllMemory, Tcl_DumpActiveMemory
KEYWORDS
memory, debug
Tcl 8.1 TCL_MEM_DEBUG(3)
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