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
       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 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
       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.

SEE ALSO

       ckalloc, memory, Tcl_ValidateAllMemory, Tcl_DumpActiveMemory

KEYWORDS

       memory, debug