TCLMEMDEBUG(3) Tcl Library Procedures TCLMEMDEBUG(3)

NAME
       TCLMEMDEBUG - Compile-time flag to enable Tcl memory debugging.



DESCRIPTION
       When Tcl is compiled with TTCCLLMMEEMMDDEEBBUUGG defined, a powerful set of mem-
       ory 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.


EENNAABBLLIINNGG MMEEMMOORRYY DDEEBBUUGGGGIINNGG
       To enable memory debugging, Tcl should be recompiled from scratch  with
       TTCCLLMMEEMMDDEEBBUUGG defined.  This will also compile in a non-stub version of
       TTccllIInniittMMeemmoorryy to add the mmeemmoorryy command to Tcl.

       TTCCLLMMEEMMDDEEBBUUGG 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 TTccllDDbbCCkkffrreeee  and  TTccllDDbbCCkkaalllloocc  or
       TTccllCCkkaalllloocc and TTccllCCkkffrreeee being undefined.

       Once  memory  debugging support has been compiled into Tcl, the C func-
       tions TTccllVVaalliiddaatteeAAllllMMeemmoorryy, and TTccllDDuummppAAccttiivveeMMeemmoorryy, and the Tcl mmeemm-
       oorryy command can be used to validate and examine memory usage.


GGUUAARRDD ZZOONNEESS
       When  memory  debugging is enabled, whenever a call to cckkaalllloocc is made,
       slightly more memory than requested is allocated so the  memory  debug-
       ging  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 LLOOWWGGUUAARRDDSSIIZZEE and #define HHIIGGHHGGUUAARRDDSSIIZZEE in the file
       generic/tclCkalloc.c  -  it can be extended if you suspect large over-
       write problems, at some cost in performance.)  A known pattern is writ-
       ten  into  the guard zones and, on a call to cckkffrreeee, the guard zones of
       the space being freed are checked to see if either zone has been  modi-
       fied  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 cckkffrreeee.  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.


DDEEBBUUGGGGIINNGG DDIIFFFFIICCUULLTT MMEEMMOORRYY CCOORRRRUUPPTTIIOONN PPRROOBBLLEEMMSS
       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 MMEEMMVVAALLIIDDAATTEE define,
       recompile tclCkalloc.c and rebuild Tcl.  This will enable memory  vali-
       dation  from  the  first call to cckkaalllloocc, again, at a large performance
       impact.

       If you are desperate and validating memory on every call to cckkaalllloocc and
       cckkffrreeee  isn't  enough,  you  can  explicitly call TTccllVVaalliiddaatteeAAllllMMeemmoorryy
       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, TclValidateAllMemory, TclDumpActiveMemory


KKEEYYWWOORRDDSS
       memory, debug





Tcl                                   8.1                     TCLMEMDEBUG(3)