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 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.

ENABLING MEMORY DEBUGGING

       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 func-
       tions Tcl_ValidateAllMemory, and Tcl_DumpActiveMemory, and the Tcl mem-
       ory 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  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 LOW_GUARD_SIZE and #define HIGH_GUARD_SIZE 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 ckfree, 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 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  vali-
       dation  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.

SEE ALSO

       ckalloc, memory, Tcl_ValidateAllMemory, Tcl_DumpActiveMemory

KEYWORDS

       memory, debug

Tcl                                   8.1                     TCL_MEM_DEBUG(3)