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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 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_Ckalloc and Tcl_Ckfree 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 prob-lem. problem.
       lem.  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



Tcl                                                  8.1                                    TCL_MEM_DEBUG(3)