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TCL_MEM_DEBUG(3)	    Tcl Library Procedures	      TCL_MEM_DEBUG(3)

______________________________________________________________________________

NAME
       TCL_MEM_DEBUG - Compile-time flag to enable Tcl memory debugging
_________________________________________________________________

DESCRIPTION
       When Tcl is compiled with TCL_MEM_DEBUG defined, a powerful set of mem‐
       ory 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	 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 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)
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