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+'\" 
+'\" Copyright (c) 1992-1999 Karl Lehenbauer and Mark Diekhans.
+'\" Copyright (c) 2000 by Scriptics Corporation.
+'\" All rights reserved.
+'\" 
+.TH TCL_MEM_DEBUG 3 8.1 Tcl "Tcl Library Procedures"
+.so man.macros
+.BS
+.SH NAME
+TCL_MEM_DEBUG \- Compile-time flag to enable Tcl memory debugging
+.BE
+.SH DESCRIPTION
+When Tcl is compiled with \fBTCL_MEM_DEBUG\fR 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.
+.SH "ENABLING MEMORY DEBUGGING"
+.PP
+To enable memory debugging, Tcl should be recompiled from scratch with
+\fBTCL_MEM_DEBUG\fR defined (e.g. by passing the
+\fI\-\-enable\-symbols=mem\fR flag to the \fIconfigure\fR script when
+building).  This will also compile in a non-stub
+version of \fBTcl_InitMemory\fR to add the \fBmemory\fR command to Tcl.
+.PP
+\fBTCL_MEM_DEBUG\fR 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 \fBTcl_DbCkfree\fR and \fBTcl_DbCkalloc\fR or
+\fBTcl_Alloc\fR and \fBTcl_Free\fR being undefined.
+.PP
+Once memory debugging support has been compiled into Tcl, the C
+functions \fBTcl_ValidateAllMemory\fR, and \fBTcl_DumpActiveMemory\fR,
+and the Tcl \fBmemory\fR command can be used to validate and examine
+memory usage.
+.SH "GUARD ZONES"
+.PP
+When memory debugging is enabled, whenever a call to \fBckalloc\fR is
+made, slightly more memory than requested is allocated so the memory
+debugging code can keep track of the allocated memory, and eight-byte
+.QW "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 \fBLOW_GUARD_SIZE\fR and #define \fBHIGH_GUARD_SIZE\fR
+in the file \fIgeneric/tclCkalloc.c\fR \(em 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 \fBckfree\fR, 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
+.QW "low guard failed"
+or
+.QW "high guard failed"
+message is issued.  The
+.QW "guard failed"
+message includes the address of the memory packet and
+the file name and line number of the code that called \fBckfree\fR.
+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.
+.SH "DEBUGGING DIFFICULT MEMORY CORRUPTION PROBLEMS"
+.PP
+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
+\fBMEM_VALIDATE\fR define, recompile tclCkalloc.c and rebuild Tcl.
+This will enable memory validation from the first call to
+\fBckalloc\fR, again, at a large performance impact.
+.PP
+If you are desperate and validating memory on every call to
+\fBckalloc\fR and \fBckfree\fR is not enough, you can explicitly call
+\fBTcl_ValidateAllMemory\fR directly at any point.  It takes a \fIchar
+*\fR and an \fIint\fR 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.
+.SH "SEE ALSO"
+ckalloc, memory, Tcl_ValidateAllMemory, Tcl_DumpActiveMemory
+.SH KEYWORDS
+memory, debug