1TCL_MEM_DEBUG(3) Tcl Library Procedures TCL_MEM_DEBUG(3)
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8 TCL_MEM_DEBUG - Compile-time flag to enable Tcl memory debugging
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12 When Tcl is compiled with TCL_MEM_DEBUG defined, a powerful set of mem‐
13 ory debugging aids is included in the compiled binary. This includes C
14 and Tcl functions which can aid with debugging memory leaks, memory
15 allocation overruns, and other memory related errors.
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18 To enable memory debugging, Tcl should be recompiled from scratch with
19 TCL_MEM_DEBUG defined (e.g. by passing the --enable-symbols=mem flag to
20 the configure script when building). This will also compile in a non-
21 stub version of Tcl_InitMemory to add the memory command to Tcl.
22 TCL_MEM_DEBUG must be either left defined for all modules or undefined
24 for all modules that are going to be linked together. If they are not,
25 link errors will occur, with either Tcl_DbCkfree and Tcl_DbCkalloc or
26 Tcl_Alloc and Tcl_Free being undefined.
27 Once memory debugging support has been compiled into Tcl, the C func‐
29 tions Tcl_ValidateAllMemory, and Tcl_DumpActiveMemory, and the Tcl mem‐
30 ory command can be used to validate and examine memory usage.
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33 When memory debugging is enabled, whenever a call to ckalloc is made,
34 slightly more memory than requested is allocated so the memory debug‐
35 ging code can keep track of the allocated memory, and eight-byte “guard
36 zones” are placed in front of and behind the space that will be
37 returned to the caller. (The sizes of the guard zones are defined by
38 the C #define LOW_GUARD_SIZE and #define HIGH_GUARD_SIZE in the file
39 generic/tclCkalloc.c — it can be extended if you suspect large over‐
40 write problems, at some cost in performance.) A known pattern is writ‐
41 ten into the guard zones and, on a call to ckfree, the guard zones of
42 the space being freed are checked to see if either zone has been modi‐
43 fied in any way. If one has been, the guard bytes and their new con‐
44 tents are identified, and a “low guard failed” or “high guard failed”
45 message is issued. The “guard failed” message includes the address of
46 the memory packet and the file name and line number of the code that
47 called ckfree. This allows you to detect the common sorts of one-off
48 problems, where not enough space was allocated to contain the data
49 written, for example.
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52 Normally, Tcl compiled with memory debugging enabled will make it easy
53 to isolate a corruption problem. Turning on memory validation with the
54 memory command can help isolate difficult problems. If you suspect (or
55 know) that corruption is occurring before the Tcl interpreter comes up
56 far enough for you to issue commands, you can set MEM_VALIDATE define,
57 recompile tclCkalloc.c and rebuild Tcl. This will enable memory vali‐
58 dation from the first call to ckalloc, again, at a large performance
59 impact.
60 If you are desperate and validating memory on every call to ckalloc and
62 ckfree is not enough, you can explicitly call Tcl_ValidateAllMemory
63 directly at any point. It takes a char * and an int which are normally
64 the filename and line number of the caller, but they can actually be
65 anything you want. Remember to remove the calls after you find the
66 problem.
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69 ckalloc, memory, Tcl_ValidateAllMemory, Tcl_DumpActiveMemory
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72 memory, debug
73Tcl 8.1 TCL_MEM_DEBUG(3)