Memory(TCL)

NAME
ckalloc, memory, ckfree, Tcl_DisplayMemory, Tcl_InitMemory, Tcl_ValidateAllMemory - Validated memory allocation interface.

SYNOPSIS
memory info
memory trace [on|off]
memory validate [on|off]
memory trace_on_at_malloc nnn
memory break_on_malloc nnn
memory display file

#include <tcl.h>

char *
ckalloc (unsigned size)

void
ckfree (char *ptr)

int
Tcl_DumpActiveMemory (char *fileName);

void
Tcl_ValidateAllMemory (char *file,
                       int   line)
void
Tcl_InitMemory (interp)

ARGUMENTS
A pointer to the Tcl interpreter. The filename of the caller of Tcl_ValidateAllMemory. The line number of the caller of Tcl_ValidateAllMemory. File to display list of active memory.

DESCRIPTION

  ckalloc

Thi macro allocates memory, in the same manner as malloc, with the following differences: One, ckalloc checks the value returned from malloc (it calls malloc for you) and panics if the allocation request fails. Two, if enabled at compile time, a version of ckalloc with special memory debugging capabilities replaces the normal version of ckalloc, which aids in detecting memory overwrites and leaks (repeated allocations not matched by corresponding frees).

Parameters:
    o size - The size of the memory block to be allocated.

Returns:
    A pointer to the allocated memory block.

  ckfree

This macro frees memory allocated by ckalloc. Like ckalloc, when memory debugging is enabled, ckfree has enhanced capabilities for detecting memory overwrites and leaks.

It is very important that you use ckalloc when you need to allocate memory, and that you use ckfree to free it. Should you use malloc to allocate and ckfree to free, spurious memory validation errors will occur when memory debugging is enabled. Should you use free to free memory allocated by ckalloc, memory corruption will occur when memory debugging is enabled. Any memory that is to be become the property of the Tcl interpreter, such as result space, must be allocated with ckalloc. If it is absolutely necessary for an application to pass back malloced memory to Tcl, it will work only if Tcl is complied with the TCL_MEM_DEBUG flag turned off. If you convert your application to use this facility, it will help you find memory over runs and lost memory. Note that memory allocated by a C library routine requiring freeing should still be freed with free, since it calls malloc rather than ckalloc to do the allocation.

Parmaters:
    o ptr - The address of a block to free, as returned by ckalloc.

  Tcl_DumpActiveMemory

This function will output a list of all currently allocated memory to the specified file. The following information is outputed for each allocated block of memory: starting and ending addresses (excluding guard zone), size, source file where ckalloc was called to allocate the block and line number in that file. It is especially useful to call Tcl_DumpActiveMemory after the Tcl interpreter has been deleted.

Parameters:
    o fileName - The name of the file to output the memory list to.

  Tcl_ValidateAllMemory

Forces a validation of the guard zones of all currently allocated blocks of memory. Normally validation of a block occures when its freed, unless full validation is enabled, in which case validation of all blocks occurs when ckalloc and ckfree are called. This function forces the validation to occur at any point.

Parameters:
    o file - The file that this routine is being called from, normally __FILE__.
    o line - The line that this routine is being called from, normally __LINE__.

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.

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 also eight-byte ``guard zones'' are placed in front of and behind the space that will be returned to the caller. (The size of the guard zone is defined by the C #define GUARD_SIZE in baseline/src/ckalloc.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.

THE MEMORY COMMAND

memory options
    The Tcl memory command gives the Tcl developer control of Tcl's memory debugging capabilities. The memory command has several suboptions, which are described below. It is only available when Tcl has been compiled with memory debugging enabled.

memory info
    Produces a report containing the total allocations and frees since Tcl began, the current packets allocated (the current number of calls to ckalloc not met by a corresponding call to ckfree), the current bytes allocated, and the maximum number of packets and bytes allocated.

memory trace [on|off]
    Turns memory tracing on or off. When memory tracing is on, every call to ckalloc causes a line of trace information to be written to stderr, consisting of the word ckalloc, followed by the address returned, the amount of memory allocated, and the C filename and line number of the code performing the allocation, for example...

       ckalloc 40e478 98 tclProc.c 1406

    Calls to ckfree are traced in the same manner, except that the word ckalloc is replaced by the word ckfree.

memory validate [on|off]
    Turns memory vaidation on or off. When memory validation is enabled, on every call to ckalloc or ckfree, the guard zones are checked for every piece of memory currently in existence that was allocated by ckalloc. This has a large performance impact and should only be used when overwrite problems are strongly suspected. The advantage of enabling memory validation is that a guard zone overwrite can be detected on the first call to ckalloc or ckfree after the overwrite occurred, rather than when the specific memory with the overwritten guard zone(s) is freed, which may occur long after the overwrite occurred.

memory trace_on_at_malloc nnn
    Enable memory tracing after nnn ckallocs have been performed. For example, if you enter memory trace_on_at_malloc 100, after the 100th call to ckalloc, memory trace information will begin being displayed for all allocations and frees. Since there can be a lot of memory activity before a problem occurs, judicious use of this option can reduce the slowdown caused by tracing (and the amount of trace information produced), if you can identify a number of allocations that occur before the problem sets in. The current number of memory allocations that have occured since Tcl started is printed on a guard zone failure.

memory break_on_malloc nnn
    After the nnn allocations have been performed, ckallocs output a message to this effect and that it is now attempting to enter the C debugger. Tcl will then issue a SIGINT signal against itself. If you are running Tcl under a C debugger, it should then enter the debugger command mode.

memory display file
    Write a list of all currently allocated memory to the specified file.

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 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 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.

KEYWORDS
ckalloc, ckfree, free, memory, malloc