ckalloc
Memory(TCL) Memory(TCL)
NAME
ckalloc, memory, ckfree, Tcl_DisplayMemory, Tcl_InitMemory, Tcl_Vali-
dateAllMemory - 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_Vali-
dateAllMemory. File to display list of active memory.
DESCRIPTION
ckalloc
Thi macro allocates memory, in the same manner as malloc, with the fol-
lowing 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 mem-
ory debugging capabilities replaces the normal version of ckalloc,
which aids in detecting memory overwrites and leaks (repeated alloca-
tions 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 mem-
ory 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.
Parmeters:
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 outputted 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 occurs 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 debug-
ging 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 per-
formance.) 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 num-
ber 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 validation 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 iden-
tify a number of allocations that occur before the problem sets
in. The current number of memory allocations that have occurred
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 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 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
Tcl Memory(TCL)
