The debug memory allocation or dmalloc library has been designed
as a drop in replacement for the system's malloc, realloc,
calloc, free and other memory management routines while
providing powerful debugging facilities configurable at runtime. These
facilities include such things as memory-leak tracking, fence-post write
detection, file/line number reporting, and general logging of
statistics.
The library is reasonably portable having been run successfully on at least the following operating systems: AIX, BSD/OS, DG/UX, FreeBSD, HPUX, Irix, Linux, MS-DOG, NeXT, OSF, SCO, Solaris, SunOS, Ultrix, Unixware, Windoze and even Unicos on a Cray Y-MP. It also provides support for the debugging of threaded programs. See Using With Threads.
The package includes the library, configuration scripts, debug utility application, test program, and documentation. Online documentation as well as the full source is available at URL http://dmalloc.com/. Details on the library's mailing list are available there as well.
My contact information is available on the web page. I can be reached with any questions or feedback. Please include the version number of the library that you are using as well as your machine and operating system types.
Gray Watson.
Copyright 1992 to 2000 by Gray Watson.
Permission to use, copy, modify, and distribute this software for any purpose and without fee is hereby granted, provided that the above copyright notice and this permission notice appear in all copies, and that the name of Gray Watson not be used in advertising or publicity pertaining to distribution of the document or software without specific, written prior permission.
Gray Watson makes no representations about the suitability of the software described herein for any purpose. It is provided "as is" without express or implied warranty.
Any program can be divided into 2 logical parts: text and data. Text is the actual program code in machine-readable format and data is the information that the text operates on when it is executing. The data, in turn, can be divided into 3 logical parts according to where it is stored: static, stack, and heap.
Static data is the information whose storage space is compiled into the program.
/* global variables are allocated as static data */
int numbers[10];
main()
{
...
}
Stack data is data allocated at runtime to hold information used inside of functions. This data is managed by the system in the space called stack space.
void foo()
{
/* this local variable is stored on the stack */
float total;
...
}
main()
{
foo();
}
Heap data is also allocated at runtime and provides a programmer with dynamic memory capabilities.
main()
{
/* the address is stored on the stack */
char * string;
...
/*
* Allocate a string of 10 bytes on the heap. Store the
* address in string which is on the stack.
*/
string = (char *)malloc(10);
...
/* de-allocate the heap memory now that we're done with it */
(void)free(string);
...
}
It is the heap data that is managed by this library.
Although the above is an example of how to use the malloc and free commands, it is not a good example of why using the heap for runtime storage is useful.
Consider this: You write a program that reads a file into memory, processes it, and displays results. You would like to handle files with arbitrary size (from 10 bytes to 1.2 megabytes and more). One problem, however, is that the entire file must be in memory at one time to do the calculations. You don't want to have to allocate 1.2 megabytes when you might only be reading in a 10 byte file because it is wasteful of system resources. Also, you are worried that your program might have to handle files of more than 1.2 megabytes.
A solution: first checkout the file's size and then, using the heap-allocation routines, get enough storage to read the entire file into memory. The program will only be using the system resources necessary for the job and you will be guaranteed that your program can handle any sized file.
All malloc libraries support 4 basic memory allocation commands. These
include malloc, calloc, realloc, and free. For
more information about their capabilities, check your system's manual
pages - in unix, do a man 3 malloc.
| Function |
|
void *malloc ( unsigned int size )
Usage: The malloc routine is the basic memory allocation routine. It allocates
an area of |
| Function |
|
void *calloc ( unsigned int number, unsigned int
size )
Usage: The calloc routine allocates a certain Also, calloc nulls the space that it returns, assuring that the memory is all zeros. |
| Function |
|
void *realloc ( void *old_pnt, unsigned int
new_size )
Usage: The realloc function expands or shrinks the memory allocation in
If the |
| Function |
|
void free ( void *pnt )
Usage: The free routine releases allocation in |
NOTE: the returned address from the memory allocation/reallocation functions should be cast to the appropriate pointer type for the variable being assigned. Also this may no longer be necessary on your system, it can still provide some documentation value to the code.
WARNING: there is a quite common myth that all of the space that
is returned by malloc libraries has already been cleared. Only
the calloc routine will zero the memory space it returns.
The debugging features that are available in this debug malloc library can be divided into a couple basic classifications:
NOTE: The library cannot notice when the program reads
from these areas, only when it writes values. Also, fence-post checking
will increase the amount of memory the program allocates.
By enabling heap-consistency checking, the library will run through its administrative structures to make sure all is in order. This will mean that problems will be caught faster and diagnosed better.
The drawback of this is, of course, that the library often takes quite a long time to do this. It is suitable to enable this only during development and debugging sessions.
NOTE: the heap checking routines cannot guarantee that the tests
will not cause a segmentation-fault if the heap administration
structures are properly (or improperly if you will) overwritten. In
other words, the tests will verify that everything is okay but may not
inform the user of problems in a graceful manner.
The library has a number of logging capabilities that can track un-freed
memory pointers as well as runtime memory usage, memory transactions,
administrative actions, and final statistics.
To combat this, the library can write special values into a block of memory after it has been freed. This serves two purposes: it will make sure that the program will get garbage data if it trying to access the area again, and it will allow the library to verify the area later for signs of overwriting.
If any of the above debugging features detect an error, the library will try to recover. If logging is enabled then an error will be logged with as much information as possible.
The error messages that the library displays are designed to give the most information for developers. If the error message is not understood, then it is most likely just trying to indicate that a part of the heap has been corrupted.
The library can be configured to quit immediately when an error is detected and to dump a core file or memory-image. This can be examined with a debugger to determine the source of the problem. The library can either stop after dumping core or continue running.
NOTE: do not be surprised if the library catches problems with your system's routines. It took me hours to finally come to the conclusion that the localtime call, included in SunOS release 4.1, overwrites one of its fence-post markers.
To configure, compile, and install the library, follow these steps carefully.
settings.dist to tune specific features of the library. The
configure script will copy this file to settings.h
which is where you should be adding per-architecture settings.
config.help file for some information about
configure. Configure should generate the Makefile and some
configuration files automatically.
NOTE: It seems that some versions of tr (especially from HP-UX)
don't understand tr '[a-z]' '[A-Z]'. Since configure uses tr
often, you may need to either get GNU's tr (in their textutils package)
or generate the Makefile and conf.h files by hand.
Makefile and conf.h files
created by configure to make sure it did its job correctly.
settings.h file to
tune the library to the local architecture. This file contains relevant
settings if you are using pthreads or another thread library.
See Using With Threads. The configure script created this
file from the settings.dist file. Any permanent changes to these
settings should made to the settings.dist file. You then can run
config.status to re-create the settings.h file.
DMALLOC_SIZE variable gets auto-configured in
dmalloc.h.2 but it may not generate correct settings for all
systems. You may have to alter the definitions in this file to get
things to stop complaining when you go to compile about the size
arguments to malloc routines. Comments on this please.
libdmalloc.a,
libdmalloclp.a, and dmalloc program. If it does not work,
please see if there are any notes in the contrib directory about your
system-type. If not and you figure your problem out, please send me
some notes so future users can profit from your experiences.
NOTE: You may experience some errors compiling some of the
return.h assembly macros which attempt to determine the callers address
for logging purposes. You may want to first try disabling any compiler
optimization flags. If this doesn't work then you may need to disable
the USE_RET_ADDRESS variable in the settings.h file.
NOTE: The code is dependent on an ANSI-C compiler. If the
configure script gives the WARNING that you do not have an ANSI-C
compiler, you may still be able to add some sort of option to your
compiler to make it ANSI. If there such is an option, please send it to
the author so it can be added to the configure script.
libdmallocth.a which is the threaded version of the
library. This may or may not work depending on the configuration
scripts ability to detect your local thread functionality. Feel free to
send me mail with improvements.
See the "Using With Threads" section for more information about the operation of the library with your threaded program. See Using With Threads.
libdmalloc++.a which is the C++ version of the library. See the
dmallocc.cc C++ file which contains basic code to overload the
new, new[], delete, and delete[] C++
operators. My apologies on the minimal C++ support. I am still living
in a mostly C world. Any help improving this interface without
sacrificing portability would be appreciated.
dmalloc_t test
program.
dmalloc_t test
program through a set of light trials. By default this will execute
dmalloc_t 5 times - each time will execute 10,000 malloc
operations in a very random manner. Anal folks can type make
heavy to up the ante. Use dmalloc_t --usage for the list of all
dmalloc_t options.
libdmalloc.a
and libdmalloc_lp.a library files in /usr/local/lib, the
dmalloc.h include file in /usr/local/include, and the
dmalloc utility in /usr/local/bin. You may also want to
type make installth to install the thread library into place
and/or make installcc to install the C++ library into place.
You may have specified a --prefix=PATH option to configure in
which case /usr/local will have been replaced with PATH.
See the "Getting Started" section to get up and running with the library. See Getting Started.
This section should give you a quick idea on how to get going. Basically, you need to do the following things to make use of the library:
on_exit or atexit
functions. If so, then the dmalloc library should be able to
automatically call dmalloc_shutdown when exit is called.
This causes the memory statistics and unfreed information to be dumped
to the log file. However, if your system has neither, you will need to
call dmalloc_shutdown yourself before your program exits.
.bashrc, .profile, or .zshrc file respectively
(notice the -b option for bourne shell output):
function dmalloc { eval `command dmalloc -b $*`; }
By the way, if you are looking for a shell, I heartily recommend trying
out zsh. It is a bourne shell written from scratch with much the same
features as tcsh without the csh crap. If you are still using
csh or tcsh, you should add the following to your .cshrc file
(notice the -C option for c-shell output):
alias dmalloc 'eval `\dmalloc -C \!*`'
dmalloc.h
in your C files and recompile. This will allow the library to report
the file/line numbers of calls that generate problems. See Allocation Macros. It should be inserted at the bottom of your include
files as to not conflict with other includes. You may want to ifdef it
as well and compile with cc -DDMALLOC ...:
/* other includes above ^^^ */ #ifdef DMALLOC #include "dmalloc.h" #endif
logfile (-l logfile)
dmalloc --usage will provide verbose usage info for the dmalloc program. See Dmalloc Program.
You may also want to install the dmallocrc file in your home
directory as .dmallocrc. This allows you to add your own
combination of debug tokens. See RC File.
log-unknown token
(dmalloc -p log-unknown) which will log non-freed information
about "unknown" memory to the log file. Unknown means memory that
does not have associated file and line information. This is necessary
if you are not including dmalloc.h in your C files or if
you want to track possible memory leaks in system functions.
This section provides some answers to some common problems and questions. Please feel free to send me mail with any additions to this list - either problems you are still having or tips that you would like to pass on.
Why does my program run so slow?
check-heap token enabled, see the -i option to the
dmalloc utility. See Dmalloc Program.
Why was a log-file not produced after I ran my program?
DMALLOC_OPTIONS variable is set in your exported environment.
See Environment Variable.
$Id:
chunk.c,v 1.152 1999/08/25 12:37:01 gray Exp $ with different versions
and date information. If this doesn't show up then chances are dmalloc
was not linked into your program.
I don't see any information about my non-freed (leaked) memory?
log-unknown token (dmalloc -p log-unknown).
This will be necessary if you are not including dmalloc.h
in all of your C files or if you are interested in tracking leaks in
system functions.
By including dmalloc.h in your C files, your calls to malloc,
calloc, realloc, recalloc, memalign, valloc, strdup, and free are
replaced with calls to _malloc_leap, _calloc_leap, _realloc_leap,
_recalloc_leap, _memalign_leap, _valloc_leap, _strdup_leap, and
_free_leap. Additionally the library replaces calls to xmalloc,
xcalloc, xrealloc, xrecalloc, xmemalign, xvalloc, xstrdup, and xfree
with associated _leap calls.
These leap macros use the c-preprocessor __FILE__ and
__LINE__ macros which get replaced at compilation time with the
current file and line-number of the source code in question. The leap
routines take this information and pass it on to the library making it
able to produce verbose reports on memory problems.
not freed: '0x38410' (22 bytes) from 'dmalloc_t.c:92'
This line from a log file shows that memory was not freed from file
dmalloc_t.c line 92. See Memory Leaks.
You may notice some non standard memory allocation functions in the above leap list. Recalloc is a routine like realloc that reallocates previously allocated memory to a new size. If the new memory size is larger than the old, recalloc initializes the new space to all zeros. This may or may not be supported natively by your operating system. Memalign is like malloc but should insure that the returned pointer is aligned to a certain number of specified bytes. Currently, the memalign function is not supported by the library. It defaults to returning possibly non-aligned memory for alignment values less than a block-size. Valloc is like malloc but insures that the returned pointer will be aligned to a page boundary. This may or may not be supported natively by your operating system but is fully supported by the library. Strdup is a string duplicating routine which takes in a null terminated string pointer and returns an allocated copy of the string that will need to be passed to free later to deallocate.
The X versions of the standard memory functions (xmalloc, xfree, etc.) will print out an error message to standard error and will stop if the library is unable to allocate any additional memory. It is useful to use these routines instead of checking everywhere in your program for allocation routines returning NULL pointers.
WARNING: If you are including the dmalloc.h file in your
sources, it is recommended that it be at the end of your include file
list because dmalloc uses macros and may try to change declarations of
the malloc functions if they come after it.
Even though the allocation macros can provide file/line information for
some of your code, there are still modules which either you can't
include dmalloc.h (such as library routines) or you just don't
want to. You can still get information about the routines that call
dmalloc function from the return-address information. To accomplish
this, you must be using this library on one of the supported
architecture/compilers. See Portability.
The library attempts to use some assembly hacks to get the the
return-address or the address of the line that called the dmalloc
function. If you have the log-unknown token enabled and you run
your program, you might see the following non-freed memory messages.
not freed: '0x38410' (22 bytes) from 'ra=0xdd2c' not freed: '0x38600' (10232 bytes) from 'ra=0x10234d' not freed: '0x38220' (137 bytes) from 'ra=0x82cc'
With the help of a debugger, these return-addresses (or ra) can then be
identified. I've provided a ra_info.pl perl script in the
contrib/ directory with the dmalloc sources which seems to work
well with gdb. You can also use the manual methods below for gdb.
(gdb) x 0x10234d 0x10234d <_findbuf+132>: 0x7fffceb7 (gdb) info line *(0x82cc) Line 1092 of argv.c starts at pc 0x7540 and ends at 0x7550.
In the above example, gdb was used to find that the two non-freed memory
pointers were allocated in _findbuf() and in file argv.c line
1092 respectively. The x address (for examine) can always be
used on the return-addresses but the info line *(address) will
only work if that file was compiled using the -g option and has
not been stripped. This limitation may not be true in later versions of
gdb.
One potential problem with the library and its multitude of checks and
diagnoses is that they only get performed when a dmalloc function is
called. One solution this is to include dmalloc.h and compile
your source code with the DMALLOC_FUNC_CHECK flag defined and
enable the check-funcs token. See Debug Tokens.
cc -DDMALLOC_FUNC_CHECK file.c
NOTE: Once you have compiled your source with DMALLOC_FUNC_CHECK enabled, you will have to recompile with it off to disconnect the library. See Disabling the Library.
WARNING: You should be sure to have dmalloc.h included at
the end of your include file list because dmalloc uses macros and may
try to change declarations of the checked functions if they come after
it.
When this is defined dmalloc will override a number of functions and
will insert a routine which knows how to check its own arguments and
then call the real function. Dmalloc can check such functions as
bcopy, index, strcat, and strcasecmp. For
the full list see the end of dmalloc.h.
When you call strlen, for instance, dmalloc will make sure the
string argument's fence-post areas have not been overwritten, its file
and line number locations are good, etc. With bcopy, dmalloc
will make sure that the destination string has enough space to store the
number of bytes specified.
For all of the arguments checked, if the pointer is not in the heap then it is ignored since dmalloc does not know anything about it.
The library has a number of variables that are not a standard part of most malloc libraries:
char * dmalloc_logpath
int dmalloc_errno
dmalloc_strerror()
(see below) to get a string version of the error. It will have a value
of zero if the library has not detected any problems.
int dmalloc_address
int dmalloc_address_count
addr address this many times, it will call
dmalloc_error().
This works well in conjunction with the STORE_SEEN_COUNT option.
See Memory Leaks.
Additionally the library provides a number of non-standard malloc routines:
| Function |
|
void dmalloc_shutdown ( void )
This function shuts the library down and logs the final statistics and
information especially the non-freed memory pointers. The library has
code to support auto-shutdown if your system has main()
{
...
dmalloc_shutdown();
exit(0);
}
|
| Function |
|
void dmalloc_log_heap_map ( void )
This routine logs to the logfile (if it is enabled) a graphical representation of the current heap space. |
| Function |
|
void dmalloc_log_stats ( void )
This routine outputs the current dmalloc statistics to the log file. |
| Function |
|
void dmalloc_log_unfreed( void )
This function dumps the unfreed-memory information to the log file. This is also useful to dump the currently allocated points to the log file to be compared against another dump later on. |
| Function |
|
int dmalloc_verify ( char * pnt )
This function verifies individual memory pointers that are suspect of memory problems. To check the entire heap pass in a NULL or 0 pointer. The routine returns DMALLOC_VERIFY_ERROR or DMALLOC_VERIFY_NOERROR. NOTE: |
| Function |
|
void dmalloc_debug ( long debug )
This routine overrides the debug setting from the environment variable
and sets the library debugging features explicitly. For instance, if
debugging should never be enabled for a program, a call to
One problem however is that some systems make calls to memory allocation
functions before |
| Function |
|
long dmalloc_debug_current ( void )
This routine returns the current debug value from the environment variable. This allows you to save a copy of the debug dmalloc settings to be changed and then restored later. |
| Function |
|
int dmalloc_examine ( char * pnt, int * size,
char ** file, int * line, void ** ret_address )
This function returns the size of a pointer's allocation as well as the file and line or the return-address from where it was allocated. It will return NOERROR or ERROR depending on whether pnt is good or not. NOTE: This function is certainly not provided by most if not all other malloc libraries. |
| Function |
|
void dmalloc_message ( char * format, ... )
Write a message into the dmalloc logfile using printf-like arguments. NOTE: this is only available is STDARG is defined by the library
and you have the |
| Function |
|
void dmalloc_vmessage ( char * format, va_list args )
Write a message into the dmalloc logfile using vprintf-like arguments. NOTE: this is only available is STDARG is defined by the library
and you have the |
| Function |
|
void dmalloc_track ( dmalloc_track_t track_func )
Register an allocation tracking function which will be called each time an allocation occurs. Pass in NULL to disable. To take a look at what information is provided, see the dmalloc_track_t function typedef in dmalloc.h. |
| Function |
|
unsigned long dmalloc_mark ( void )
Return to the caller the current "mark" which can be used later to dmalloc_log_changed pointers since this point. Multiple marks can be saved and used. |
| Function |
|
unsigned long dmalloc_log_changed ( unsigned long mark,
int not_freed_b, int freed_b, int details_b )
Dump the pointers that have changed since the mark which was returned by dmalloc_mark. If not_freed_b is set to non-0 then log the new pointers that are non-freed. If free_b is set to non-0 then log the new pointers that are freed. If details_b set to non-0 then dump the individual pointers that have changed otherwise just dump the summaries. |
| Function |
|
const char * dmalloc_strerror ( int errnum )
This function returns the string representation of the error value in errnum (which probably should be dmalloc_errno). This allows the logging of more verbose memory error messages. You can also display the string representation of an error value by a
call to the |
For those people using the C++ language, some special things need to be
done to get the library to work. The problem exists with the fact that
the dynamic memory routines in C++ are new() and delete()
as opposed to malloc() and free().
The file dmalloc.cc is provided in the distribution which
effectively redirects new to the more familiar malloc and
delete to the more familiar free. Compile and link this
file in with the C++ program you want to debug.
NOTE: The author is not a C++ hacker so feedback in the form of other hints and ideas for C++ users would be much appreciated.
When you are finished with the development and debugging sessions, you may want to disable the dmalloc library and put in its place either the system's memory-allocation routines, gnu-malloc, or maybe your own. Attempts have been made to make this a reasonably painless process. The ease of the extraction depends heavily on how many of the library's features your made use of during your coding.
Reasonable suggestions are welcome as to how to improve this process while maintaining the effectiveness of the debugging.
dmalloc.h while
defining DMALLOC_DISABLE. This will cause the dmalloc leap
macros to not be applied. See Allocation Macros.
cc -g -DDMALLOC_DISABLE main.c
DMALLOC_FUNC_CHECK
defined then you must first recompile all those modules without the flag
enabled.
DMALLOC_DISABLED
defined then you need to link your program with the
libdmalloclp.a library.
cc main.o -L/usr/local/lib -ldmalloclp -lgmalloc
If you have disabled dmalloc with the DMALLOC_DISABLED flag or
never included dmalloc.h in any of your C files, then you will
not need the libdmalloclp.a library.
cc -g main.o -L/usr/local/lib -lgmalloc
If you get unresolved references like _malloc_leap or
_dmalloc_bcopy then something was not disabled as it should have
been.
Here are a number of possible scenarios for using the dmalloc library to track down problems with your program.
You should first enable a logfile filename (I use dmalloc) and
turn on a set of debug features. You can use dmalloc -l dmalloc
low to accomplish this. If you are interested in having the error
messages printed to your terminal as well, enable the print-error
token by typing dmalloc -p print-error afterwards. See Dmalloc Program.
Now you can enter your debugger (I use the excellent GNU debugger
gdb), and put a break-point in dmalloc_error() which is the
internal error routine for the library. When your program is run, it
will stop there if a memory problem is detected.
If you are using GDB, I would recommend adding the contents of
dmalloc.gdb in the contrib subdirectory to your
.gdbinit file in your home directory. This enables the
dmalloc command which will prompt you for the arguments to the
dmalloc command and will set a break point in dmalloc_error()
automatically.
If your program stops at the dmalloc_error() routine then one of
a number of problems could be happening. Incorrect arguments could have
been passed to a malloc call: asking for negative number of bytes,
trying to realloc a non-heap pointer, etc.. There also could be a
problem with the system's allocations: you've run out of memory, some
other function in your program is using sbrk, etc. However, it
is most likely that some code that has been executed was naughty.
To get more information about the problem, first print via the debugger the dmalloc_errno variable to get the library's internal error code. You can suspend your debugger and run dmalloc -e value-returned-from-print to get an English translation of the error. A number of the error messages are designed to indicate specific problems with the library administrative structures and may not be user-friendly.
If the problem was due to the arguments or system allocations then the
source of the problem has been found. However, if some code did
something wrong, you may have some more work to do to locate the actual
problem. The check-heap token should be enabled and the interval
setting disabled or set to a low value so that the library can find the
problem as close as possible to its source. The code that was execute
right before the library halted, can then be examined closely for
irregularities. See Debug Tokens, See Dmalloc Program.
You may also want to put calls to dmalloc_verify(0) in your code
before the section which generated the error. This should locate the
problem faster by checking the library's structures at that point.
See Extensions.
So you've run your program, examined the log-file and discovered (to your horror) some un-freed memory. Memory leaks can become large problems since even the smallest and most insignificant leak can starve the program given the right circumstances.
not freed: '0x45008' (12 bytes) from 'ra=0x1f8f4' not freed: '0x45028' (12 bytes) from 'unknown' not freed: '0x45048' (10 bytes) from 'argv.c:1077' known memory not freed: 1 pointer, 10 bytes unknown memory not freed: 2 pointers, 24 bytes
Above you will see a sample of some non-freed memory messages from the
logfile. In the first line the 0x45008 is the pointer that was
not freed, the 12 bytes is the size of the unfreed block, and the
ra=0x1f8f4 or return-address shows where the allocation
originated from. See Return Address.
The systems which cannot provide return-address information show
unknown instead, as in the 2nd line in the sample above.
The argv.c:1077 information from the 3rd line shows the file and
line number which allocated the memory which was not freed. This
information comes from the calls from C files which included
dmalloc.h. See Allocation Macros.
At the bottom of the sample it totals the memory for you and breaks it down to known memory (those calls which supplied the file/line information) and unknown (the rest).
Often, you may allocate memory in via strdup() or another
routine, so the logfile listing where in the strdup routine the
memory was allocated does not help locate the true source of the memory
leak - the routine that called strdup. Without a mechanism to
trace the calling stack, there is no way for the library to see who the
caller of the caller (so to speak) was.
However, there is a way to track down unfreed memory in this
circumstance. You need to compile the library with
STORE_SEEN_COUNT defined in conf.h. The library will then
record how many times a pointer has been allocated or freed. It will
display the unfreed memory as:
not freed: '0x45008|s3' (12 bytes) from 'ra=0x1f8f4'
The STORE_SEEN_COUNT option adds a |s# qualifier to the
address. This means that the address in question was seen # many
times. In the above example, the address 0x45008 was seen
3 times. The last time it was allocated, it was not freed.
How can a pointer be "seen" 3 times? Let say you strdup a
string of 12 characters and get address 0x45008 - this is #1
time the pointer is seen. You then free the pointer (seen #2) but later
strdup another 12 character string and it gets the 0x45008
address from the free list (seen #3).
So to find out who is allocating this particular 12 bytes the 3rd time,
try dmalloc -a 0x45008:3. The library will stop the program the
third time it sees the 0x45008 address. You then enter a
debugger and put a break point at dmalloc_error. Run the program
and when the breakpoint is reached you can examine the stack frame to
determine who called strdup to allocate the pointer.
To not bother with the STORE_SEEN_COUNT feature, you can also run
your program with the never-reuse token enabled. This token will
cause the library to never reuse memory that has been freed. Unique
addresses are always generated. This should be used with caution since
it may cause your program to run out of memory.
For a definition of fence-posts please see the "Features" section. See Features.
If you have encountered a fence-post memory error, the logfile should be able to tell you the offending address.
free: failed UNDER picket-fence magic-number checking: pointer '0x1d008' from 'dmalloc_t.c:427' Dump of proper fence-bottom bytes: '\e\253\300\300\e\253\300\300' Dump of '0x1d008'-8: '\e\253\300\300WOW!\003\001pforger\023\001\123'
The above sample shows that the pointer 0x1d008 has had its lower
fence-post area overwritten. This means that the code wrote below the
bottom of the address or above the address right below this one. In the
sample, the string that did it was WOW!.
The library first shows you what the proper fence-post information
should look like, and then shows what the pointer's bad information was.
If it cannot print the character, it will display the value as
\ddd where ddd are three octal digits.
By enabling the check-heap debugging token and assigning the
interval setting to a low number, you should be able to locate
approximately when this problem happened. See Debug Tokens,
See Dmalloc Program.
This is the newest section of the library and therefore contains the least information about a subject on which I probably could write a book. My apologies.
Threads are special operating system facilities which allow your programs to have multiple threads of execution (hence the name). In effect your program can be doing a number of things "at the same time". This allows you to take full advantage of modern operating system scheduling and multi-processor hardware. If I've already lost you or if any of the terminology below does not make sense, see manuals about POSIX threads (pthreads) before going any further. O'Reilly publishes a pretty good pthreads manual for example.
The support for threads in dmalloc is minimal although probably adequate for most if not all testing scenarios. It provides support for mutex locking itself to protect against race conditions that result in multiple simultaneous execution. One of the major problems is that most thread libraries uses malloc themselves. Since all of dmalloc's initialization happens when a call to malloc is made, we may be attempting to initialize or lock the mutex while the thread library is booting up. A baaaad thing since thread libraries aren't reentrant.
The solution to this problem is to have the library not initialize or lock its mutex variable until after a certain number of allocation calls have been completed. If the library does not wait before initializing the locks, the thread library will probably core dump. If it waits too long then it can't protect itself from multiple execution and it will abort or other bad things might happen. You adjust the number of times to wait at runtime with the "lock-on" option to the dmalloc program (for example dmalloc -o 20). See Dmalloc Program. Times values between 2 and 30 are probably good although operating systems will vary significantly. You know its too low if your program core dumps and too high if the library generates an exception.
An additional complexity is when we are initializing the lock before
mutex locking around the library. As mentioned, the initialization
itself may generate a malloc call causing the library to go recursive
and the pthread library to possibly core dump. With the THREAD_INIT_LOCK
setting defined in settings.h, you can tune how many times before
we start locking to try and initialize the mutex lock. It defaults to 2
which seems to work for me. If people need to have this runtime
configurable or would like to present an alternative default, please let
me know.
In addition to the "lock-on" option, you probably will need to enable
the allow-nonlinear token if you thread library needs to call
sbrk directly to allocate some storage. See Debug Tokens.
To build the library with the threaded stubs type make threads
which should build libdmallocth.a. The auto-configurations for
these functions is in its infancy and may or may not work depending on
the script's ability to detect your local thread functionality. Feel
free to send me mail with comments or improvements to them.
So that's it. If you have any specific questions or would like addition information posted in this section, please let me know. Experienced thread programmers only please.
The dmalloc program is designed to assist in the setting of the
environment variable DMALLOC_OPTIONS. See Environment Variable. It is designed to print the shell commands necessary to make
the appropriate changes to the environment. Unfortunately, it cannot
make the changes on its own so the output from dmalloc should be sent
through the eval shell command which will do the commands.
The dmalloc program is designed to assist in the setting of the
environment variable DMALLOC_OPTIONS. See Environment Variable. It is designed to print the shell commands necessary to make
the appropriate changes to the environment. Unfortunately, it cannot
make the changes on its own so the output from dmalloc should be sent
through the eval shell command which will do the commands.
With shells that have aliasing or macro capabilities: csh, bash, ksh,
tcsh, zsh, etc., setting up an alias to dmalloc to do the eval call
is recommended. Csh/tcsh users (for example) should put the following
in their .cshrc file:
alias dmalloc 'eval `\dmalloc -C \!*`'
Bash and Zsh users on the other hand should put the following in their
.zshrc file:
function dmalloc { eval `command dmalloc -b $*` }
This allows the user to execute the dmalloc command as dmalloc
arguments.
The most basic usage for the program is dmalloc [-bC] tag. The
-b or -C (either but not both flags used at a time) are
for generating Bourne or C shell type commands respectively. dmalloc
will try and use the SHELL environment variable to determine
whether bourne or C shell commands should be generated but you may want
to explicitly specify the correct flag.
The tag argument to dmalloc should match a line from the user's
runtime configuration file or should be one of the built-in tags.
See RC File. If no tag is specified and no other option-commands
used, dmalloc will display the current settings of the environment
variable. It is useful to specify one of the verbose options when doing
this.
To find out the usage for the debug malloc program try dmalloc
--usage-long. The standardized usage message that will be displayed is
one of the many features of the argv library included with this package.
It is available on the web at http://256.com/sources/argv/. See the documentation there for more information.
Here is a detailed list of the flags that can passed to dmalloc:
-a address
addr part of the DMALLOC_OPTIONS variable to
address (or alternatively address:number).
-b
-C
-c
NOTE: clear will never unset the debug setting. Use
-d 0 or a tag to none to achieve this.
-d bitmask
debug part of the DMALLOC_OPTIONS env variable to
the bitmask value which should be in hex. This is overridden (and
unnecessary) if a tag is specified.
-D
-e errno
-f filename
$HOME/.dmallocrc.
-i number
check-heap token is
enabled, this causes the library to only check the heap every Nth time
which can significantly increase the running speed of your
program. If a problem is found, however, this limits your ability to
determine when the problem occurred. Try values of 50 or 100 initially.
-k
-l filename
-L
-m token(s)
-n
-o times
-p token(s)
-r
-s number
start part of the DMALLOC_OPTIONS env variable to
number (alternatively file:line).
-S
-t
-v
If no arguments are specified, dmalloc dumps out the current settings that you have for the environment variable. For example:
Debug-Flags '0x40005c7' (runtime) Address 0x1f008, count = 3 Interval 100 Logpath 'malloc' Start-File not-set
With a -v option and no arguments, dmalloc dumps out the current settings in a verbose manner. For example:
Debug-Flags '0x40005c7' (runtime) log-stats, log-non-free, log-blocks, log-unknown, log-bad-space, check-fence, catch-null Address 0x1f008, count = 10 Interval 100 Logpath 'malloc' Start-File not-set
Here are some examples of dmalloc usage:
# start tough debugging, check the heap every 100 times, # send the log information to file 'dmalloc' dmalloc high -i 100 -l dmalloc # find out what error code 20 is (from the logfile) dmalloc -e 20 # cause the library to halt itself when it sees the address 0x34238 # for the 6th time. dmalloc -a 0x34238:6 # return to the normal 'runtime' settings and clear out all # other settings dmalloc -c runtime # enable basic 'low' settings plus (-p) the logging of # transactions (log-trans) to file 'dmalloc' dmalloc low -p log-trans -l dmalloc # print out the current settings with Very-verbose output dmalloc -V # list the available debug malloc tokens with Very-verbose output dmalloc -DV # list the available tags from the rc file with verbose output dmalloc -tv
An environment variable is a variable that is part of the user's
working environment and is shared by all the programs. The
DMALLOC_OPTIONS variable is used by the dmalloc library to enable
or disable the memory debugging features, at runtime. It can be set
either by hand or with the help of the dmalloc program. See Dmalloc Program.
To set it by hand, C shell (csh or tcsh) users need to invoke:
setenv DMALLOC_OPTIONS value
Bourne shell (sh, bash, ksh, or zsh) users should use:
DMALLOC_OPTIONS=value export DMALLOC_OPTIONS
The value in the above examples is a comma separated list of tokens each having a corresponding value. The tokens are described below:
debug
0x008) and wanted to check fence-post memory
(value 0x400) then debug should be set to 0x408
(0x008 + 0x400).
NOTE: You don't have to worry about remembering all the hex values of the tokens because the dmalloc program automates the setting of this variable especially.
NOTE: You can also specify the debug tokens directly, separated
by commas. See Debug Tokens. If debug and the tokens are
both used, the token values will be added to the debug value.
log
debug has logging enabled, the
library can log transactions, administration information, and/or errors
to the file so memory problems and usage can be tracked.
To get different logfiles for different processes, you can assign
log to a string with %d in it (for instance
logfile.%d). This will be replaced with the pid of the running
process (for instance logfile.2451).
WARNING: it is easy to core dump any program with dmalloc, if
you send in a format with arguments other than the one %d.
addr
The address can also have an :number argument. For instance, if
it was set it to 0x3e45:10, the library will kill itself the 10th
time it sees address 0x3e45. By setting the number argument to
0, the program will never stop when it sees the address. This is useful
for logging all activity on the address and makes it easier to track
down specific addresses not being freed.
This works well in conjunction with the STORE_SEEN_COUNT option.
See Memory Leaks.
NOTE: dmalloc will also log all activity on this address along
with a count.
inter
A setting of 100 works well with reasonably memory intensive
programs. This of course means that the library will not catch errors
exactly when they happen but possibly 100 library calls later.
start
start also has the format file:line. For instance, if it is set
to dmalloc_t.c:126 dmalloc will start checking the heap after it
sees a dmalloc call from the dmalloc_t.c file, line number 126.
If line number is 0 then dmalloc will start checking the heap after it
sees a call from anywhere in the dmalloc_t.c file.
This allows the intensive debugging to be started after a certain routine or file has been reached in the program.
Some examples are:
# turn on transaction and stats logging and set 'malloc' as the log-file setenv DMALLOC_OPTIONS log-trans,log-stats,log=malloc # enable debug flags 0x1f as well as heap-checking and set the interval # to be 100 setenv DMALLOC_OPTIONS debug=0x1f,check-heap,inter=100 # enable 'malloc' as the log-file, watch for address '0x1234', and start # checking when we see file.c line 123 setenv DMALLOC_OPTIONS log=malloc,addr=0x1234,start=file.c:123
The below tokens and their corresponding descriptions are for the
setting of the debug library setting in the environment variable.
See Environment Variable. They should be specified in the user's
.dmallocrc file. See RC File.
Each token, when specified, enables a specific debugging feature. For instance, if you have the log-stats token enabled, the library will log general statistics to the logfile.
To get this information on the fly, use dmalloc -DV. This will print out the Debug tokens in Very-verbose mode. See Dmalloc Program.
none
log-stats
log-non-free
log-thread-id
conf.h).
log-trans
log-stamp
log-admin
log-blocks
log-unknown
log-bad-space
log-nonfree-space
log-elapsed-time
conf.h).
log-current-time
conf.h).
check-fence
check-heap
check-lists
check-blank
check-funcs
realloc-copy
free-blank
error-abort
error-dump below.
alloc-blank
heap-check-map
print-messages
catch-null
never-reuse
allow-nonlinear
sbrk
directly. This is now enabled by default since an increasing number of
operating system functions seem to be doing this - one example is the
pthreads package.
WARNING: This should be used with caution since it may hide
certain heap problems.
allow-free-null
settings.h file which change the default behavior.
NOTE: This does not impact the ALLOW_REALLOC_NULL compilation
options which can be adjusted in conf.h.
error-dump
error-abort above.
NOTE: This will only work if your system supports the fork
system call and the configuration utility was able to fork without going
recursive.
By using a RC File (or runtime configuration file) you can alias tags to combinations of debug tokens. See Debug Tokens.
NOTE: For beginning users, the dmalloc program has a couple of tags built into it so it is not necessary for you to setup a RC file:
runtime
low
medium
high
all
For expert users, a sample dmallocrc file has been provided but
you are encouraged to roll your own combinations. The name of default
rc-file is $HOME/.dmallocrc. The $HOME environment
variable should be set by the system to point to your home-directory.
The file should contain lines in the general form of:
tag token1, token2, ...
tag is to be matched with the tag argument passed to the dmalloc
program, while token1, token2, ... are debug capability
tokens. See Dmalloc Program, Debug Tokens.
A line can be finished with a \ meaning it continues onto the
next line. Lines beginning with # are treated as comments and
are ignored along with empty lines.
Here is an example of a .dmallocrc file:
#
# Dmalloc runtime configuration file for the debug malloc library
#
# no debugging
none none
# basic debugging
debug1 log-stats, log-non-free, check-fence
# more logging and some heap checking
debug2 log-stats, log-non-free, log-trans, \
check-fence, check-heap, check-lists, error-abort
# good utilities
debug3 log-stats, log-non-free, log-trans, \
log-admin, check-fence, check-heap, check-lists, realloc-copy, \
free-blank, error-abort
...
For example, with the above file installed, you can type dmalloc
debug1 after setting up your shell alias. See Dmalloc Program.
This enables the logging of statistics, the logging of non-freed memory,
and the checking of fence-post memory areas.
Enter dmalloc none to disable all memory debugging features.
Here are a couple definitions and other information for those interested in "picking the brain" of the library. The code is a little ugly here and there and it conforms to the Gray-Watson handbook of coding standards only.
For more information about administration structures, see the code and
comments from chunk_loc.h.
conf.h file.
conf.h for some manual compilation
options to handle this.
General portability issues center around:
The library has not been tested on a system whose heap grows towards low memory. If you are trying to run the library on such a system please contact the author.
dmalloc.h: C library calls for
instance.
See return.h for the available architecture/compiler
combinations. You may want to examine the assembly code from gcc (GNUs
superior c-compiler) version 2+ being run on the following code. It
should give you a good start on building a hack for your box.
static char * x;
a()
{
x = __builtin_return_address(0);
}
main()
{
a();
}