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Debug Malloc Library

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, Free/OpenBSD, 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.

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1 Library Copying and Licensing Conditions

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.

Node:Overview, Next:, Previous:Copying, Up:Top

2 Description of Features and How to Get Started

Node:Allocation Basics, Next:, Previous:Overview, Up:Overview

2.1 Basic Description of Terms and Functions

Node:Basic Definitions, Next:, Previous:Allocation Basics, Up:Allocation Basics

2.1.1 General Memory Terms and Concepts

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.

Node:Malloc Functions, Previous:Basic Definitions, Up:Allocation Basics

2.1.2 Functionality Supported by All Malloc Libraries

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: pnt = (type *)malloc(size)

The malloc routine is the basic memory allocation routine. It allocates an area of size bytes. It will return a pointer to the space requested.

Function
void *calloc ( unsigned int number, unsigned int size )

Usage: pnt = (type *)calloc(number, size)

The calloc routine allocates a certain number of items, each of size bytes, and returns a pointer to the space. It is appropriate to pass in a sizeof(type) value as the size argument.

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: new_pnt = (type *)realloc(old_pnt, new_size)

The realloc function expands or shrinks the memory allocation in old_pnt to new_size number of bytes. Realloc copies as much of the information from old_pnt as it can into the new_pnt space it returns, up to new_size bytes. If there is a problem allocating this memory, 0L will be returned.

If the old_pnt is 0L then realloc will do the equivalent of a malloc(new_size). If new_size is 0 and old_pnt is not 0L, then it will do the equivalent of free(old_pnt) and will return 0L.

Function
void free ( void *pnt )

Usage: free(pnt)

The free routine releases allocation in pnt which was returned by malloc, calloc, or realloc back to the heap. This allows other parts of the program to re-use memory that is not needed anymore. It guarantees that the process does not grow too big and swallow a large portion of the system resources.

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.

Node:Features, Next:, Previous:Allocation Basics, Up:Overview

2.2 General Features of the Library

The debugging features that are available in this debug malloc library can be divided into a couple basic classifications:

file and line number information
One of the nice things about a good debugger is its ability to provide the file and line number of an offending piece of code. This library attempts to give this functionality with the help of cpp, the C preprocessor. See Allocation Macros.
return-address information
To debug calls to the library from external sources (i.e. those files that could not use the allocation macros), some facilities have been provided to supply the caller's address. This address, with the help of a debugger, can help you locate the source of a problem. See Return Address.
fence-post (i.e. bounds) checking
Fence-post memory is the area immediately above or below memory allocations. It is all too easy to write code that accesses above or below an allocation - especially when dealing with arrays or strings. The library can write special values in the areas around every allocation so it will notice when these areas have been overwritten. See Fence-Post Overruns.

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.

heap-constancy verification
The administration of the library is reasonably complex. If any of the heap-maintenance information is corrupted, the program will either crash or give unpredictable results.

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.

logging statistics
One of the reasons why the debug malloc library was initially developed was to track programs' memory usage - specifically to locate memory leaks which are places where allocated memory is never getting freed. See Memory Leaks.

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.

examining unfreed memory
Another common problem happens when a program frees a memory pointer but goes on to use it again by mistake. This can lead to mysterious crashes and unexplained problems.

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.

Node:Installation, Next:, Previous:Features, Up:Overview

2.3 How to Install the Library

To configure, compile, and install the library, follow these steps carefully.

  1. Make sure you have the latest version of the library available from the home page http://dmalloc.com/.
  2. You probably will want to edit the settings in 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.
  3. Type sh ./configure to configure the library. You may want to first examine the config.help file for some information about configure. You may want to use the --disable-cxx option if you do not want the Makefile to build the C++ version of dmalloc. You may want to use the --enable-threads option to build the threaded version of dmalloc. You may want to use the --enable-shlib option to build the shared versions of the dmalloc libraries. Configure should generate the Makefile and 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.

  4. You may want to examine the Makefile and conf.h files created by configure to make sure it did its job correctly.
  5. You might want to tune the settings in 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.
  6. The 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.
  7. Typing make should be enough to build 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_RETURN_MACROS 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.

  8. If you use threads, typing make threads should be enough to build 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.

  9. If you have a C++ compiler installed, the library should have automatically built libdmallocxx.a which is the C++ version of the library. If it was not done automatically, you can build it by typing make cxx. You should link this library into your C++ programs instead of libdmalloc.a. 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.
  10. Typing make tests should build the dmalloc_t test program.
  11. Typing make light should run the 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.
  12. Typing make install should install the 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.

Node:Getting Started, Next:, Previous:Installation, Up:Overview

2.4 Getting Started with the Library

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:

  1. Make sure you have the latest version of the library available from the home page http://dmalloc.com/.
  2. Follow the installation instructions on how to configure and make and install the library (i.e. type: make install). See Installation.
  3. You need to make sure that the dmalloc building process above was able to locate one of the the 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.
  4. Add an alias for dmalloc to your shell's rc file if supported. Bash, ksh, and zsh users should add the following to their .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 \!*`'

  5. Although not necessary, you may want to include 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
  6. Link the dmalloc library into your program.
  7. Enable the debugging features by typing dmalloc -l logfile -i 100 low (for example). This will:
    • set the malloc log path to logfile (-l logfile)
    • have the library check itself every 100 iterations (-i 100)
    • enable a number of debug features (low). You can also try runtime for minimal checking or medium or high for more extensive heap verification. all is also provided but generates a multitude of log messages without many more tests.

    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.

  8. You may also want to enable the 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.
  9. Run your program, examine the logfile that should have been created by dmalloc_shutdown, and use its information to help debug your program. See the next section for help with this. See Troubleshooting.

Node:Troubleshooting, Previous:Getting Started, Up:Overview

2.5 Some Solutions to Common Problems

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?
This library has never been (and maybe never will be) optimized for space nor speed. Some of its features make it unable to use some of the organizational methods of other more efficient heap libraries. If you have the 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?
This could be caused by a number of different problems.

  1. Are you sure you followed all of the items in the "Getting Started" section? Please review them if there is any doubt. See Getting Started.
  2. Use the env or printenv commands to make sure that the DMALLOC_OPTIONS variable is set in your exported environment. See Environment Variable.
  3. Make sure that your program has been compiled correctly with the dmalloc library. The ident program should show chunk.c and other dmalloc files compiled into your program. You can also do strings -a your-program | grep chunk.c and look for something like $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.
  4. If your program changes its working directory, it may write the dmalloc log-file somewhere else in the filesystem. You will need to check both where the program was started and to where it might change directory.

I don't see any information about my non-freed (leaked) memory?
The library will not (by default) report on "unknown" non-freed memory. Unknown means memory that does not have associated file and line information. To log information about unknown pointers you should enable the 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.

Node:Programming, Next:, Previous:Overview, Up:Top

3 How to Program with the Library

Node:Allocation Macros, Next:, Previous:Programming, Up:Programming

3.1 Macros Providing File and Line Information

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.

Node:Return Address, Next:, Previous:Allocation Macros, Up:Programming

3.2 Getting Caller Address Information

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.

Node:Argument Checking, Next:, Previous:Return Address, Up:Programming

3.3 Checking of Function Arguments

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.

Node:Extensions, Next:, Previous:Argument Checking, Up:Programming

3.4 Additional Non-standard Routines

The library has a number of variables that are not a standard part of most malloc libraries:

char * dmalloc_logpath
This variable can be used to set the dmalloc log filename. The env variable DMALLOC_LOGFILE overrides this variable.
int dmalloc_errno
This variable stores the internal dmalloc library error number like errno does for the system calls. It can be passed to 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
This variable holds the address to be specifically looked for when allocating or freeing by the library.
int dmalloc_address_count
This variable stores the argument to the address library setting. If it is set to a greater than 0 value then after the library has seen the 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 on_exit() or atexit() calls (see conf.h). If you do not have these routines, then dmalloc_shutdown should be called right before exit() or as the last function in main(). 

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: dmalloc_verify() can only check the heap with the functions that have been enabled. For example, if fence-post checking is not enabled, dmalloc_verify() cannot check the fence-post areas in the heap.

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 dmalloc_debug(0) as the first call in main() will disable all the memory debugging from that point on.

One problem however is that some systems make calls to memory allocation functions before main() is reached therefore before dmalloc_debug() can be called meaning some debugging information may be generated regardless.

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 stdarg.h include file.

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 stdarg.h include file.

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 dmalloc program with a -e # option. See Dmalloc Program.

Node:C++ and the Library, Next:, Previous:Extensions, Up:Programming

3.5 Using the Library with C++

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 dmallocc.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.

Node:Disabling the Library, Next:, Previous:C++ and the Library, Up:Programming

3.6 How to Disable the library

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.

  • If you want to totally disable the dmalloc library then you will need to recompile all the C files that include 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
  • If you compiled any of your source modules with DMALLOC_FUNC_CHECK defined then you must first recompile all those modules without the flag enabled.
  • If you have not compiled all your source with 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.

Node:Using With a Debugger, Next:, Previous:Disabling the Library, Up:Programming

3.7 Using Dmalloc With a Debugger

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.

Node:General Errors, Next:, Previous:Using With a Debugger, Up:Using With a Debugger

3.7.1 Diagnosing General Problems with a Debugger

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.

Node:Memory Leaks, Next:, Previous:General Errors, Up:Using With a Debugger

3.7.2 Tracking Down Non-Freed Memory

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.

Node:Fence-Post Overruns, Previous:Memory Leaks, Up:Using With a Debugger

3.7.3 Diagnosing Fence-Post Overwritten 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.

Node:Using With Threads, Previous:Using With a Debugger, Up:Programming

3.8 Using the Library with a Thread Package

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.

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.

Node:Dmalloc Program, Next:, Previous:Programming, Up:Top

4 Dmalloc Utility Program

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.

Node:Shell Alias, Next:, Previous:Dmalloc Program, Up:Dmalloc Program

4.1 Using a Shell Alias with the Utility

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. Bash, ksh, and zsh users should add the following to their .bashrc, .profile, or .zshrc file respectively (notice the -b option for bourne shell output): 

function dmalloc { eval `command dmalloc -b $*`; }

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 \!*`'

This allows the user to execute the dmalloc command as dmalloc arguments.

Users of versions of the Bourne shell (usually known as /bin/sh) that don't have command functions will need to send the output to a temporary file and the read it back in with the "." command: 

$  dmalloc -b arguments... > /tmp/out
$  . /tmp/out

Node:Utility Usage, Next:, Previous:Shell Alias, Up:Dmalloc Program

4.2 How to Use the Dmalloc Program

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
Set the addr part of the DMALLOC_OPTIONS variable to address (or alternatively address:number).
-b
Output Bourne shell type commands.
-C
Output C shell type commands.
-c
Clear/unset all of the settings not specified with other arguments. You can do this automatically when you set to a new tag with the -r option.

NOTE: clear will never unset the debug setting. Use -d 0 or a tag to none to achieve this.

-d bitmask
Set the 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
List all of the debug-tokens. Useful for finding a token to be used with the -p or -m options. Use with -v or -V verbose options.
-e errno
Print the dmalloc error string that corresponds to the error number errno.
-f filename
Use this configuration file instead of the RC file $HOME/.dmallocrc.
-i number
Set the checking interval to number. If the 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
Do not reset all of the settings when a tag is specified. This specifically overrides the -r option and is provided here to override -r if it has been added to the dmalloc alias.
-l filename
Write the debugging output and other log-file information to filename.
-L
Output the debug-value not in hex but by individual debug-tokens in long form.
-m token(s)
Remove (minus) the debug capabilities of token(s) from the current debug setting or from the selected tag (or -d value). Multiple -m options can be specified.
-n
Without changing the environment, output the commands resulting from the supplied options.
-o times
Set the "lock-on" period which dictates with the threaded version of the library to not initialize or lock the mutex lock around the library until after a certain number of allocation calls have been made. Some number between 2 and 30 is probably good. See the "Using With Threads" section for more information about the operation of the library with threads. See Using With Threads.
-p token(s)
Add (plus) the debug capabilities of token(s) to the current debug setting or to the selected tag (or -d value). Multiple -p options can be specified.
-r
Remove (unset) all settings when using a tag. This is useful when you are returning to a standard development tag and want the logfile, address, and interval settings to be cleared automatically. If you want this behavior by default, this can be put into the dmalloc alias.
-s number
Set the start part of the DMALLOC_OPTIONS env variable to number (alternatively file:line).
-S
Output the debug-value not in hex but by individual debug-tokens in short form.
-t
List all of the tags in the rc-file. Use with -v or -V verbose options.
-v
Give verbose output. Especially useful when dumping current settings or listing all of the tags.

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

Node:Environment Variable, Next:, Previous:Utility Usage, Up:Dmalloc Program

4.3 Environment Variable Name and Features

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, Bourne shell (sh, bash, ksh, or zsh) users should use: 

DMALLOC_OPTIONS=value
export DMALLOC_OPTIONS

C shell (csh or tcsh) users need to invoke: 

setenv DMALLOC_OPTIONS value

The value in the above examples is a comma separated list of tokens each having a corresponding value. The tokens are described below:

debug
This should be set to a value in hexadecimal which corresponds to the functionality token values added together. See Debug Tokens. For instance, if the user wanted to enable the logging of memory transactions (value 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.

lockon
Set this to a number which is the "lock-on" period. This dictates with the threaded version of the the library to not initialize or lock the mutex lock around the library until after a certain number of allocation calls have been made. See the "Using With Threads" section for more information about the operation of the library with threads. See Using With Threads.
log
Set this to a filename so that if 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
When this is set to a hex address (taken from the dmalloc log-file for instance) dmalloc will abort when it finds itself either allocating or freeing that address.

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
By setting this to a number X, dmalloc will only check the heap every X times. This means a number of debugging features can be enabled while still running the program within a finite amount of time.

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
Set this to a number X and dmalloc will begin checking the heap after X times. This means the intensive debugging can be started after a certain point in a program.

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

Node:Debug Tokens, Next:, Previous:Environment Variable, Up:Dmalloc Program

4.4 Description of the Debugging Tokens

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
No debugging functionality
log-stats
Log general statistics when dmalloc_shutdown or dmalloc_log_stats is called.
log-non-free
Log non-freed memory pointers when dmalloc_shutdown or dmalloc_log_unfreed is called.
log-thread-id
For systems that have multi-threaded programs (don't worry if this does not make sense to you), log thread-id for allocated pointer (see conf.h).
log-trans
Log general memory transactions (quite verbose).
log-stamp
Log a time stamp for all messages.
log-admin
Log administrative information (quite verbose).
log-blocks
Log detailed block information when dmalloc_log_heap_map is called.
log-unknown
Like log-non-free but logs non-freed memory pointers that did not have file/line information associated with them.
log-bad-space
Log actual bytes in and around bad pointers.
log-nonfree-space
Log actual bytes in non-freed pointers.
log-elapsed-time
Log elapsed-time for allocated pointers (see conf.h).
log-current-time
Log current-time for allocated pointers (see conf.h).
check-fence
Check fence-post memory areas.
check-heap
Verify heap administrative structure.
check-lists
Examine internal heap linked-lists.
check-blank
Check to see if space that was blanked by free-blank or alloc-blank has been overwritten. If this is enabled then it will enable free-blank and alloc-blank automatically.
check-funcs
Check the arguments of some functions (mostly string operations) looking for bad pointers.
force-linear
Have the heap complain when additional program functionality seems to have made use of the system's heap-allocation routine sbrk directly behind the dmalloc library's back. This is disabled by default since an increasing number of operating system functions seem to be doing this, especially pthreads packages.
catch-signals
Shutdown the library automatically on SIGHUP, SIGINT, or SIGTERM. This will cause the library to dump its statistics (if requested) when you press control-c on the program (for example).
realloc-copy
Always copy data to a new pointer when realloc.
free-blank
Write special bytes (decimal 197, octal 0305, hex 0xc5) into space when it is freed. The same bytes are written when memory is allocated if alloc-blank is enabled.
error-abort
Abort the program (and dump core) on errors. See error-dump below.
alloc-blank
Write special bytes (decimal 197, octal 0305, hex 0xc5) into space when it is allocated. The same bytes are written when memory is freed if free-blank is enabled.
heap-check-map
Log a heap-map to the logfile every time the heap is checked.
print-messages
Log any errors and messages to the screen via standard-error.
catch-null
Abort the program immediately if the library fails to get more heap space from sbrk.
never-reuse
Have the heap never use space that has been used before and freed. See Memory Leaks. WARNING: This should be used with caution since you may run out of heap space.
allow-free-null
The library will not generate an error when a program tries to free a NULL pointer. See ALLOW_FREE_NULL and ALLOW_FREE_NULL_MESSAGE settings in the 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
Dump core on error and then continue. Later core dumps overwrite earlier ones if the program encounters more than one error. See 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.

Node:RC File, Previous:Debug Tokens, Up:Dmalloc Program

4.5 Format of the Runtime Configuration File

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
enables basic runtime tests
low
turns on minimal checking of heap structures
medium
significant checking of heap areas
high
extensive checking of heap areas
all
turns on all the checking possible. This generates a multitude of log messages without many more tests than high.

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.

Node:Source Code, Next:, Previous:Dmalloc Program, Up:Top

5 Information on the Source Code

Node:Definitions, Next:, Previous:Source Code, Up:Source Code

5.1 Definition of Terms and other Information

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.

bblock
basic block containing 2 ^ BASIC_BLOCK bytes of info
bblock_adm
administration for a set of basic blocks
dblock
divided block containing some base 2 number of blocks smaller than a basic block.
dblock_adm
administration for a set of divided blocks
chunk
some anonymous amount of memory

For more information about administration structures, see the code and comments from chunk_loc.h.

Node:Compatibility, Next:, Previous:Definitions, Up:Source Code

5.2 General Compatibility Concerns

  • Realloc() backwards compatibility with being able to realloc from the last freed block is not supported. The author is interested to know who is using this (cough, cough) feature and for what reason.
  • Realloc() of a NULL pointer is supported in which case the library will just make a call to malloc(). This can be disabled with the help of a manual compilation option in the conf.h file.
  • Some systems allow free(0) to not be an error for some reason. Since 0 is not a valid address returned by the malloc call, it is debatable that this should be allowed. See conf.h for some manual compilation options to handle this.
  • Aside from possibly being slower than the system's memory allocation functions, the library should be fully compatible with the standard memory routines. If this is not the case, please bring this to my attention.

Node:Portability, Previous:Compatibility, Up:Source Code

5.3 Issues Important for Porting the Library

General portability issues center around:

  • sbrk or compatible function usages
  • Whether the systems' heap grows towards high or low memory. The chunk.c code is designed (loosely) around the fact that consecutive calls to sbrk should give higher memory addresses.

    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.

  • The locating of the caller's address from the dmalloc functions. This is useful in locating problems from dmalloc functions called from C files which did not include 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();
}

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Index of Concepts

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