170 lines
6.3 KiB
Plaintext
170 lines
6.3 KiB
Plaintext
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Using memwatch
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==============
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What is it?
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Memwatch is primarily a memory leak detector for C. Besides
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detecting leaks, it can do a bunch of other stuff, but lets
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stay to the basics. If you _really_ want to know all the
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gory details, you should check out the header file,
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memwatch.h, and the source code. It's actually got some
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comments! (Whoa, what a concept!)
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How do I get the latest version?
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http://www.link-data.com/sourcecode.html
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ftp://ftp.link-data.com/pub/memwatch/
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How does it work?
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Using the C preprocessor, memwatch replaces all your
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programs calls to ANSI C memory allocation functions with
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calls to it's own functions, which keeps a record of all
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allocations.
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Memwatch is very unobtrusive; unless the define MEMWATCH is
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defined, memwatch removes all traces of itself from the
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code (using the preprocessor).
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Memwatch normally writes it's data to the file
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memwatch.log, but this can be overridden; see the section
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on I/O, later.
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Initialization and cleanup
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In order to do it's work in a timely fashion, memwatch
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needs to do some startup and cleanup work. mwInit()
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initializes memwatch and mwTerm() terminates it. Memwatch
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can auto-initialize, and will do so if you don't call
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mwInit() yourself. If this is the case, memwatch will use
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atexit() to register mwTerm() to the atexit-queue.
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The auto-init technique has a caveat; if you are using
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atexit() yourself to do cleanup work, memwatch may
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terminate before your program is done. To be on the safe
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side, use mwInit() and mwTerm().
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mwInit() and mwTerm() is nestable, so you can call mwInit()
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several times, requiring mwTerm() to be called an equal
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number of times to terminate memwatch.
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In case of the program aborting in a controlled way, you
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may want to call mwAbort() instead of mwTerm(). mwAbort()
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will terminate memwatch even if there are outstanding calls
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to mwTerm().
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I/O operations
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During normal operations, memwatch creates a file named
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memwatch.log. Sometimes, memwatch.log can't be created;
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then memwatch tries to create files name memwatNN.log,
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where NN is between 01 and 99. If that fails, no log will
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be produced.
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If you can't use a file log, or don't want to, no worry.
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Just call mwSetOutFunc() with the address of a "void
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func(int c)" function, and all output will be directed
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there, character by character.
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Memwatch also has an Abort/Retry/Ignore handler that is
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used when an ASSERT or VERIFY fails. The default handler
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does no I/O, but automatically aborts the program. You can
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use any handler you want; just send the address of a "int
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func(const char*)" to mwSetAriFunc(). For more details on
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that, see memwatch.h.
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TRACE/ASSERT/VERIFY macros
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Memwatch defines (if not already defined) the macros TRACE,
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ASSERT and VERIFY. If you are already using macros with
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these names, memwatch 2.61 and later will not override
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them. Memwatch 2.61 and later will also always define the
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macros mwTRACE, mwASSERT and mwVERIFY, so you can use these
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to make sure you're talking to memwatch. Versions previous
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to 2.61 will OVERRIDE TRACE, ASSERT and VERIFY.
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To make sure that existing TRACE, ASSERT and VERIFY macros
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are preserved, you can define MW_NOTRACE, MW_NOASSERT and
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MW_NOVERIFY. All versions of memwatch will abide by these.
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Stress-testing the application
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You can simulate low-memory conditions using mwLimit().
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mwLimit() takes the maximum number of bytes to be
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allocated; when the limit is hit, allocation requests will
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fail, and a "limit" message will be logged.
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If you hit a real low-memory situation, memwatch logs that
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too. Memwatch itself has some reserve memory tucked away so
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it should continue running even in the worst conditions.
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Hunting down wild writes and other Nasty Things
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Wild writes are usually caused by using pointers that arent
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initialized, or that were initialized, but then the memory
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they points to is moved or freed. The best way to avoid
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these kind of problems is to ALWAYS initialize pointers to
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NULL, and after freeing a memory buffer, setting all
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pointers that pointed to it to NULL.
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To aid in tracking down uninitialized pointers memwatch
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zaps all memory with certain values. Recently allocated
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memory (unless calloc'd, of course), contains 0xFE.
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Recently freed memory contains 0xFD. So if your program
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crashes when using memwatch and not without memwatch, it's
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most likely because you are not initializing your allocated
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buffers, or using the buffers after they've been freed.
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In the event that a wild pointer should damage memwatch's
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internal data structures, memwatch employs checksums,
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multiple copies of some values, and can also repair it's
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own data structures.
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If you are a paranoid person, and as programmer you should
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be, you can use memwatch's mwIsReadAddr() and
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mwIsSafeAddr() functions to check the accessibility of
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memory. These are implemented for both ANSI C systems and
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Win32 systems. Just put an mwASSERT() around the check and
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forget about it.
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Can I help?
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Well, sure. For instance, I like memwatch to compile
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without any warnings or errors. If you are using an ANSI C
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compliant compiler, and are getting warnings or errors,
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please mail me the details and instructions on how to fix
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them, if you can.
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Another thing you can do if you decide to use memwatch is
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to mail me the name of the project(s) (and URL, if any),
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hardware and operating system, compiler and what user
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(organization). I will then post this info on the list of
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memwatch users.
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(http://www.link-data.com/memwatchusers.html)
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Top five problems using memwatch
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5. Passed a non-memwatch allocated pointer to memwatch's
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free(). Symtom: Causes an erroneous "WILD free" log
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entry to appear. Cure: Either include memwatch.h for
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the file that allocates, or use mwFree_() to free it.
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4. Relied on auto-initialization when using atexit().
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Symptom: Causes incorrect "unfreed" and "WILD free"
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messages. Cure: Use mwInit() and mwTerm().
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3. Forgot to include memwatch.h in all files. Symptom:
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Tends to generate "WILD free" and "unfreed" messages.
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Cure: Make sure to include memwatch.h!
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2. No write permissions in currect directory. Symptom:
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Seems like memwatch 'just aint working'. Cure: Use
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mwSetOutFunc() to redirect output.
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...and the number one problem is...
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1. Didn't define MEMWATCH when compiling. Symptom:
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Memwatch dutifully disables itself. Cure: Try adding
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-DMEMWATCH to the command line.
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