Advanced Memory Allocation
Tracing the history of memory blocks helps in finding problems related to memory leaks and usage or release of already freed blocks. For this purpose, the GNU C library offers a tracing facility that is enabled by calling the mtrace() function. Once this call is made, every heap operation is logged to a file whose name must be specified in the environment variable MALLOC_TRACE. Analysis of the log file then can be performed off-line using a Perl script that is provided with the library and called, not surprisingly, mtrace. Logging can be stopped by calling muntrace(), but keep in mind that applying tracing to portions of your program may invalidate the result of post-processing. For example, false leaks may be detected if you allocate one block while tracing and then free it after muntrace().
Here is a sample tracing session using the program in Listing 3:
$ gcc -g Listing_3.c -o Listing_3 $ MALLOC_TRACE="trace.log" ./Listing_3 $ mtrace trace.log Memory not freed: ----------------- Address Size Caller 0x08049718 0xa at malloc_debug/Listing_3.c:9
Memory tracing has nothing to do with protection from errors; calling mtrace() won't prevent the program from crashing. Even worse, if the program segfaults, the trace file is likely to be truncated and tracing may be inconsistent. To protect against this risk, it is always a good idea to install a SIGSEGV handler that calls muntrace(), because it closes the trace file before aborting (Listing 4). More information on memory tracing can be found on the libc info page.
Sometimes the standard debugging facilities provided by the GNU C library may not be suited to the particular needs of your program. In this case, you can resort either to an external memory debugging tool (see Resources) or carve your own inside the library. Doing this is simply a matter of writing three functions and hooking them to these predefined variables:
__malloc_hook points to a function to be called when the user calls malloc(). You can do your own checks and accounting here, and then call the real malloc() to get the memory that was requested.
__free_hook points to a function called instead of the standard free().
__malloc_initialize_hook points to a function called when the memory management system is initialized. This allows you to perform some operations, say, setting the values of the previous hooks, before any memory-related operation takes place.
Hooks also are available for other memory-related calls, including realloc(), calloc() and so on. Be sure to save the previous values of the hooks and restore them before calling malloc() or free() inside your routines. If you fail to do so, infinite recursion prevents your code from working. Have a look at the example given in the libc info page for memory debugging to see all the nifty details.
As a final note, consider that these hooks also are used by the mcheck and mtrace systems. It's a good idea to be careful when using all of them combined.
The GNU C library offers several extensions that turn out to be quite useful when dealing with memory. If you want to fine-tune your application's memory usage or build a memory debugging solution tailored to your needs, you probably will find these tools helpful or, at least, a good starting point to develop your own mechanisms.
Gianluca Insolvibile has been a Linux enthusiast since kernel 0.99pl4. He currently deals with networking and digital video research and development.
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