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9.2 Programs that crash in malloc or free.

Q: Since I upgraded to DJGPP v2.02, my program started to crash, and the traceback points to library function free. This program worked flawlessly with v2.01, so I guess there's a bug in the new version of free, right?

A: Such problems are a tell-tale sign of programs that overwrite buffers allocated by malloc or calloc, or call free more than once with the same pointer, or pass to free a pointer that didn't originate from a call to malloc or calloc. If the program that crashes is a C++ program, you might have several objects that share the same data, and the object destructor crashes when it calls free several time with the same memory chunk.

These crashes happen inside the memory-allocation functions because these functions maintain some crucial information about the allocated and free memory blocks right before the beginning and beyond the end of the allocated buffers. For speed considerations, this information is not protected by any means like CRC or parity, so if you overwrite this information, malloc and free will become confused and eventually will blow up.

The version of malloc in DJGPP library before v2.02 left some slack space beyond the end of the allocated buffer (this was a side-effect of the algorithm it used, which was optimized for speed, but wasted some memory). Thus, a program could overrun the allocated buffer and still get away uncaught. The new version of malloc introduced with v2.02 doesn't waste memory, and because of this is much less tolerant to such bugs.

Bottom line: you should debug your program to find the offending code that overwrites the end of an allocated buffer. One way of doing that is to put a data breakpoint (a.k.a. watchpoint) inside a debugger at the address which gets overwritten; then, when the program overwrites it, the debugger will kick in and you will see whodunit.

Another possibility to debug such problems is to use the YAMD package, written and maintained by Nate Eldredge. YAMD is a malloc debugger which will catch and report many problems related to allocating, freeing, and using allocated memory. YAMD is available from Nate's home page.

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