Memory Leak Detection in C++
An earlier article [“Memory Leak Detection in Embedded Systems”, LJ, September 2002, available at www.linuxjournal.com/article/6059] discussed the detection of memory leaks when using C as the programming language. This article discusses the problem of detecting memory leaks in C++ programs. The tools discussed here detect application program errors, not kernel memory leaks. All of these tools have been used with the MontaVista Linux Professional Edition 2.1 and 3.0 products, and one of them, dmalloc, ships with MontaVista Linux.
When developing application programs for embedded systems, designers and programmers must take great care with using system memory resources. Unlike workstations, embedded systems have a finite memory source. Typically, no swap area is available to idle programs. When the system uses up all of its resources, nothing is left to do but panic and start over or kill some programs to make room for the needed resources. Therefore, it is important to write programs that do not leak memory. Many tools aid programmers in finding these resource leaks. All of the tools discussed here come with their own test programs.
One method of testing, which I have seen used successfully by application developers, involves using a workstation to develop prototype code and debugging as much as possible on it. Using memory leak tools in this manner is strongly advised. By debugging on a workstation, the application programmer can be assured that the transition to the target processor will be easier. A major reason for using workstations is they are cheap, and everybody involved has one. Targets, on the other hand, are usually few and in great demand.
Most memory leak detection programs are available as full source. They typically have been built on an x86-based platform. Running them on non-x86 targets requires some porting. This porting effort could be as simple as a recompile, link and run, or it could require changing some assembler code from one platform to another. Some of the tools come with hints and suggestions for use in cross-compiling environments.
The author of dmalloc, a tool I covered in detail in the September 2002 article, states that his knowledge of C++ is limited, and thus the C++ detection of memory leaks also is limited. In order to use dmalloc with C++ and threads, it has been necessary to link the application as static.
The ccmalloc tool is a memory profiler with a simple usage model that supports dynamically linked libraries but not dlopen. It detects memory leaks, multiple de-allocation of the same data, underwrites and overwrites and writes to already de-allocated data. It displays allocation and de-allocation statistics. It is applicable to optimized and stripped code and supports C++. It also provides file and line number information for the whole call chain, not only for the immediate caller of malloc/free, and it supports C++. No recompilation is needed to use ccmalloc; simply link it with -lccmalloc -ldl or ccmalloc.o -ldl. ccmalloc provides efficient representation of call chains, customizable printing of call chains, selective printing of call chains, a compressed log file and a startup file called .ccmalloc. The major documentation is found in a file named ccmalloc.cfg. The test files included with the program provide more documentation. nm and gdb are required to get information about symbols and gzip or to compress log files.
NJAMD is, as the author states, “not just another malloc debugger”. As with most memory allocation debuggers, the standard allocation functions are replaced with new ones that perform various checks as memory is used. Specifically, it looks for dynamic buffer over/underflows and detects memory reuse after it is freed. The library built for NJAMD can be LD_PRELOADed, or it can be linked to the program. It creates a large memory buffer on the first memory allocation, 20MB, and it then carves this up as the program needs memory.
NJAMD can be used alone, with a front end or from within gdb. It has a utility that allows postmortem heap analysis. Another feature allows the application being debugged to skip recompilation; simply preload the library. NJAMD also is capable of tracing leaks in library functions that wrap malloc and free, GUI widget allocators and C++ new and delete. Often a memory leak is not discovered immediately but lurks, waiting to strike at the most visible moment. Tracking this down can take a long time. NJAMD has many environment variables that allow setting varying levels of detection. As with most debugging tools, performance can be an issue with NJAMD, so the tool should be used only during development. Deploying with the tool enabled can result in slower systems.