Memory safety is the state of being protected from various software bugs and security vulnerabilities when dealing with memory access, such as buffer overflows and dangling pointers. For example, Java is said to be memory-safe because its runtime error detection checks array bounds and pointer dereferences. In contrast, C and C++ allow arbitrary pointer arithmetic with pointers implemented as direct memory addresses with no provision for bounds checking, and thus are potentially memory-unsafe.
Memory errors were first considered in the context of resource management and time-sharing systems, in an effort to avoid problems such as fork bombs. Developments were mostly theoretical until the Morris worm, which exploited a buffer overflow in fingerd. The field of computer security developed quickly thereafter, escalating with multitudes of new attacks such as the return-to-libc attack and defense techniques such as the non-executable stack and address space layout randomization. Randomization prevents most buffer overflow attacks and requires the attacker to use heap spraying or other application-dependent methods to obtain addresses, although its adoption has been slow. However, deployments of the technology are typically limited to randomizing libraries and the location of the stack.
DieHard, its redesign DieHarder, and the Allinea Distributed Debugging Tool are special heap allocators that allocate objects in their own random virtual memory page, allowing invalid reads and writes to be stopped and debugged at the exact instruction that causes them. Protection relies upon hardware memory protection and thus overhead is typically not substantial, although it can grow significantly if the program makes heavy use of allocation. Randomization provides only probabilistic protection against memory errors, but can often be easily implemented in existing software by relinking the binary.
The memcheck tool of Valgrind uses an instruction set simulator and runs the compiled program in a memory-checking virtual machine, providing guaranteed detection of a subset of runtime memory errors. However, it typically slows the program down by a factor of 40, and furthermore must be explicitly informed of custom memory allocators.
With access to the source code, libraries exist that collect and track legitimate values for pointers ("metadata") and check each pointer access against the metadata for validity, such as the Boehm garbage collector. In general, memory safety can be safely assured using tracing garbage collection and the insertion of runtime checks on every memory access; this approach has overhead, but less than that of Valgrind. All garbage-collected languages take this approach. For C and C++, many tools exist that perform a compile-time transformation of the code to do memory safety checks at runtime, such as CheckPointer and AddressSanitizer which imposes an average slowdown factor of 2.
Another approach uses static program analysis and automated theorem proving to ensure that the program is free of memory errors. For example, the Rust programming language implements a borrow checker to ensure memory safety. Tools such as Coverity offer static memory analysis for C. C++'s smart pointers are a limited form of this approach.
Many different types of memory errors can occur: