Tombstones are a mechanism to detect

dangling pointer Dangling pointers and wild pointers in computer programming are pointers that do not point to a valid object of the appropriate type. These are special cases of memory safety violations. More generally, dangling references and wild references a ...

s and mitigate the problems they can cause in computer programs. Dangling pointers can appear in certain computer

programming language A programming language is a system of notation for writing computer programs. Programming languages are described in terms of their Syntax (programming languages), syntax (form) and semantics (computer science), semantics (meaning), usually def ...

s, e.g. C, C++ and

assembly language In computing, assembly language (alternatively assembler language or symbolic machine code), often referred to simply as assembly and commonly abbreviated as ASM or asm, is any low-level programming language with a very strong correspondence bet ...

s. A tombstone is a structure that acts as an intermediary between a pointer and its target, often heap-dynamic data in

memory Memory is the faculty of the mind by which data or information is encoded, stored, and retrieved when needed. It is the retention of information over time for the purpose of influencing future action. If past events could not be remembe ...

. The pointer – sometimes called the

handle A handle is a part of, or an attachment to, an object that allows it to be grasped and object manipulation, manipulated by hand. The design of each type of handle involves substantial ergonomics, ergonomic issues, even where these are dealt wi ...

– points only at tombstones and never to its actual target. When the data is deallocated, the tombstone is set to a

null Null may refer to: Science, technology, and mathematics Astronomy *Nuller, an optical tool using interferometry to block certain sources of light Computing *Null (SQL) (or NULL), a special marker and keyword in SQL indicating that a data value do ...

(or, more generally, to a value that is illegal for a pointer in the given runtime environment), indicating that the variable no longer exists. This mechanism prevents the use of invalid pointers, which would otherwise access the memory area that once belonged to the now deallocated variable, although it may already contain other data, in turn leading to corruption of in-memory data. Depending on the

operating system An operating system (OS) is system software that manages computer hardware and software resources, and provides common daemon (computing), services for computer programs. Time-sharing operating systems scheduler (computing), schedule tasks for ...

, the CPU can automatically detect such an invalid access (e.g. for the null value: a ''null pointer dereference error''). This supports in analyzing the actual reason, a programming error, in

debugging In engineering, debugging is the process of finding the Root cause analysis, root cause, workarounds, and possible fixes for bug (engineering), bugs. For software, debugging tactics can involve interactive debugging, control flow analysis, Logf ...

, and it can also be used to abort the program in production use, to prevent it from continuing with invalid data structures. In more generalized terms, a tombstone can be understood as a marker for "this data is no longer here". For example, in filesystems it may be efficient when deleting files to mark them as "dead" instead of immediately reclaiming all their data blocks. The downsides of using tombstones include a

computational overhead Overhead in computer systems consists of shared functions that benefit all users or processes but are not directly attributable to any specific task. It is thus similar to overhead in organizations. Computer system overhead shows up as slower pr ...

and additional memory consumption: extra processing is necessary to follow the path from the pointer to data through the tombstone, and extra memory is necessary to retain tombstones for every pointer throughout the program. One other problem is that all the code that needs to work with the pointers in question needs to be implemented to use the tombstone mechanism. Among popular programming languages, C++ implements the tombstone pattern in its standard library as a weak pointer using std::weak_ptr. Built–in support by programming languages or the compiler is not necessary to use this mechanism.

References

Programming constructs {{Compu-lang-stub

See also

References