Callclobbered

Callclobbered is a term used in software development to describe a situation where a function call can overwrite or modify data in the caller’s context. This includes registers, stack slots, or memory that the caller expects to remain unchanged. The term is used informally in discussions of calling conventions, inline assembly, and foreign function interfaces to highlight potentially unintended side effects of a call.

Causes of callclobbered states typically relate to violations or misunderstandings of the calling convention. If the callee uses or overwrites registers that the caller assumes to be preserved, or if callee-saved registers are not properly saved and restored, the caller’s state can be corrupted. Inline assembly or foreign code often requires explicit clobber declarations; omitting or incorrect reporting of clobbers can produce callclobbered behavior. Asynchronous events such as signals, longjmp/setjmp, or thread context switches can also clobber the caller’s context. Compiler optimizations that assume values persist across calls without side effects can contribute if the code relies on those assumptions.

Mitigation focuses on adhering to conventions and clearly communicating side effects. Follow the platform’s Application Binary Interface (ABI) for which registers are caller- or callee-saved, and ensure callee-saved registers are preserved. In inline assembly, declare all clobbers and memory dependencies. Avoid relying on non-volatile state across calls, and use volatile semantics or memory barriers when appropriate. Be cautious with asynchronous events and long-running callbacks, as they can introduce callclobbered conditions even in otherwise well-behaved code.