setjmplongjmp

Setjmplongjmp is a reference to the pair of C standard library facilities setjmp and longjmp, used to perform non-local jumps in C programs. They provide a mechanism to save the current execution state and later restore it, allowing control to return to an outer function call from a deeply nested level or from an error path.

The basic usage is that setjmp saves the program’s execution context (stack pointer, program counter, and other registers) in a jmp_buf. When longjmp is later invoked with that same jmp_buf, execution resumes at the point where setjmp was called. The return value of setjmp when invoked directly is 0; when longjmp transfers control, setjmp returns the value supplied to longjmp, unless that value is 0, in which case it is treated as 1. The exact saved state and the contents of jmp_buf are implementation-defined and can vary between compilers and platforms.

What state is preserved and what is not is also important. The environment saved by setjmp/longjmp does not perform a normal stack unwinding, so automatic objects with non-trivial destructors (in languages like C++) are not properly destroyed. In C, this means resources may need explicit cleanup before performing a longjmp. For portability and correctness, only a subset of state is guaranteed to be preserved, and values of automatic variables modified between setjmp and longjmp are indeterminate unless they are declared volatile.

POSIX provides variants such as sigsetjmp and siglongjmp that also save and restore the process’s signal mask. These functions, along with the standard setjmp/longjmp, are primarily used for implementing error handling and lightweight coroutine-like behavior in C, but they are typically avoided in modern code in favor of structured error handling or other abstractions due to their complexity and potential for resource leaks.