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In computer programming, the word trampoline has a number of meanings, and is generally associated with jumps (i.e., moving to different code paths).
Trampolines (sometimes referred to as indirect jump vectors) are memory locations holding addresses pointing to interrupt service routines, I/O routines, etc. Execution jumps into the trampoline and then immediately jumps out, or bounces, hence the term trampoline. They have many uses:
- Trampoline can be used to overcome the limitations imposed by a CPU architecture that expects to always find vectors in fixed locations.
- When an operating system is booted on a symmetric multiprocessing (SMP) machine, only one processor, the boot-strap processor, will be active. After the operating system has configured itself, it will instruct the other processors to jump to a piece of trampoline code that will initialize the processors and wait for the operating system to start scheduling threads on them.
- As used in some Lisp implementations, a trampoline is a loop that iteratively invokes thunk-returning functions (continuation-passing style). A single trampoline suffices to express all control transfers of a program; a program so expressed is trampolined, or in trampolined style; converting a program to trampolined style is trampolining. Programmers can use trampolined functions to implement tail-recursive function calls in stack-oriented programming languages.
- In Java, trampoline refers to using reflection to avoid using inner classes, for example in event listeners. The time overhead of a reflection call is traded for the space overhead of an inner class. Trampolines in Java usually involve the creation of a GenericListener to pass events to an outer class.
No Execute Stacks
Some implementations of trampolines cause a loss of No Execute Stacks (NX Stack). In particular, GCC's nested function builds a trampoline on the stack at runtime, and then calls the nested function through the data on stack. The trampoline requires the stack to be executable.
No execute stacks and nested functions are mutually exclusive under GCC. If a nested function is used in the development of a program, then the NX Stack is silently lost. GCC offers the -Wtrampoline warning to alert of the condition.
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