setcontext is one of a family of C library functions (the others being getcontext, makecontext and swapcontext) used for context control. The family allows the implementation in C of advanced control flow patterns such as iterators, fibers, and coroutines. They may be viewed as an advanced version of setjmp/longjmp; whereas the latter allows only a single non-local jump up the stack, allows the creation of multiple cooperative threads of control, each with its own stack.

Specification

was specified in POSIX.1-2001 and the Single Unix Specification, version 2, but not all Unix-like operating systems provide them. POSIX.1-2004 obsoleted these functions, and in POSIX.1-2008 they were removed, with POSIX Threads indicated as a possible replacement.

Definitions

The functions and associated types are defined in the system header file. This includes the type, with which all four functions operate: points to the context which will be resumed when the current context exits, if the context was created with (a secondary context). is used to store the set of signals blocked in the context, and is the stack used by the context. stores execution state, including all registers and CPU flags, the instruction pointer, and the stack pointer; is an opaque type. The functions are:

Example

The example below demonstrates an iterator using. NOTE: this example is not correct, but may work as intended in some cases. The function requires additional parameters to be type , but the example passes pointers. Thus, the example may fail on 64-bit machines (specifically LP64-architectures, where ). This problem can be worked around by breaking up and reconstructing 64-bit values, but that introduces a performance penalty. "On architectures where int and pointer types are the same size (e.g., x86-32, where both types are 32 bits), you may be able to get away with passing pointers as arguments to makecontext following argc. However, doing this is not guaranteed to be portable, is undefined according to the standards, and won't work on architectures where pointers are larger than ints. Nevertheless, starting with version 2.8, glibc makes some changes to, to permit this on some 64-bit architectures (e.g., x86-64)." For get and set context, a smaller context can be handy: This makes an infinite loop because context holds the program counter.