NAME
getcontext, setcontext - get or set the user context SYNOPSIS
#include
int getcontext(ucontextt *ucp); int setcontext(const ucontextt *ucp); DESCRIPTION In a System V-like environment, one has the two types mcontextt and ucontextt defined in
and the four functions getcontext(), setcontext(), makecontext(3) and swapcontext(3) that allow user-level context switching between multiple threads of control within a process.
The mcontextt type is machine-dependent and opaque. The ucontextt type is a structure that has at least the following fields: typedef struct ucontext { struct ucontext *uclink; sigsett ucsigmask; stackt ucstack; mcontextt ucmcontext; ... } ucontextt; with sigsett and stackt defined in
. Here uclink points to the context that will be resumed when the current context terminates (in case the current context was created using makecontext(3)), ucsig‐ mask is the set of signals blocked in this context (see sigproc‐ mask(2)), ucstack is the stack used by this context (see sigalt‐ stack(2)), and ucmcontext is the machine-specific representation of the saved context, that includes the calling thread's machine regis‐ ters. The function getcontext() initializes the structure pointed at by ucp to the currently active context. The function setcontext() restores the user context pointed at by ucp. A successful call does not return. The context should have been obtained by a call of getcontext(), or makecontext(3), or passed as third argument to a signal handler. If the context was obtained by a call of getcontext(), program execu‐ tion continues as if this call just returned. If the context was obtained by a call of makecontext(3), program execu‐ tion continues by a call to the function func specified as the second argument of that call to makecontext(3). When the function func returns, we continue with the uclink member of the structure ucp spec‐ ified as the first argument of that call to makecontext(3). When this member is NULL, the thread exits. If the context was obtained by a call to a signal handler, then old standard text says that "program execution continues with the program instruction following the instruction interrupted by the signal". How‐ ever, this sentence was removed in SUSv2, and the present verdict is "the result is unspecified". RETURN VALUE When successful, getcontext() returns 0 and setcontext() does not
return. On error, both return -1 and set errno appropriately. ERRORS None defined. ATTRIBUTES For an explanation of the terms used in this section, see attributes(7). ┌───────────────────────────┬───────────────┬──────────────────┐ │Interface │ Attribute │ Value │ ├───────────────────────────┼───────────────┼──────────────────┤
│getcontext(), setcontext() │ Thread safety │ MT-Safe race:ucp │ └───────────────────────────┴───────────────┴──────────────────┘ CONFORMING TO
SUSv2, POSIX.1-2001. POSIX.1-2008 removes the specification of getcon‐ text(), citing portability issues, and recommending that applications be rewritten to use POSIX threads instead. NOTES The earliest incarnation of this mechanism was the setjmp(3)/longjmp(3) mechanism. Since that does not define the handling of the signal con‐ text, the next stage was the sigsetjmp(3)/siglongjmp(3) pair. The present mechanism gives much more control. On the other hand, there is no easy way to detect whether a return from getcontext() is from the first call, or via a setcontext() call. The user has to invent her own bookkeeping device, and a register variable won't do since registers are restored. When a signal occurs, the current user context is saved and a new con‐ text is created by the kernel for the signal handler. Do not leave the handler using longjmp(3): it is undefined what would happen with con‐ texts. Use siglongjmp(3) or setcontext() instead. SEE ALSO sigaction(2), sigaltstack(2), sigprocmask(2), longjmp(3), makecon‐ text(3), sigsetjmp(3) COLOPHON
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Linux 2009-03-15 GETCONTEXT(3)