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NAME |LIBRARY |SYNOPSIS |DESCRIPTION |RETURN VALUE |ERRORS |ATTRIBUTES |STANDARDS |HISTORY |NOTES |SEE ALSO |COLOPHON | |
getcontext(3) Library Functions Manualgetcontext(3)getcontext, setcontext - get or set the user context
Standard C library (libc,-lc)
#include <ucontext.h>int getcontext(ucontext_t *ucp);int setcontext(const ucontext_t *ucp);
In a System V-like environment, one has the two typesmcontext_t anducontext_t defined in<ucontext.h> and the four functionsgetcontext(),setcontext(),makecontext(3), andswapcontext(3) that allow user-level context switching between multiple threads of control within a process. Themcontext_t type is machine-dependent and opaque. Theucontext_t type is a structure that has at least the following fields: typedef struct ucontext_t { struct ucontext_t *uc_link; sigset_t uc_sigmask; stack_t uc_stack; mcontext_t uc_mcontext; ... } ucontext_t; withsigset_t andstack_t defined in<signal.h>. Hereuc_link points to the context that will be resumed when the current context terminates (in case the current context was created usingmakecontext(3)),uc_sigmask is the set of signals blocked in this context (seesigprocmask(2)),uc_stack is the stack used by this context (seesigaltstack(2)), anduc_mcontext is the machine- specific representation of the saved context, that includes the calling thread's machine registers. The functiongetcontext() initializes the structure pointed to byucp to the currently active context. The functionsetcontext() restores the user context pointed to byucp. A successful call does not return. The context should have been obtained by a call ofgetcontext(), ormakecontext(3), or received as the third argument to a signal handler (see the discussion of theSA_SIGINFOflag insigaction(2)). If the context was obtained by a call ofgetcontext(), program execution continues as if this call just returned. If the context was obtained by a call ofmakecontext(3), program execution continues by a call to the functionfunc specified as the second argument of that call tomakecontext(3). When the functionfunc returns, we continue with theuc_link member of the structureucp specified as the first argument of that call tomakecontext(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". However, this sentence was removed in SUSv2, and the present verdict is "the result is unspecified".When successful,getcontext() returns 0 andsetcontext() does not return. On error, both return -1 and seterrno to indicate the error.
None defined.
For an explanation of the terms used in this section, seeattributes(7). ┌─────────────────────────────┬───────────────┬──────────────────┐ │Interface│Attribute│Value│ ├─────────────────────────────┼───────────────┼──────────────────┤ │getcontext(),setcontext() │ Thread safety │ MT-Safe race:ucp │ └─────────────────────────────┴───────────────┴──────────────────┘
None.
SUSv2, POSIX.1-2001. POSIX.1-2008 removes these functions, citing portability issues, and recommending that applications be rewritten to use POSIX threads instead.
The earliest incarnation of this mechanism was thesetjmp(3)/longjmp(3) mechanism. Since that does not define the handling of the signal context, the next stage was thesigsetjmp(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 fromgetcontext() is from the first call, or via asetcontext() call. The user has to invent their 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 context is created by the kernel for the signal handler. Do not leave the handler usinglongjmp(3): it is undefined what would happen with contexts. Usesiglongjmp(3) orsetcontext() instead.
sigaction(2),sigaltstack(2),sigprocmask(2),longjmp(3),makecontext(3),sigsetjmp(3),signal(7)
This page is part of theman-pages (Linux kernel and C library user-space interface documentation) project. Information about the project can be found at ⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report for this manual page, see ⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩. This page was obtained from the tarball man-pages-6.15.tar.gz fetched from ⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on 2025-08-11. If you discover any rendering problems in this HTML version of the page, or you believe there is a better or more up- to-date source for the page, or you have corrections or improvements to the information in this COLOPHON (which isnot part of the original manual page), send a mail to man-pages@man7.orgLinux man-pages 6.15 2025-05-17getcontext(3)Pages that refer to this page:PR_SET_SYSCALL_USER_DISPATCH(2const), sigaction(2), sigreturn(2), makecontext(3)
HTML rendering created 2025-09-06 byMichael Kerrisk, author ofThe Linux Programming Interface. For details of in-depthLinux/UNIX system programming training courses that I teach, lookhere. Hosting byjambit GmbH. | |