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| .TH GETCONTEXT 3 2015-03-02 "Linux" "Linux Programmer's Manual" |
| .SH NAME |
| getcontext, setcontext \- get or set the user context |
| .SH SYNOPSIS |
| .B #include <ucontext.h> |
| .sp |
| .BI "int getcontext(ucontext_t *" ucp ); |
| .br |
| .BI "int setcontext(const ucontext_t *" ucp ); |
| .SH DESCRIPTION |
| In a System V-like environment, one has the two types |
| .I mcontext_t |
| and |
| .I ucontext_t |
| defined in |
| .I <ucontext.h> |
| and the four functions |
| .BR getcontext (), |
| .BR setcontext (), |
| .BR makecontext (3), |
| and |
| .BR swapcontext (3) |
| that allow user-level context switching between multiple |
| threads of control within a process. |
| .LP |
| The |
| .I mcontext_t |
| type is machine-dependent and opaque. |
| The |
| .I ucontext_t |
| type is a structure that has at least |
| the following fields: |
| .in +4 |
| .nf |
| |
| typedef struct ucontext { |
| struct ucontext *uc_link; |
| sigset_t uc_sigmask; |
| stack_t uc_stack; |
| mcontext_t uc_mcontext; |
| ... |
| } ucontext_t; |
| |
| .fi |
| .in |
| with |
| .IR sigset_t |
| and |
| .I stack_t |
| defined in |
| .IR <signal.h> . |
| Here |
| .I uc_link |
| points to the context that will be resumed |
| when the current context terminates (in case the current context |
| was created using |
| .BR makecontext (3)), |
| .I uc_sigmask |
| is the |
| set of signals blocked in this context (see |
| .BR sigprocmask (2)), |
| .I uc_stack |
| is the stack used by this context (see |
| .BR sigaltstack (2)), |
| and |
| .I uc_mcontext |
| is the |
| machine-specific representation of the saved context, |
| that includes the calling thread's machine registers. |
| .LP |
| The function |
| .BR getcontext () |
| initializes the structure |
| pointed at by |
| .I ucp |
| to the currently active context. |
| .LP |
| The function |
| .BR setcontext () |
| restores the user context |
| pointed at by |
| .IR ucp . |
| A successful call does not return. |
| The context should have been obtained by a call of |
| .BR getcontext (), |
| or |
| .BR makecontext (3), |
| or passed as third argument to a signal |
| handler. |
| .LP |
| If the context was obtained by a call of |
| .BR getcontext (), |
| program execution continues as if this call just returned. |
| .LP |
| If the context was obtained by a call of |
| .BR makecontext (3), |
| program execution continues by a call to the function |
| .I func |
| specified as the second argument of that call to |
| .BR makecontext (3). |
| When the function |
| .I func |
| returns, we continue with the |
| .I uc_link |
| member of the structure |
| .I ucp |
| specified as the |
| first argument of that call to |
| .BR makecontext (3). |
| When this member is NULL, the thread exits. |
| .LP |
| 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". |
| .SH RETURN VALUE |
| When successful, |
| .BR getcontext () |
| returns 0 and |
| .BR setcontext () |
| does not return. |
| On error, both return \-1 and set |
| .I errno |
| appropriately. |
| .SH ERRORS |
| None defined. |
| .SH ATTRIBUTES |
| For an explanation of the terms used in this section, see |
| .BR attributes (7). |
| .TS |
| allbox; |
| lbw26 lb lb |
| l l l. |
| Interface Attribute Value |
| T{ |
| .BR getcontext (), |
| .BR setcontext () |
| T} Thread safety MT-Safe race:ucp |
| .TE |
| .SH CONFORMING TO |
| SUSv2, POSIX.1-2001. |
| POSIX.1-2008 removes the specification of |
| .BR getcontext (), |
| citing portability issues, and |
| recommending that applications be rewritten to use POSIX threads instead. |
| .SH NOTES |
| The earliest incarnation of this mechanism was the |
| .BR setjmp (3)/ longjmp (3) |
| mechanism. |
| Since that does not define |
| the handling of the signal context, the next stage was the |
| .BR 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 |
| .BR getcontext () |
| is from the first call, or via a |
| .BR setcontext () |
| call. |
| The user has to invent her own bookkeeping device, and a register |
| variable won't do since registers are restored. |
| .LP |
| 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 using |
| .BR longjmp (3): |
| it is undefined what would happen with contexts. |
| Use |
| .BR siglongjmp (3) |
| or |
| .BR setcontext () |
| instead. |
| .SH SEE ALSO |
| .BR sigaction (2), |
| .BR sigaltstack (2), |
| .BR sigprocmask (2), |
| .BR longjmp (3), |
| .BR makecontext (3), |
| .BR sigsetjmp (3) |