getcontext(2)getcontext(2)NAME
getcontext, setcontext - Initiates and restores user level context
switching
SYNOPSIS
#include <ucontext.h>
int getcontext(
ucontext_t *ucp ); int setcontext(
const ucontext_t *ucp );
STANDARDS
Interfaces documented on this reference page conform to industry stan‐
dards as follows:
getcontext(), setcontext(): XSH5.0
Refer to the standards(5) reference page for more information about
industry standards and associated tags.
PARAMETERS
Provides a pointer to a ucontext structure, defined in the <ucontext.h>
header file. The ucontext structure contains the signal mask, execu‐
tion stack, and machine registers. (See ucontext(5) for more informa‐
tion about the format of the ucontext structure.)
DESCRIPTION
Using both the getcontext() and setcontext() functions enables you to
initiate user level context control, switching between multiple threads
of control within a single process.
When you call getcontext(), it initializes the ucp argument to the cur‐
rent user context of the calling process.
Use the setcontext() function to restore the state of the user context
pointed to by the ucp argument. The setcontext() function, if success‐
ful, does not return; application execution continues from the point
specified by the ucontext structure you pass to the setcontext() func‐
tion.
The ucontext structure that you pass to the setcontext() function must
have been created by a call to the getcontext() function or the make‐
context() function, or have been passed as the third argument to a sig‐
nal handler. (The third argument in a call to the sigaction() function
determines the action to be performed when a signal is delivered. For
more information, see sigaction(2).)
When a context structure is created by the getcontext() function, exe‐
cution of the program continues as if the corresponding call of the
getcontext() function had just returned.
When a context structure is created by the makecontext() function, pro‐
gram execution continues with the function passed to makecontext().
When that function returns, the thread continues as if after a call to
setcontext() with the context structure argument that was input to
makecontext().
If the uc_link member of the ucontext_t structure pointed to by the ucp
argument is 0 (zero), then this context is the main context, and the
thread will exit when this context returns. The effects of passing a
ucp argument from any other source are unspecified.
NOTES
When a signal handler executes, the current user context is saved and a
new context is created by the kernel. If the process leaves the signal
handler using the longjmp() function, the original context cannot be
restored, and the result of future calls to the getcontext() function
are unpredictable. Use the siglongjmp() or setcontext() functions in
signal handlers, instead of the longjmp() function.
RETURN VALUES
The setcontext() function does not return upon success. The getcon‐
text() function returns 0 (zero) upon success. Upon failure, both the
setcontext() and getcontext() functions return a value of -1.
SEE ALSO
Functions: bsd_signal(2), makecontext(2), sigaction(2), sigalt‐
stack(2), sigprocmask(2), setjmp(3), sigsetjmp(3)
Files: ucontext(5)
Standards: standards(5)getcontext(2)
