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MAKECONTEXT(3) NetBSD Library Functions Manual MAKECONTEXT(3)
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makecontext, swapcontext -- manipulate user contexts
Standard C Library (libc, -lc)
makecontext(ucontext_t *ucp, void (*func)(), int argc, ...);
swapcontext(ucontext_t * restrict oucp, ucontext_t * restrict ucp);
The makecontext() function modifies the object pointed to by ucp, which
has been initialized using getcontext(2). When this context is resumed
using swapcontext() or setcontext(2), program execution continues as if
func had been called with the arguments specified after argc in the call
of makecontext(). The value of argc must be equal to the number of inte-
ger arguments following it, and must be equal to the number of integer
arguments expected by func; otherwise, the behavior is undefined.
Before being modified using makecontext(), a stack must be allocated for
the context (in the uc_stack member), and a context to resume after func
has returned must be determined (pointed to by the uc_link member); oth-
erwise, the behavior is undefined. If uc_link is a null pointer, then
the context is the main context, and the process will exit with an exit
status of 0 upon return.
The swapcontext() function saves the current context in the object
pointed to by oucp, sets the current context to that specified in the
object pointed to by ucp, and resumes execution. When a context saved by
swapcontext() is restored using setcontext(2), execution will resume as
if the corresponding invocation of swapcontext() had just returned (suc-
The makecontext() function returns no value.
On success, swapcontext() returns a value of 0, Otherwise, -1 is returned
and errno is set to indicate the error.
The swapcontext() function will fail if:
[EFAULT] The oucp argument or the ucp argument points to an
[EINVAL] The contents of the datum pointed to by ucp are
_exit(2), getcontext(2), setcontext(2), ucontext(2)
The makecontext() and swapcontext() functions conform to X/Open System
Interfaces and Headers Issue 5 (``XSH5'') and IEEE Std 1003.1-2001
The IEEE Std 1003.1-2004 (``POSIX.1'') revision marked the functions
makecontext() and swapcontext() as obsolete, citing portability issues
and recommending the use of POSIX threads instead. The IEEE Std
1003.1-2008 (``POSIX.1'') revision removed the functions from the speci-
The standard does not clearly define the type of integer arguments passed
to func via makecontext(); portable applications should not rely on the
implementation detail that it may be possible to pass pointer arguments
to functions. This may be clarified in a future revision of the stan-
The makecontext() and swapcontext() functions first appeared in AT&T
System V.4 UNIX.
NetBSD 10.99 May 4, 2012 NetBSD 10.99