MAKECONTEXT(3) Library Functions Manual MAKECONTEXT(3)

makecontext, swapcontext — manipulate user contexts

Standard C Library (libc, -lc)

#include <ucontext.h>

void
makecontext(ucontext_t *ucp, void (*func)(), int argc, ...);

int
swapcontext(ucontext_t * restrict oucp, ucontext_t * restrict ucp);

The () 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 integer 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 (), 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); otherwise, 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 () 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 (successfully).

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:

[]
The oucp argument or the ucp argument points to an invalid address.
[]
The contents of the datum pointed to by ucp are invalid.

_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 (“POSIX.1”).

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 specification.

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 standard.

The makecontext() and swapcontext() functions first appeared in AT&T System V Release 4 UNIX.

May 4, 2012 NetBSD 11.0