MAKECONTEXT(3)

NAME
makecontext, swapcontext - manipulate user contexts

LIBRARY
libc

SYNOPSIS
#include <ucontext.h>
void
makecontext(ucontext_t *ucp, void (*func)(), int argc, ...);
int
swapcontext(ucontext_t * restrict oucp, ucontext_t * restrict ucp);

DESCRIPTION
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 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 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); 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 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 (successfully).

RETURN VALUES
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.

ERRORS
The swapcontext() function will fail if:

[EFAULT]
    The oucp argument or the ucp argument points to an invalid address.

[EINVAL]
    The contents of the datum pointed to by ucp are invalid.

SEE ALSO
_exit(2), getcontext(2), setcontext(2), ucontext(2)

STANDARDS
The makecontext() and swapcontext() functions conform to xsh5 and p1003.1-2001.

The p1003.1-2004 revision marked the functions makecontext() and swapcontext() as obsolete, citing portability issues and recommending the use of POSIX threads instead. The p1003.1-2008 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.

HISTORY
The makecontext() and swapcontext() functions first appeared in AT&T UNIX.

CAVEATS
Due to limitations in the current pthread implementation, makecontext should not be used in programs which link against the pthread(3) library (whether threads are used or not).