MAKECONTEXT(3C) Standard C Library Functions MAKECONTEXT(3C)


NAME


makecontext, swapcontext - manipulate user contexts

SYNOPSIS


#include <ucontext.h>

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


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


DESCRIPTION


The makecontext() function modifies the context specified by ucp, which
has been initialized using getcontext(2). When this context is resumed
using swapcontext() or setcontext(2), execution continues by calling the
function func, passing it the arguments that follow argc in the
makecontext() call. The value of argc must match the number of pointer-
sized integer arguments passed to func, otherwise the behavior is
undefined.


Before a call is made to makecontext(), the context being modified should
have a stack allocated for it. The stack is assigned to the context by
initializing the uc_stack member.


The uc_link member is used to determine the context that will be resumed
when the context being modified by makecontext() returns. The uc_link
member should be initialized prior to the call to makecontext(). If the
uc_link member is initialized to NULL, the thread executing func will
exit when func returns. See pthread_exit(3C).


The swapcontext() function saves the current context in the context
structure pointed to by oucp and sets the context to the context
structure pointed to by ucp.


If the ucp or oucp argument points to an invalid address, the behavior is
undefined and errno may be set to EFAULT.

RETURN VALUES


On successful completion, swapcontext() returns 0. Otherwise, -1 is
returned and errno is set to indicate the error.

ERRORS


The swapcontext() function will fail if:

ENOMEM
The ucp argument does not have enough stack left to complete
the operation.


The swapcontext() function may fail if:

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


EXAMPLES


Example 1: Alternate execution context on a stack whose memory was


allocated using mmap().

#include <stdio.h>
#include <ucontext.h>
#include <sys/mman.h>

void
assign(long a, int *b)
{
*b = (int)a;
}

int
main(int argc, char **argv)
{
ucontext_t uc, back;
size_t sz = 0x10000;
int value = 0;

getcontext(&uc);

uc.uc_stack.ss_sp = mmap(0, sz,
PROT_READ | PROT_WRITE | PROT_EXEC,
MAP_PRIVATE | MAP_ANON, -1, 0);
uc.uc_stack.ss_size = sz;
uc.uc_stack.ss_flags = 0;

uc.uc_link = &back;

makecontext(&uc, assign, 2, 100L, &value);
swapcontext(&back, &uc);

printf("done %d\n", value);

return (0);
}


USAGE


These functions are useful for implementing user-level context switching
between multiple threads of control within a process (co-processing).
More effective multiple threads of control can be obtained by using
native support for multithreading. See threads(5).

ATTRIBUTES


See attributes(5) for descriptions of the following attributes:


+--------------------+-----------------+
| ATTRIBUTE TYPE | ATTRIBUTE VALUE |
+--------------------+-----------------+
|Interface Stability | Standard |
+--------------------+-----------------+
|MT-Level | MT-Safe |
+--------------------+-----------------+

SEE ALSO


mmap(2), getcontext(2), sigaction(2), sigprocmask(2), pthread_exit(3C),
ucontext.h(3HEAD), attributes(5), standards(5), threads(5)

NOTES


The semantics of the uc_stack member of the ucontext_t structure have
changed as they apply to inputs to makecontext(). Prior to Solaris 10,
the ss_sp member of the uc_stack structure represented the high memory
address of the area reserved for the stack. The ss_sp member now
represents the base (low memory address), in keeping with other uses of
ss_sp.


This change in the meaning of ss_sp is now the default behavior. The
-D__MAKECONTEXT_V2_SOURCE compilation flag used in Solaris 9 update
releases to access this behavior is obsolete.


Binary compatibility has been preserved with releases prior to Solaris
10. Before recompiling, applications that use makecontext() must be
updated to reflect this behavior change. The example below demonstates a
typical change that must be applied:

--- example1_s9.c Thu Oct 3 11:58:17 2002
+++ example1.c Thu Jun 27 13:28:16 2002
@@ -27,12 +27,9 @@
uc.uc_stack.ss_sp = mmap(0, sz,
PROT_READ | PROT_WRITE | PROT_EXEC,
MAP_PRIVATE | MAP_ANON, -1, 0);
- uc.uc_stack.ss_sp = (char *)uc.uc_stack.ss_sp + sz - 8;
uc.uc_stack.ss_size = sz;
uc.uc_stack.ss_flags = 0;

uc.uc_link = &back

makecontext(&uc, assign, 2, 100L, &value);


March 8, 2004 MAKECONTEXT(3C)