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makecontext(3c)
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 unde‐
fined.
Before a call is made to makecontext(), the context being modified
should have a stack allocated for it. The stack is assigned to the con‐
text by initializing the uc_stack member.
The uc_link member is used to determine the context that will be re‐
sumed when the context being modified by makecontext() returns. The
uc_link member should be initialized prior to the call to makecon‐
text(). If the uc_link member is initialized to NULL, the thread exe‐
cuting 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 struc‐
ture 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 re‐
turned 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 allo‐
cated 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 switch‐
ing between multiple threads of control within a process (co-process‐
ing). More effective multiple threads of control can be obtained by us‐
ing native support for multithreading. See threads(7).
ATTRIBUTES
See attributes(7) for descriptions of the following attributes:
tab() box; cw(2.75i) |cw(2.75i) lw(2.75i) |lw(2.75i) ATTRIBUTE TYPEAT‐
TRIBUTE VALUE _ Interface StabilityCommitted _ MT-LevelMT-Safe _ Stan‐
dardSee standards(7).
SEE ALSO
getcontext(2), mmap(2), sigaction(2), sigprocmask(2), pthread_exit(3C),
ucontext.h(3HEAD), attributes(7), standards(7), threads(7)
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 repre‐
sents 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 re‐
leases 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 up‐
dated to reflect this behavior change. The example below demonstrates 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);
Oracle Solaris 11.4 27 Nov 2017 makecontext(3C)