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NAME
| | makecontext, swapcontext - manipulate user contexts |
SYNOPSIS
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cc -D__MAKECONTEXT_V2_SOURCE [ flag... ] file... [ library... ]
#include <ucontext.h> void makecontext(ucontext_t *ucp, void(*func)(), int argc, ...); |
| | int swapcontext(ucontext_t *oucp, const ucontext_t *ucp); |
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These functions are useful for implementing user-level context switching between multiple threads of control within a process.
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)), program 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 value of argc must match the number of integer arguments passed to func, otherwise the behavior is undefined.
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(3THR).
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 illegal address, the behavior is undefined and errno may be set to EFAULT.
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Upon successful completion, swapcontext() returns 0. Otherwise, -1 is returned and errno is set to indicate the error.
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The swapcontext() function will fail if:
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ENOMEM
- The ucp argument does not have enough stack left to complete the operation.
The swapcontext() function may fail if:
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EFAULT
- The ucp or oucp argument points to an invalid address.
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| | Example 1. Alternate execution context on a stack whose memory was allocated using mmap(2).
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#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);
}
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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(3THR).
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See attributes(5) for descriptions of the following attributes:
| ATTRIBUTE TYPE | ATTRIBUTE VALUE |
| Interface Stability | Standard |
| MT-Level | MT-Safe |
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The legacy implementation of makecontext() for sparc and sparcv9 was in violation of the standard. To use the updated version with the corrected behavior, specify -D__MAKECONTEXT_V2_SOURCE when invoking the compiler. See the EXAMPLES section for the correct usage.
Future releases of Solaris will enable the corrected behavior by default, thereby eleminating the need to define __MAKECONTEXT_V2_SOURCE.
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Company Info
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Contact
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Copyright 2003 Sun Microsystems, Inc. All rights reserved.
Use is subject to license terms.
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