The Open Group Base Specifications Issue 6
IEEE Std 1003.1, 2004 Edition
Copyright © 2001-2004 The IEEE and The Open Group, All Rights reserved.
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makecontext, swapcontext - manipulate user contexts


[OB XSI] [Option Start] #include <ucontext.h>

void makecontext(ucontext_t *
ucp, void (*func)(),
argc, ...);
int swapcontext(ucontext_t *restrict
       const ucontext_t *restrict
ucp); [Option End]


The makecontext() function shall modify the context specified by ucp, which has been initialized using getcontext(). When this context is resumed using swapcontext() or setcontext(), program execution shall continue by calling func, passing it the arguments that follow argc in the makecontext() call.

Before a call is made to makecontext(), the application shall ensure that the context being modified has a stack allocated for it. The application shall ensure that the value of argc matches the number of arguments of type int passed to func; otherwise, the behavior is undefined.

The uc_link member is used to determine the context that shall be resumed when the context being modified by makecontext() returns. The application shall ensure that the uc_link member is initialized prior to the call to makecontext().

The swapcontext() function shall save the current context in the context structure pointed to by oucp and shall set the context to the context structure pointed to by ucp.


Upon successful completion, swapcontext() shall return 0. Otherwise, -1 shall be returned and errno set to indicate the error.


The swapcontext() function shall fail if:

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

The following sections are informative.


The following example illustrates the use of makecontext():

#include <stdio.h>
#include <ucontext.h>

static ucontext_t ctx[3];
static void f1 (void) { puts("start f1"); swapcontext(&ctx[1], &ctx[2]); puts("finish f1"); }
static void f2 (void) { puts("start f2"); swapcontext(&ctx[2], &ctx[1]); puts("finish f2"); }
int main (void) { char st1[8192]; char st2[8192];
getcontext(&ctx[1]); ctx[1].uc_stack.ss_sp = st1; ctx[1].uc_stack.ss_size = sizeof st1; ctx[1].uc_link = &ctx[0]; makecontext(&ctx[1], f1, 0);
getcontext(&ctx[2]); ctx[2].uc_stack.ss_sp = st2; ctx[2].uc_stack.ss_size = sizeof st2; ctx[2].uc_link = &ctx[1]; makecontext(&ctx[2], f2, 0);
swapcontext(&ctx[0], &ctx[2]); return 0; }


The obsolescent functions getcontext(), makecontext(), and swapcontext() can be replaced using POSIX threads functions.


With the incorporation of the ISO/IEC 9899:1999 standard into this specification it was found that the ISO C standard (Subclause 6.11.6) specifies that the use of function declarators with empty parentheses is an obsolescent feature. Therefore, using the function prototype:

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

is making use of an obsolescent feature of the ISO C standard. Therefore, a strictly conforming POSIX application cannot use this form. Therefore, use of getcontext(), makecontext(), and swapcontext() is marked obsolescent.

There is no way in the ISO C standard to specify a non-obsolescent function prototype indicating that a function will be called with an arbitrary number (including zero) of arguments of arbitrary types (including integers, pointers to data, pointers to functions, and composite types).

Replacing makecontext() with a number of ISO C standard-compatible functions handing various numbers and types of arguments would have forced all existing uses of makecontext() to be rewritten for little or no gain. There are very few applications today that use the *context() routines. Those that do use them are almost always using them to implement co-routines. By maintaining the XSH, Issue 5 specification for makecontext(), existing applications will continue to work, although they won't be able to be classified as strictly conforming applications.

There is no way in the ISO C standard (without using obsolescent behavior) to specify functionality that was standard, strictly conforming behavior in the XSH, Issue 5 specification using the ISO C standard. Threads can be used to implement the functionality provided by makecontext(), getcontext(), and swapcontext() but they are more complex to use. It was felt inventing new ISO C standard-compatible interfaces that describe what can be done with the XSH, Issue 5 functions and then converting applications to use them would cause more difficulty than just converting applications that use them to use threads instead.




exit(), getcontext(), sigaction(), sigprocmask(), the Base Definitions volume of IEEE Std 1003.1-2001, <ucontext.h>


First released in Issue 4, Version 2.

Issue 5

Moved from X/OPEN UNIX extension to BASE.

In the ERRORS section, the description of [ENOMEM] is changed to apply to swapcontext() only.

Issue 6

The DESCRIPTION is updated to avoid use of the term "must" for application requirements.

The restrict keyword is added to the swapcontext() prototype for alignment with the ISO/IEC 9899:1999 standard.

IEEE Std 1003.1-2001/Cor 1-2002, item XSH/TC1/D6/33 is applied, clarifying that the arguments passed to func are of type int.

IEEE Std 1003.1-2001/Cor 2-2004, item XSH/TC2/D6/57 is applied, obsoleting the functions and giving advice on the alternatives. Changes are made to the SYNOPSIS, APPLICATION USAGE, and RATIONALE sections.

End of informative text.

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