MAKECONTEXT(3) Linux Programmer's Manual MAKECONTEXT(3) NAME
makecontext, swapcontext - manipulate user context SYNOPSIS
#include
void makecontext(ucontextt *ucp, void (*func)(), int argc, ...); int swapcontext(ucontextt *oucp, ucontextt *ucp); DESCRIPTION In a System V-like environment, one has the type ucontextt defined in
and the four functions getcontext(3), setcontext(3), make‐ context() and swapcontext() that allow user-level context switching between multiple threads of control within a process. For the type and the first two functions, see getcontext(3). The makecontext() function modifies the context pointed to by ucp (which was obtained from a call to getcontext(3)). Before invoking makecontext(), the caller must allocate a new stack for this context
and assign its address to ucp->ucstack, and define a successor context
and assign its address to ucp->uclink. When this context is later activated (using setcontext(3) or swapcon‐ text()) the function func is called, and passed the series of integer (int) arguments that follow argc; the caller must specify the number of these arguments in argc. When this function returns, the successor context is activated. If the successor context pointer is NULL, the thread exits. The swapcontext() function saves the current context in the structure pointed to by oucp, and then activates the context pointed to by ucp. RETURN VALUE When successful, swapcontext() does not return. (But we may return later, in case oucp is activated, in which case it looks like swapcon‐
text() returns 0.) On error, swapcontext() returns -1 and sets errno appropriately. ERRORS ENOMEM Insufficient stack space left. VERSIONS makecontext() and swapcontext() are provided in glibc since version 2.1. ATTRIBUTES For an explanation of the terms used in this section, see attributes(7). ┌──────────────┬───────────────┬────────────────────────────┐ │Interface │ Attribute │ Value │ ├──────────────┼───────────────┼────────────────────────────┤
│makecontext() │ Thread safety │ MT-Safe race:ucp │ ├──────────────┼───────────────┼────────────────────────────┤
│swapcontext() │ Thread safety │ MT-Safe race:oucp race:ucp │ └──────────────┴───────────────┴────────────────────────────┘ CONFORMING TO
SUSv2, POSIX.1-2001. POSIX.1-2008 removes the specifications of make‐ context() and swapcontext(), citing portability issues, and recommend‐ ing that applications be rewritten to use POSIX threads instead. NOTES
The interpretation of ucp->ucstack is just as in sigaltstack(2), namely, this struct contains the start and length of a memory area to be used as the stack, regardless of the direction of growth of the stack. Thus, it is not necessary for the user program to worry about this direction. On architectures where int and pointer types are the same size (e.g.,
x86-32, where both types are 32 bits), you may be able to get away with passing pointers as arguments to makecontext() following argc. How‐ ever, doing this is not guaranteed to be portable, is undefined accord‐ ing to the standards, and won't work on architectures where pointers are larger than ints. Nevertheless, starting with version 2.8, glibc
makes some changes to makecontext(), to permit this on some 64-bit
architectures (e.g., x86-64). EXAMPLE The example program below demonstrates the use of getcontext(3), make‐ context(), and swapcontext(). Running the program produces the follow‐ ing output:
$ ./a.out main: swapcontext(&uctxmain, &uctxfunc2) func2: started func2: swapcontext(&uctxfunc2, &uctxfunc1) func1: started func1: swapcontext(&uctxfunc1, &uctxfunc2) func2: returning func1: returning main: exiting Program source
#include
#include
#include
static ucontextt uctxmain, uctxfunc1, uctxfunc2; #define handleerror(msg) \ do { perror(msg); exit(EXITFAILURE); } while (0) static void func1(void) { printf("func1: started\n"); printf("func1: swapcontext(&uctxfunc1, &uctxfunc2)\n");
if (swapcontext(&uctxfunc1, &uctxfunc2) == -1) handleerror("swapcontext"); printf("func1: returning\n"); } static void func2(void) { printf("func2: started\n"); printf("func2: swapcontext(&uctxfunc2, &uctxfunc1)\n");
if (swapcontext(&uctxfunc2, &uctxfunc1) == -1) handleerror("swapcontext"); printf("func2: returning\n"); } int main(int argc, char *argv[]) { char func1stack[16384]; char func2stack[16384];
if (getcontext(&uctxfunc1) == -1) handleerror("getcontext"); uctxfunc1.ucstack.sssp = func1stack; uctxfunc1.ucstack.sssize = sizeof(func1stack); uctxfunc1.uclink = &uctxmain; makecontext(&uctxfunc1, func1, 0);
if (getcontext(&uctxfunc2) == -1) handleerror("getcontext"); uctxfunc2.ucstack.sssp = func2stack; uctxfunc2.ucstack.sssize = sizeof(func2stack); /* Successor context is f1(), unless argc > 1 */ uctxfunc2.uclink = (argc > 1) ? NULL : &uctxfunc1; makecontext(&uctxfunc2, func2, 0); printf("main: swapcontext(&uctxmain, &uctxfunc2)\n");
if (swapcontext(&uctxmain, &uctxfunc2) == -1) handleerror("swapcontext"); printf("main: exiting\n"); exit(EXITSUCCESS); } SEE ALSO sigaction(2), sigaltstack(2), sigprocmask(2), getcontext(3), sigsetjmp(3) COLOPHON
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GNU 2013-02-12 MAKECONTEXT(3)