makecontext - Online Linux Manual PageSection : 3
Updated : 2022-12-15
Source : Linux man-pages 6.03

NAMEmakecontext, swapcontext − manipulate user context

LIBRARYStandard C library (libc, −lc)

SYNOPSIS#include <ucontext.h>void makecontext(ucontext_t *ucp, void (*func)(), int argc, ...); int swapcontext(ucontext_t *restrict oucp, const ucontext_t *restrict ucp);

DESCRIPTIONIn a System V-like environment, one has the type ucontext_t (defined in <ucontext.h> and described in getcontext(3)) and the four functions getcontext(3), setcontext(3), makecontext(), and swapcontext() that allow user-level context switching between multiple threads of control within a process. 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−>uc_stack, and define a successor context and assign its address to ucp−>uc_link. When this context is later activated (using setcontext(3) or swapcontext()) 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 VALUEWhen successful, swapcontext() does not return. (But we may return later, in case oucp is activated, in which case it looks like swapcontext() returns 0.) On error, swapcontext() returns −1 and sets errno to indicate the error.

ERRORSENOMEM  Insufficient stack space left.

VERSIONSmakecontext() and swapcontext() are provided since glibc 2.1.

ATTRIBUTESFor an explanation of the terms used in this section, see attributes(7). InterfaceAttributeValue makecontext() Thread safety MT-Safe race:ucp swapcontext() Thread safety MT-Safe race:oucp race:ucp

STANDARDSSUSv2, POSIX.1-2001. POSIX.1-2008 removes the specifications of makecontext() and swapcontext(), citing portability issues, and recommending that applications be rewritten to use POSIX threads instead.

NOTESThe interpretation of ucp−>uc_stack 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. However, doing this is not guaranteed to be portable, is undefined according to the standards, and won't work on architectures where pointers are larger than ints. Nevertheless, starting with glibc 2.8, glibc makes some changes to makecontext(), to permit this on some 64-bit architectures (e.g., x86-64).

EXAMPLESThe example program below demonstrates the use of getcontext(3), makecontext(), and swapcontext(). Running the program produces the following output: $ ./a.out main: swapcontext(&uctx_main, &uctx_func2) func2: started func2: swapcontext(&uctx_func2, &uctx_func1) func1: started func1: swapcontext(&uctx_func1, &uctx_func2) func2: returning func1: returning main: exiting

Program source#include <stdio.h> #include <stdlib.h> #include <ucontext.h> static ucontext_t uctx_main, uctx_func1, uctx_func2; #define handle_error(msg) \ do { perror(msg); exit(EXIT_FAILURE); } while (0) static void func1(void) { printf("%s: started\n", __func__); printf("%s: swapcontext(&uctx_func1, &uctx_func2)\n", __func__); if (swapcontext(&uctx_func1, &uctx_func2) == −1) handle_error("swapcontext"); printf("%s: returning\n", __func__); } static void func2(void) { printf("%s: started\n", __func__); printf("%s: swapcontext(&uctx_func2, &uctx_func1)\n", __func__); if (swapcontext(&uctx_func2, &uctx_func1) == −1) handle_error("swapcontext"); printf("%s: returning\n", __func__); } int main(int argc, char *argv[]) { char func1_stack[16384]; char func2_stack[16384]; if (getcontext(&uctx_func1) == −1) handle_error("getcontext"); uctx_func1.uc_stack.ss_sp = func1_stack; uctx_func1.uc_stack.ss_size = sizeof(func1_stack); uctx_func1.uc_link = &uctx_main; makecontext(&uctx_func1, func1, 0); if (getcontext(&uctx_func2) == −1) handle_error("getcontext"); uctx_func2.uc_stack.ss_sp = func2_stack; uctx_func2.uc_stack.ss_size = sizeof(func2_stack); /* Successor context is f1(), unless argc > 1 */ uctx_func2.uc_link = (argc > 1) ? NULL : &uctx_func1; makecontext(&uctx_func2, func2, 0); printf("%s: swapcontext(&uctx_main, &uctx_func2)\n", __func__); if (swapcontext(&uctx_main, &uctx_func2) == −1) handle_error("swapcontext"); printf("%s: exiting\n", __func__); exit(EXIT_SUCCESS); }

SEE ALSOsigaction(2), sigaltstack(2), sigprocmask(2), getcontext(3), sigsetjmp(3)
0
Johanes Gumabo
Data Size   :   20,611 byte
man-swapcontext.3Build   :   2024-12-05, 20:55   :  
Visitor Screen   :   x
Visitor Counter ( page / site )   :   2 / 237,618
Visitor ID   :     :  
Visitor IP   :   3.141.201.46   :  
Visitor Provider   :   AMAZON-02   :  
Provider Position ( lat x lon )   :   39.962500 x -83.006100   :   x
Provider Accuracy Radius ( km )   :   1000   :  
Provider City   :   Columbus   :  
Provider Province   :   Ohio ,   :   ,
Provider Country   :   United States   :  
Provider Continent   :   North America   :  
Visitor Recorder   :   Version   :  
Visitor Recorder   :   Library   :  
Online Linux Manual Page   :   Version   :   Online Linux Manual Page - Fedora.40 - march=x86-64 - mtune=generic - 24.12.05
Online Linux Manual Page   :   Library   :   lib_c - 24.10.03 - march=x86-64 - mtune=generic - Fedora.40
Online Linux Manual Page   :   Library   :   lib_m - 24.10.03 - march=x86-64 - mtune=generic - Fedora.40
Data Base   :   Version   :   Online Linux Manual Page Database - 24.04.13 - march=x86-64 - mtune=generic - fedora-38
Data Base   :   Library   :   lib_c - 23.02.07 - march=x86-64 - mtune=generic - fedora.36

Very long time ago, I have the best tutor, Wenzel Svojanovsky . If someone knows the email address of Wenzel Svojanovsky , please send an email to johanes_gumabo@yahoo.co.id .
If error, please print screen and send to johanes_gumabo@yahoo.co.id
Under development. Support me via PayPal.