Unix ucontext_t Operations on Windows Platforms

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Introduction

Background

Most modern Unix environments provide to the programmer two options that allow user-level context switching between multiple threads of control within a process: either with the type jmpbuf defined in <setjmp.h> and the setjmp/longjmp pair of functions, or with the type ucontext_t defined in <ucontext.h> and the four functions getcontext, setcontext, makecontext, and swapcontext. For more information on the usage of these functions, you can look at [3]. On Windows platforms, however, the Microsoft C Runtime Library provides only the first set of functions [4].

Using the code

The following program (testcontext.c - included in the demo project) is a typical example that makes use of the ucontex_t operations. It runs successfully on both Unix and Windows platforms. On Windows, the programmer has only to include the two files (ucontext.[c,h]) in his application code. In the demo project, these two files are stored in the unix2nt directory, in order to make straightforward the compilation and execution of the test program on Unix platforms.

/* testcontext.c : demo of ucontex_t operations */
#include <stdio.h>
#include <stdlib.h>
#include <ucontext.h>

ucontext_t auc,buc,mainuc;

void a()
{
    int i;

    for (i = 0; i < 10; i++)
    {
        printf("a");
        swapcontext(&auc, &buc);        /* switch to thread B */
    }

    printf("\nswitching to main\n");
    swapcontext(&auc, &mainuc);         /* switch to main thread */
}

void b()
{
    int i;

    for (i = 0; i < 10; i++)
    {
        printf("b");
        swapcontext(&buc, &auc);        /* switch to thread A */
    }
}

int main(void)
{
    printf("start\n");                  /* main thread starts */

    /* Set up context for thread A (Unix code, see manpages) */
    getcontext(&auc);
    auc.uc_stack.ss_size = 16 * 1024;

    if ((auc.uc_stack.ss_sp = malloc(auc.uc_stack.ss_size)) == NULL)
        perror("malloc"), exit(1);

    auc.uc_stack.ss_flags = 0;
    makecontext(&auc, a, 0);

    /* Set up context for thread B */
    getcontext(&buc);
    buc.uc_stack.ss_size = 16 * 1024;

    if ((buc.uc_stack.ss_sp = malloc(buc.uc_stack.ss_size)) == NULL)
        perror("malloc"), exit(1);

    buc.uc_stack.ss_flags = 0;
    makecontext(&buc, b, 0);

    /* Switch to A */
    getcontext(&mainuc);           /* Save the context of main thread */
    swapcontext(&mainuc, &auc);    /* Switch to thread A */

    printf("\ndone\n");  /* Execution control returned to main thread */
    return 0;
}

If you compile and run the above program (testcontext.dsp), you get the following output:

C:\>testcontext.exe
start
abababababababababab
switching to main

done
C:\>

Implementation

We implemented the above functionality based on the CONTEXT data structure and the GetThreadContext and SetThreadContext functions [2]. In ucontext.h, we defined the appropriate data structures and function prototypes. The implementation of getcontext and setcontext is straightforward, using the corresponding GetThreadContext and SetThreadContext functions. In swapcontext, the current thread saves its context (getcontext) and then restores a new context (setcontext). The trickiest point is in the implementation of makecontext, where we reserve stack space for the function arguments; then we set appropriately the instruction (Eip) and the stack pointer (Esp) fields, and finally, we copy the function arguments in the reserved space. Although the latest specification of makecontext [4] restricts the arguments of the function to be of type int, according to our implementation, the maximum allowed size of an argument is 64 bits, enabling thus the passing of variables of type double. For more implementation details, see the ucontext.c and ucontext.h files.

History

• May 22, 2003 - Initial release.
• May 25, 2004 - Revised source code.
• Mar 6, 2007 - Distributed under the LGPL license.

References

• [1] Cygwin
• [2] U/Win
• [3] The Open Group Base Specifications Issue 6, IEEE Std. 1003.1, 2003 Edition
• [4] Microsoft Corporation, Microsoft Developer Network Library

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