why does this program crash while using setcontext?

["Lee Chin" <leechin@mail.com>]

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

ucontext_t mainucp;
ucontext_t fooucp;
int flag = 0;

int foo()
{
    printf("(2) foo setting context\n");
    getcontext(&fooucp);
    if (!flag)
    {
        flag = 1;
        setcontext(&mainucp);
    }
    printf("(3) foo done\n");
    return 0;
}

void eleven()
{
    printf("(1) calling foo\n");
    foo();
    printf("(4) foo now finished\n");
}

void ten()
{
    eleven();
}

void nine()
{
    ten();
}

void eight()
{
    nine();
}

void seven()
{
    eight();
}

void six()
{
    seven();
}

void five()
{
    six();
}

void four()
{
    five();
}

void three()
{
    four();
}

void two()
{
    three();
}

void one()
{
    two();
}

int main()
{
    getcontext(&mainucp);
    if (!flag)
    {
        one();
    }
    else
    {
        printf("(2.5) main again\n");
        setcontext(&fooucp);
    }
}

~                                               
-- 
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