SetJmp/LongJmp:为什么这会引发段错误?

The following code summarizes the problem I have at the moment. My current execution flow is as follows and a I'm running in GCC 4.3.

jmp_buf a_buf;
jmp_buf b_buf;

void b_helper()
{
    printf("entering b_helper");
    if(setjmp(b_buf) == 0)
    {
        printf("longjmping to a_buf");
        longjmp(a_buf, 1);
    }
    printf("returning from b_helper");
    return; //segfaults right here
}
void b()
{
    b_helper();
}
void a()
{
    printf("setjmping a_buf");
    if(setjmp(a_buf) == 0)
    {
        printf("calling b");
        b();
    }
    printf("longjmping to b_buf");
    longjmp(b_buf, 1);
}
int main()
{
    a();
}

上面的执行流程在 b_helper 中返回后立即创建了一个段错误.就好像只有 b_helper 栈帧是有效的，下面的栈都被擦除了.