使用getcontext/setcontext切换堆栈/向下堆栈

Question

我想了解getcontext/setcontext是否可以在特定场景中正常工作。

我可以看到如何使用setcontext（）将堆栈展开回历史中的某个位置。

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

int rollback = 0; 
ucontext_t context; 

void func(void) 
{ 
    setcontext(cp); 
} 

int main(void) 
{ 
    getcontext(&context); 
    if (rollback == 0) 
    { 
     printf("getcontext has been called\n"); 
     rollback++; 
     func(); 
    } 
    else 
    { 
     printf("setcontext has been called\n"); 
    } 
} 

但是我想知道如果放松后你能重新回到未来的地方吗？我想这取决于getcontext()调用捕获堆栈的副本，​​我无法在文档中找到确切的详细信息。

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

int rollback  = 0; 
int backToFuture = 0; 
ucontext_t context; 
ucontext_t futureContext; 

void func(void) 
{ 
    // Some complex calc 
    if (some-condition) 
    { 
     getcontext(&futureContext); // After returning I want to come back 
            // here to carry on with my work. 
     if (backToFuture == 0) 
     { 
      setcontext(&context); // rewind to get stuff-done 
     } 
    } 
    // Finishe work 
} 

int main(void) 
{ 
    getcontext(&context); 
    if (rollback == 0) 
    { 
     printf("getcontext has been called\n"); 
     rollback++; 
     func(); 

     // eventually always return here. 
    } 
    else 
    { 
     printf("setcontext has been called\n"); 
     // Do specialized work that needed to be done 
     // May involve function calls. 
     // 
     // I worry that anything the adds new stack frames 
     // will disrupt the saved state of futureContext 
     // 
     // But without detailed information I can not be sure 
     // if this is an allowed senario. 
     backToFuture = 1; 
     setcontext(&futureContext); 
    } 
} 

Answer 1

getcontext不复制栈，当你跳下它无效顶部上下文堆栈它只转储寄存器（包括堆栈指针）和类似的信号掩模一点上下文数据等

。即使你不会做任何函数调用，也要考虑可以在那里执行的信号处理程序。如果你想在两个堆栈之间跳转，你需要makecontext。

我添加变量，以证明你的代码是无效的：

void func(void) 
{ 
    // Some complex calc 
    if (1) 
    { 
     volatile int neverChange = 1; 
     getcontext(&futureContext); // After returning I want to come back 
            // here to carry on with my work. 
     printf("neverchange = %d\n", neverChange); 
     if (backToFuture == 0) 
     { 
      setcontext(&context); // rewind to get stuff-done 
     } 
    } 
    // Finishe work 
} 

在我的机器它会导致：

getcontext has been called 
neverchange = 1 
setcontext has been called 
neverchange = 32767