使用getcontext/setcontext切換堆棧/向下堆棧

Question

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

我可以看到如何使用setcontext（）將堆棧展開回歷史中的某個位置。

#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鬆後你能重新回到未來的地方嗎？我想這取決於getcontext()調用捕獲堆棧的副本，​​我無法在文檔中找到確切的詳細信息。

#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不復制棧，當你跳下它無效頂部上下文堆棧它只轉儲寄存器（包括堆棧指針）和類似的信號掩模一點上下文數據等

。即使你不會做任何函數調用，也要考慮可以在那裏執行的信號處理程序。如果你想在兩個堆棧之間跳轉，你需要makecontext。

我添加變量，以證明你的代碼是無效的：

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 
} 

在我的機器它會導致：

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