swapcontext() and Segmentation Fault

Applications Programming
1

I am in the process of trying to analysing a code snippet I just wrote, and am facing a segmentation fault when I try to swap context an a point...

I get a segmentation fault when I call swapcontext() function...

Please guide me.

2

Can you post the code snippet where you call the "swapcontext" function?

3

I'm having the same problem-here's my code:

//code snippet from main:
/* context names for scheduler and threads, TODO maybe set up threads as an array? */
ucontext_t sched;
ucontext_t T1;


  /* Set up context and memory for threads, not sure what
   * the flags do
   */
    if(DEBUG){ printf("create context and memory for threads\n");}
    getcontext(&T1);
    if ((T1.uc_stack.ss_sp = malloc(THREAD_STK)) == NULL){
        perror("T1 malloc"), exit(1);      
    }
    T1.uc_stack.ss_flags = 0;
    T1.uc_link = &sched;
    makecontext(&T1, produce_sm, 0);
           
   
    
    /* save main, swap to a thread */
    if(DEBUG){ printf("save sched, swap to T1\n");}
    if(getcontext(&sched) == -1){ /* save main */
        perror("sched get error");
    }

    if ((sched.uc_stack.ss_sp = malloc(THREAD_STK)) == NULL){ /* main stack space */
        die("sched malloc");      
    }
//FAILS HERE
    if(swapcontext(&sched, &T1) == -1){ /* swap to T1 */
        die("sched swap error");
    }

this is the function it's supposed to swap to:

void produce_sm(){
   printf("I'm T1\n"); 
    
   
   printf("swap from T1 to sched\n");
   if(swapcontext(&T1, &sched) == -1){die("T1 swap");}
}

If I ever get this right, its supposed to be a threaded implementation of the
producer-consumer problem. This is my first venture into threads, I'd appreciate any advice.

4

gebuh, what OS are you using? Most OS'es these days have kernel support for threads. If your kernel does, it will have allocated a thread for you to use. And it looks like you are trying to implement your own user-level threads within that kernel thread. Why take this approach rather than just using pthreads?

5

I'm on redhat linux, its a class project- we're supposed to implement our own threads, we're limited to make/set/getcontext or set/longjump.

6

We have some rules here and one of them is:
(6) Do not post classroom or homework problems.

But once in a while we bend the rule a bit, so try setting ss_size when you define the stack.

7

duly chastened [walk of shame]gonna go read the rules now[/end walk of shame]

8

Guh.

I'm gonna bend the rules since I feel obliged to point out that longjmp and setcontext are not signal-safe! See 'man signal' for the explicit list of functions that are. So if you're using signals as a trigger to switch contexts you're going to have to throw them out.