CODEWITHSUNDEEP
cmultithreading
pypep278
pypep278

Reputation: 307

Segmentation Fault using getcontext() in thread library

I am trying to implement a user level thread library in C using systems calls such as get context, swap context , etc

I have a thread control block that looks like this :

struct tcb {
    int thread_id;
    int thread_pri;
    ucontext_t *thread_context;
    struct tcb *next;
}

And I have a function called init() that looks like this:

void t_init()
{
  tcb *tmp;
  tmp = malloc(sizeof(tcb));

  getcontext(tmp->thread_context);    /* let tmp be the context of main() */
 running_head = tmp;
}

I used gdb and I got a segmentation fault during runtime at the getcontext(tmp->thread_context) function.

I have read the man pages for getcontext() but am unsure as to why this is returning a segmentation fault to me!

Any suggestions please?

Upvotes: 2

Views: 2293

Answers (2)

Mantosh Kumar
Mantosh Kumar

Reputation: 5741

We can get the following information about getcontext/setcontext "The GNU C Library Reference Manual Chapter:23 Non Locals Exits, Page 622)", and found the following

While allocating the memory for the stack one has to be careful. Most modern processors keep track of whether a certain memory region is allowed to contain code which is executed or not. Data segments and heap memory is normally not tagged to allow this. The result is that programs would fail. Examples for such code include the calling sequences the GNU C compiler generates for calls to nested functions. Safe ways to allocate stacks correctly include using memory on the original threads stack or explicitly allocate memory tagged for execution using memory mapped I/O.

This is causing the problem and you should use the recommended step to allocate the memory(using memory mapped I/O For more information, Please refer the libc manual).

Upvotes: 0

austinmarton
austinmarton

Reputation: 2368

You haven't allocated any space for thread_context, try

void t_init()
{
    struct tcb *tmp;
    tmp = malloc(sizeof(struct tcb));
    if (!tmp)
        return -1;

    memset(&tmp, 0, sizeof(struct tcb));
    tmp->thread_context = malloc(sizeof(ucontext_t));
    if (!tmp->thread_context)
        return -1;

    getcontext(tmp->thread_context);
}

Upvotes: 3

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