System V contexts (The GNU C Library)

The Unix standard provides one more set of functions to control the execution path and these functions are more powerful than those discussed in this chapter so far. These functions were part of the original System V API and by this route were added to the Unix API. Besides on branded Unix implementations these interfaces are not widely available. Not all platforms and/or architectures the GNU C Library is available on provide this interface. Use configure to detect the availability.

Similar to the jmp_buf and sigjmp_buf types used for the variables to contain the state of the longjmp functions the interfaces of interest here have an appropriate type as well. Objects of this type are normally much larger since more information is contained. The type is also used in a few more places as we will see. The types and functions described in this section are all defined and declared respectively in the ucontext.h header file.

Objects of this type have to be created by the user. The initialization and modification happens through one of the following functions:

The getcontext function is similar to setjmp but it does not provide an indication of whether getcontext is returning for the first time or whether an initialized context has just been restored. If this is necessary the user has to determine this herself. This must be done carefully since the context contains registers which might contain register variables. This is a good situation to define variables with volatile.

Once the context variable is initialized it can be used as is or it can be modified using the makecontext function. The latter is normally done when implementing co-routines or similar constructs.

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 are 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 thread’s stack or explicitly allocating memory tagged for execution using (see Memory-mapped I/O).

Compatibility note: The current Unix standard is very imprecise about the way the stack is allocated. All implementations seem to agree that the uc_stack element must be used but the values stored in the elements of the stack_t value are unclear. The GNU C Library and most other Unix implementations require the ss_sp value of the uc_stack element to point to the base of the memory region allocated for the stack and the size of the memory region is stored in ss_size. There are implementations out there which require ss_sp to be set to the value the stack pointer will have (which can, depending on the direction the stack grows, be different). This difference makes the makecontext function hard to use and it requires detection of the platform at compile time.

The setcontext function simply replaces the current context with the one described by the ucp parameter. This is often useful but there are situations where the current context has to be preserved.

Example for SVID Context Handling

The easiest way to use the context handling functions is as a replacement for setjmp and longjmp. The context contains on most platforms more information which may lead to fewer surprises but this also means using these functions is more expensive (besides being less portable).

int
random_search (int n, int (*fp) (int, ucontext_t *))
{
  volatile int cnt = 0;
  ucontext_t uc;

  /* Safe current context.  */
  if (getcontext (&uc) < 0)
    return -1;

  /* If we have not tried n times try again.  */
  if (cnt++ < n)
    /* Call the function with a new random number
       and the context.  */
    if (fp (rand (), &uc) != 0)
      /* We found what we were looking for.  */
      return 1;

  /* Not found.  */
  return 0;
}

Using contexts in such a way enables emulating exception handling. The search functions passed in the fp parameter could be very large, nested, and complex which would make it complicated (or at least would require a lot of code) to leave the function with an error value which has to be passed down to the caller. By using the context it is possible to leave the search function in one step and allow restarting the search which also has the nice side effect that it can be significantly faster.

Something which is harder to implement with setjmp and longjmp is to switch temporarily to a different execution path and then resume where execution was stopped.


#include <signal.h>
#include <stdio.h>
#include <stdlib.h>
#include <ucontext.h>
#include <sys/time.h>

/* Set by the signal handler. */
static volatile int expired;

/* The contexts. */
static ucontext_t uc[3];

/* We do only a certain number of switches. */
static int switches;


/* This is the function doing the work.  It is just a
   skeleton, real code has to be filled in. */
static void
f (int n)
{
  int m = 0;
  while (1)
    {
      /* This is where the work would be done. */
      if (++m % 100 == 0)
        {
          putchar ('.');
          fflush (stdout);
        }

      /* Regularly the expire variable must be checked. */
      if (expired)
        {
          /* We do not want the program to run forever. */
          if (++switches == 20)
            return;

          printf ("\nswitching from %d to %d\n", n, 3 - n);
          expired = 0;
          /* Switch to the other context, saving the current one. */
          swapcontext (&uc[n], &uc[3 - n]);
        }
    }
}

/* This is the signal handler which simply set the variable. */
void
handler (int signal)
{
  expired = 1;
}


int
main (void)
{
  struct sigaction sa;
  struct itimerval it;
  char st1[8192];
  char st2[8192];

  /* Initialize the data structures for the interval timer. */
  sa.sa_flags = SA_RESTART;
  sigfillset (&sa.sa_mask);
  sa.sa_handler = handler;
  it.it_interval.tv_sec = 0;
  it.it_interval.tv_usec = 1;
  it.it_value = it.it_interval;

  /* Install the timer and get the context we can manipulate. */
  if (sigaction (SIGPROF, &sa, NULL) < 0
      || setitimer (ITIMER_PROF, &it, NULL) < 0
      || getcontext (&uc[1]) == -1
      || getcontext (&uc[2]) == -1)
    abort ();

  /* Create a context with a separate stack which causes the
     function f to be call with the parameter 1.
     Note that the uc_link points to the main context
     which will cause the program to terminate once the function
     return. */
  uc[1].uc_link = &uc[0];
  uc[1].uc_stack.ss_sp = st1;
  uc[1].uc_stack.ss_size = sizeof st1;
  makecontext (&uc[1], (void (*) (void)) f, 1, 1);

  /* Similarly, but 2 is passed as the parameter to f. */
  uc[2].uc_link = &uc[0];
  uc[2].uc_stack.ss_sp = st2;
  uc[2].uc_stack.ss_size = sizeof st2;
  makecontext (&uc[2], (void (*) (void)) f, 1, 2);

  /* Start running. */
  swapcontext (&uc[0], &uc[1]);
  putchar ('\n');

  return 0;
}

This an example how the context functions can be used to implement co-routines or cooperative multi-threading. All that has to be done is to call every once in a while swapcontext to continue running a different context. It is not recommended to do the context switching from the signal handler directly since leaving the signal handler via setcontext if the signal was delivered during code that was not asynchronous signal safe could lead to problems. Setting a variable in the signal handler and checking it in the body of the functions which are executed is a safer approach. Since swapcontext is saving the current context it is possible to have multiple different scheduling points in the code. Execution will always resume where it was left.

23.4 Complete Context Control

Example for SVID Context Handling