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(libc.info)System V contexts
Complete Context Control
========================
The Unix standard one more set of function to control the execution
path and these functions are more powerful than those discussed in this
chapter so far. These function were part of the original System V API
and by this route were added to the Unix API. Beside 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.
- Data Type: ucontext_t
The `ucontext_t' type is defined as a structure with as least the
following elements:
`ucontext_t *uc_link'
This is a pointer to the next context structure which is used
if the context described in the current structure returns.
`sigset_t uc_sigmask'
Set of signals which are blocked when this context is used.
`stack_t uc_stack'
Stack used for this context. The value need not be (and
normally is not) the stack pointer. Note: Signal Stack.
`mcontext_t uc_mcontext'
This element contains the actual state of the process. The
`mcontext_t' type is also defined in this header but the
definition should be treated as opaque. Any use of knowledge
of the type makes applications less portable.
Objects of this type have to be created by the user. The
initialization and modification happens through one of the following
functions:
- Function: int getcontext (ucontext_t *UCP)
The `getcontext' function initializes the variable pointed to by
UCP with the context of the calling thread. The context contains
the content of the registers, the signal mask, and the current
stack. Executing the contents would start at the point where the
`getcontext' call just returned.
The function returns `0' if successful. Otherwise it returns `-1'
and sets ERRNO accordingly.
The `getcontext' function is similar to `setjmp' but it does not
provide an indication of whether the function returns for the first
time or whether the initialized context was used and the execution is
resumed at just that point. If this is necessary the user has to take
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. The latter is normally done to implement co-routines
or similar constructs. The `makecontext' function is what has to be
used to do that.
- Function: void makecontext (ucontext_t *UCP, void (*FUNC) (void),
int ARGC, ...)
The UCP parameter passed to the `makecontext' shall be initialized
by a call to `getcontext'. The context will be modified to in a
way so that if the context is resumed it will start by calling the
function `func' which gets ARGC integer arguments passed. The
integer arguments which are to be passed should follow the ARGC
parameter in the call to `makecontext'.
Before the call to this function the `uc_stack' and `uc_link'
element of the UCP structure should be initialized. The
`uc_stack' element describes the stack which is used for this
context. No two contexts which are used at the same time should
use the same memory region for a stack.
The `uc_link' element of the object pointed to by UCP should be a
pointer to the context to be executed when the function FUNC
returns or it should be a null pointer. See `setcontext' for more
information about the exact use.
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 (Note: 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 implements 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.
- Function: int setcontext (const ucontext_t *UCP)
The `setcontext' function restores the context described by UCP.
The context is not modified and can be reused as often as wanted.
If the context was created by `getcontext' execution resumes with
the registers filled with the same values and the same stack as if
the `getcontext' call just returned.
If the context was modified with a call to `makecontext' execution
continues with the function passed to `makecontext' which gets the
specified parameters passed. If this function returns execution is
resumed in the context which was referenced by the `uc_link'
element of the context structure passed to `makecontext' at the
time of the call. If `uc_link' was a null pointer the application
terminates in this case.
Since the context contains information about the stack no two
threads should use the same context at the same time. The result
in most cases would be disastrous.
The `setcontext' function does not return unless an error occurred
in which case it returns `-1'.
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.
- Function: int swapcontext (ucontext_t *restrict OUCP, const
ucontext_t *restrict UCP)
The `swapcontext' function is similar to `setcontext' but instead
of just replacing the current context the latter is first saved in
the object pointed to by OUCP as if this was a call to
`getcontext'. The saved context would resume after the call to
`swapcontext'.
Once the current context is saved the context described in UCP is
installed and execution continues as described in this context.
If `swapcontext' succeeds the function does not return unless the
context OUCP is used without prior modification by `makecontext'.
The return value in this case is `0'. If the function fails it
returns `-1' and set ERRNO accordingly.
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 might lead to less surprises but this
also means using these functions is more expensive (beside 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 allowed to do the context switching from
the signal handler directly since neither `setcontext' nor
`swapcontext' are functions which can be called from a signal handler.
But setting a variable in the signal handler and checking it in the
body of the functions which are executed. 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.
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