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Implementing events using futex calls

From: Marin Ramesa
Subject: Implementing events using futex calls
Date: Sun, 22 Sep 2013 22:04:39 +0200

This is a first in the series of constructs that can be used to test 
the new futex call (that is, when it's finished). It's based on Ulrich
Drepper's paper "Futexes Are Tricky" and assumes the futex.h from my 
patch, with the exception that the corrected futex prototype is:

int futex(int *address, int value, int operation);

where 'value' in the case of FUTEX_WAKE operation denotes the number of 
threads to be resumed.

Objects of type event_t can be used to synchronize a number of threads 
inside a task, where threads which are to be synchronized use the same 
event_t object.

The idea is that each thread calls event_wait() with the same object as 
the argument. Then a thread outside of the synchronization circle calls 
event_signal() to send a wakeup signal. event_signal() first modifies 
the event's value, which closes the synch circle and then calls 
FUTEX_WAKE with the INT_MAX argument (which means that all suspended 
threads at the same address should be resumed).

Here's the code:

#include <limits.h>
#include <mach/futex.h>

struct event {
        int value;

typedef struct event *event_t;

void event_init(event_t ev);
void event_signal(event_t ev);
void event_wait(event_t ev);

void event_init(event_t ev)
        ev->value = 0;

void event_signal(event_t ev)
        futex(&(ev->value), INT_MAX, FUTEX_WAKE);

void event_wait(event_t ev)
        futex(&(ev->value), ev->value, FUTEX_WAIT);

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