libcruft-util/thread/semaphore_linux.cpp

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/*
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* This Source Code Form is subject to the terms of the Mozilla Public
* License, v. 2.0. If a copy of the MPL was not distributed with this
* file, You can obtain one at http://mozilla.org/MPL/2.0/.
*
* Copyright 2018 Danny Robson <danny@nerdcruft.net>
*/
#include "semaphore.hpp"
#include "../cast.hpp"
#include "../posix/except.hpp"
#include <cerrno>
#include <linux/futex.h>
#include <unistd.h>
#include <sys/syscall.h>
#include <limits>
using cruft::thread::semaphore;
///////////////////////////////////////////////////////////////////////////////
static long
sys_futex (void *addr1, int op, int val1, struct timespec *timeout, void *addr2, int val3)
{
return syscall (SYS_futex, addr1, op | FUTEX_PRIVATE_FLAG, val1, timeout, addr2, val3);
}
///////////////////////////////////////////////////////////////////////////////
semaphore::semaphore ():
semaphore (1)
{ ; }
//-----------------------------------------------------------------------------
semaphore::semaphore (int initial):
m_value (initial)
{ ; }
///////////////////////////////////////////////////////////////////////////////
int
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semaphore::acquire (int count)
{
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CHECK_GE (count, 0);
do {
int now = m_value;
// if our value is positive then attempt to decrement it and return,
// else retry because someone interfered with us.
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if (now - count >= 0) {
if (m_value.compare_exchange_weak (now, now - count))
return now - count;
continue;
}
// the count doesn't appear to allow us to acquire. sleep until
// there's been a modification and retry.
if (-1 == sys_futex (&m_value, FUTEX_WAIT, now, nullptr, nullptr, 0)) {
switch (errno) {
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case EAGAIN: break;
case EINTR: break;
default:
posix::error::throw_code ();
}
}
} while (1);
}
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//-----------------------------------------------------------------------------
int
semaphore::acquire (void)
{
return acquire (1);
}
//-----------------------------------------------------------------------------
int
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semaphore::release (int count)
{
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auto res = m_value += count;
if (sys_futex (&m_value, FUTEX_WAKE, count, nullptr, nullptr, 0) < 0)
posix::error::throw_code ();
return res;
}
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//-----------------------------------------------------------------------------
int
semaphore::release (void)
{
return release (1);
}
///////////////////////////////////////////////////////////////////////////////
int
semaphore::value (void) const
{
return m_value;
}
//-----------------------------------------------------------------------------
int
semaphore::operator++ (void)
{
return release ();
}
//-----------------------------------------------------------------------------
int
semaphore::operator-- (void)
{
// we don't need to wake anyone because this will only serve to delay
// their wakeup.
return --m_value;
}