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9bebea882a
SCM_DEFER_INTS. (scm_release_arbiter): Use scm_i_misc_mutex so return value can be guaranteed if multiple threads compete to unlock.
139 lines
4.2 KiB
C
139 lines
4.2 KiB
C
/* Copyright (C) 1995,1996, 1997, 2000, 2001, 2004 Free Software Foundation, Inc.
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*
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* This library is free software; you can redistribute it and/or
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* modify it under the terms of the GNU Lesser General Public
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* License as published by the Free Software Foundation; either
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* version 2.1 of the License, or (at your option) any later version.
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*
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* This library is distributed in the hope that it will be useful,
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* but WITHOUT ANY WARRANTY; without even the implied warranty of
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* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
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* Lesser General Public License for more details.
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*
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* You should have received a copy of the GNU Lesser General Public
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* License along with this library; if not, write to the Free Software
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* Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
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*/
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#include "libguile/_scm.h"
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#include "libguile/ports.h"
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#include "libguile/smob.h"
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#include "libguile/validate.h"
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#include "libguile/arbiters.h"
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/* ENHANCE-ME: If the cpu has an atomic test-and-set instruction it could be
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used instead of a mutex in try-arbiter and release-arbiter.
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For the i386 family, cmpxchg would suit but it's only available on 80486
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and higher so that would have to be checked, perhaps at run-time when
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setting up the definitions of the scheme procedures, or at compile time
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if we interpret a host cpu type like "i686" to mean not less than that
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chip. */
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static scm_t_bits scm_tc16_arbiter;
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#define SCM_ARB_LOCKED(arb) ((SCM_CELL_WORD_0 (arb)) & (1L << 16))
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#define SCM_LOCK_ARB(arb) (SCM_SET_CELL_WORD_0 ((arb), scm_tc16_arbiter | (1L << 16)));
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#define SCM_UNLOCK_ARB(arb) (SCM_SET_CELL_WORD_0 ((arb), scm_tc16_arbiter));
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static int
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arbiter_print (SCM exp, SCM port, scm_print_state *pstate)
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{
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scm_puts ("#<arbiter ", port);
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if (SCM_ARB_LOCKED (exp))
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scm_puts ("locked ", port);
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scm_iprin1 (SCM_PACK (SCM_SMOB_DATA (exp)), port, pstate);
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scm_putc ('>', port);
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return !0;
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}
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SCM_DEFINE (scm_make_arbiter, "make-arbiter", 1, 0, 0,
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(SCM name),
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"Return an object of type arbiter and name @var{name}. Its\n"
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"state is initially unlocked. Arbiters are a way to achieve\n"
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"process synchronization.")
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#define FUNC_NAME s_scm_make_arbiter
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{
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SCM_RETURN_NEWSMOB (scm_tc16_arbiter, SCM_UNPACK (name));
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}
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#undef FUNC_NAME
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/* The mutex here is so two threads can't both see the arbiter unlocked and
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both proceed to lock and return #t. The arbiter itself wouldn't be
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corrupted by this, but two threads both getting #t would be entirely
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contrary to the intended semantics. */
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SCM_DEFINE (scm_try_arbiter, "try-arbiter", 1, 0, 0,
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(SCM arb),
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"Return @code{#t} and lock the arbiter @var{arb} if the arbiter\n"
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"was unlocked. Otherwise, return @code{#f}.")
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#define FUNC_NAME s_scm_try_arbiter
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{
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SCM_VALIDATE_SMOB (1, arb, arbiter);
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scm_mutex_lock (&scm_i_misc_mutex);
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if (SCM_ARB_LOCKED(arb))
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arb = SCM_BOOL_F;
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else
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{
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SCM_LOCK_ARB(arb);
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arb = SCM_BOOL_T;
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}
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scm_mutex_unlock (&scm_i_misc_mutex);
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return arb;
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}
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#undef FUNC_NAME
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/* The mutex here is so two threads can't both see the arbiter locked and
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both proceed to unlock and return #t. The arbiter itself wouldn't be
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corrupted by this, but we don't want two threads both thinking they were
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the unlocker. The intended usage is for the code which locked to be
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responsible for unlocking, but we guarantee the return value even if
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multiple threads compete. */
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SCM_DEFINE (scm_release_arbiter, "release-arbiter", 1, 0, 0,
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(SCM arb),
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"Return @code{#t} and unlock the arbiter @var{arb} if the\n"
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"arbiter was locked. Otherwise, return @code{#f}.")
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#define FUNC_NAME s_scm_release_arbiter
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{
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SCM ret;
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SCM_VALIDATE_SMOB (1, arb, arbiter);
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scm_mutex_lock (&scm_i_misc_mutex);
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if (!SCM_ARB_LOCKED(arb))
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ret = SCM_BOOL_F;
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else
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{
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SCM_UNLOCK_ARB (arb);
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ret = SCM_BOOL_T;
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}
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scm_mutex_unlock (&scm_i_misc_mutex);
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return ret;
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}
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#undef FUNC_NAME
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void
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scm_init_arbiters ()
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{
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scm_tc16_arbiter = scm_make_smob_type ("arbiter", 0);
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scm_set_smob_mark (scm_tc16_arbiter, scm_markcdr);
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scm_set_smob_print (scm_tc16_arbiter, arbiter_print);
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#include "libguile/arbiters.x"
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}
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/*
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Local Variables:
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c-file-style: "gnu"
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End:
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*/
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