diff options
Diffstat (limited to 'sem_getvalue.c')
| -rw-r--r-- | sem_getvalue.c | 116 |
1 files changed, 73 insertions, 43 deletions
diff --git a/sem_getvalue.c b/sem_getvalue.c index 297bc6c..9d37c38 100644 --- a/sem_getvalue.c +++ b/sem_getvalue.c @@ -11,32 +11,32 @@ * * ------------------------------------------------------------- * - * Pthreads-win32 - POSIX Threads Library for Win32 - * Copyright(C) 1998 John E. Bossom - * Copyright(C) 1999,2002 Pthreads-win32 contributors + * Pthreads-win32 - POSIX Threads Library for Win32 + * Copyright(C) 1998 John E. Bossom + * Copyright(C) 1999,2002 Pthreads-win32 contributors * - * Contact Email: rpj@ise.canberra.edu.au + * Contact Email: rpj@ise.canberra.edu.au * - * The current list of contributors is contained - * in the file CONTRIBUTORS included with the source - * code distribution. The list can also be seen at the - * following World Wide Web location: - * http://sources.redhat.com/pthreads-win32/contributors.html + * The current list of contributors is contained + * in the file CONTRIBUTORS included with the source + * code distribution. The list can also be seen at the + * following World Wide Web location: + * http://sources.redhat.com/pthreads-win32/contributors.html * - * This library is free software; you can redistribute it and/or - * modify it under the terms of the GNU Lesser General Public - * License as published by the Free Software Foundation; either - * version 2 of the License, or (at your option) any later version. + * This library is free software; you can redistribute it and/or + * modify it under the terms of the GNU Lesser General Public + * License as published by the Free Software Foundation; either + * version 2 of the License, or (at your option) any later version. * - * This library is distributed in the hope that it will be useful, - * but WITHOUT ANY WARRANTY; without even the implied warranty of - * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU - * Lesser General Public License for more details. + * This library is distributed in the hope that it will be useful, + * but WITHOUT ANY WARRANTY; without even the implied warranty of + * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU + * Lesser General Public License for more details. * - * You should have received a copy of the GNU Lesser General Public - * License along with this library in the file COPYING.LIB; - * if not, write to the Free Software Foundation, Inc., - * 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA + * You should have received a copy of the GNU Lesser General Public + * License along with this library in the file COPYING.LIB; + * if not, write to the Free Software Foundation, Inc., + * 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA */ #include "pthread.h" @@ -55,20 +55,20 @@ sem_getvalue(sem_t * sem, int * sval) * * Return value * - * 0 sval has been set. - * -1 failed, error in errno + * 0 sval has been set. + * -1 failed, error in errno * * in global errno * - * EINVAL 'sem' is not a valid semaphore, - * ENOSYS this function is not supported, + * EINVAL 'sem' is not a valid semaphore, + * ENOSYS this function is not supported, * * * PARAMETERS * - * sem pointer to an instance of sem_t + * sem pointer to an instance of sem_t * - * sval pointer to int. + * sval pointer to int. * * DESCRIPTION * This function stores the current count value of the semaphore @@ -86,27 +86,57 @@ sem_getvalue(sem_t * sem, int * sval) { #ifdef NEED_SEM - + result = ENOSYS; - + #else - + + /* + * There appears to be NO atomic means of determining the + * value of the semaphore. Using only ReleaseSemaphore() + * with either a zero or oversized count parameter has been + * suggested but this trick doesn't produce consistent results + * across Windows versions (the technique uses values that are + * knowingly illegal but hopes to extract the current value + * anyway - the zero parameter appears to work for Win9x but + * neither work reliably for WinNT). + * + * The intrusive method below will give false results + * at times but it at least errs on the side of + * caution. Competing threads might occasionally believe + * the semaphore has a count of one less than it actually + * would have and possibly block momentarily unecessarily, + * but they will never see a higher semaphore value than + * there should be. + * + * + * Multiple threads calling sem_getvalue() at the same time + * may not all return the same value (assuming no calls to + * other semaphore routines). They will always return the + * correct value or a lesser value. This problem could be fixed + * with a global or a per-semaphore critical section here. + * + * An equally approximate but IMO slightly riskier approach + * would be to keep a separate per-semaphore counter and + * decrement/increment it inside of sem_wait() and sem_post() + * etc using the Interlocked* functions. + */ if ( WaitForSingleObject( (*sem)->sem, 0 ) == WAIT_TIMEOUT ) - { - /* Failed - must be zero. */ - value = 0; - } + { + /* Failed - must be zero */ + value = 0; + } else - { - /* Decremented sema - release it and note the value. */ - (void) ReleaseSemaphore( (*sem)->sem, 1L, &value ); - value++; - } - + { + /* Decremented semaphore - release it and note the value */ + (void) ReleaseSemaphore( (*sem)->sem, 1L, &value); + value++; + } + #endif } - - + + if ( result != 0 ) { errno = result; @@ -115,5 +145,5 @@ sem_getvalue(sem_t * sem, int * sval) *sval = value; return 0; - + } /* sem_getvalue */ |