@c @c COPYRIGHT(c) 1988-1998. @c On-Line Applications Research Corporation(OAR). @c All rights reserved. @c @c $Id$ @c @chapter Semaphore Manager @section Introduction The semaphore manager provides functions to allocate, delete, and control semaphores. This manager is based on the POSIX 1003.1 standard. The directives provided by the semaphore manager are: @itemize @bullet @item @code{sem_init} - Initialize an unnamed semaphore @item @code{sem_destroy} - Destroy an unnamed semaphore @item @code{sem_open} - Open a named semaphore @item @code{sem_close} - Close a named semaphore @item @code{sem_unlink} - Remove a named semaphore @item @code{sem_wait} - Lock a semaphore @item @code{sem_trywait} - Lock a semaphore @item @code{sem_post} - Unlock a semaphore @item @code{sem_getvalue} - Get the value of a semeaphore @end itemize @section Background @subsection Theory Semaphores are used for synchronization and mutual exclusion by indicating the availability and number of resources. The task (the task which is returning resources) notifying other tasks of an event increases the number of resources held by the semaphore by one. The task (the task which will obtain resources) waiting for the event decreases the number of resources held by the semaphore by one. If the number of resources held by a semaphore is insufficient (namely 0), the task requiring resources will wait until the next time resources are returned to the semaphore. If there is more than one task waiting for a semaphore, the tasks will be placed in the queue. @subsection "sem_t" Structure The "sem_t" structure is used to represent semaphores. It is passed as an argument to the semaphore directives and is defined as follows: @example typedef int sem_t @end example @subsection Building a Semaphore Attribute Set @section Operations @subsection Using as a Binary Semaphore Although POSIX supports mutexes, they are only visible between threads. To work between processes, a binary semaphore must be used. Creating a semaphore with a limit on the count of 1 effectively restricts the semaphore to being a binary semaphore. When the binary semaphore is available, the count is 1. When the binary semaphore is unavailable, the count is 0. Since this does not result in a true binary semaphore, advanced binary features like the Priority Inheritance and Priority Ceiling Protocols are not available. There is currently no text in this section. @section Directives This section details the semaphore manager's directives. A subsection is dedicated to each of this manager's directives and describes the calling sequence, related constants, usage, and status codes. @c @c @c @page @subsection sem_init - Initialize an unnamed semaphore @findex sem_init @cindex initialize an unnamed semaphore @subheading CALLING SEQUENCE: @ifset is-C @example int sem_init( sem_t *sem, int pshared, unsigned int value ); @end example @end ifset @ifset is-Ada @end ifset @subheading STATUS CODES: @table @b @item EINVAL The value argument exceeds SEM_VALUE_MAX @item ENOSPC A resource required to initialize the semaphore has been exhausted The limit on semaphores (SEM_VALUE_MAX) has been reached @item ENOSYS The function sem_init is not supported by this implementation @item EPERM The process lacks appropriate privileges to initialize the semaphore @end table @subheading DESCRIPTION: The sem_init function is used to initialize the unnamed semaphore referred to by "sem". The value of the initialized semaphore is the parameter "value". The semaphore remains valid until it is destroyed. ADD MORE HERE XXX @subheading NOTES: If the functions completes successfully, it shall return a value of zero. Otherwise, it shall return a value of -1 and set "errno" to specify the error that occurred. Multiprocessing is currently not supported in this implementation. @c @c @c @page @subsection sem_destroy - Destroy an unnamed semaphore @findex sem_destroy @cindex destroy an unnamed semaphore @subheading CALLING SEQUENCE: @ifset is-C @example int sem_destroy( sem_t *sem ); @end example @end ifset @ifset is-Ada @end ifset @subheading STATUS CODES: @table @b @item EINVAL The value argument exceeds SEM_VALUE_MAX @item ENOSYS The function sem_init is not supported by this implementation @item EBUSY There are currently processes blocked on the semaphore @end table @subheading DESCRIPTION: The sem_destroy function is used to destroy an unnamed semaphore refered to by "sem". sem_destroy can only be used on a semaphore that was created using sem_init. @subheading NOTES: If the functions completes successfully, it shall return a value of zero. Otherwise, it shall return a value of -1 and set "errno" to specify the error that occurred. Multiprocessing is currently not supported in this implementation. @c @c @c @page @subsection sem_open - Open a named semaphore @findex sem_open @cindex open a named semaphore @subheading CALLING SEQUENCE: @ifset is-C @example int sem_open) const char *name, int oflag ); @end example @end ifset @ifset is-Ada @end ifset @subheading ARGUMENTS: The following flag bit may be set in oflag: @code{O_CREAT} - Creates the semaphore if it does not already exist. If O_CREAT is set and the semaphore already exists then O_CREAT has no effect. Otherwise, sem_open() creates a semaphore. The O_CREAT flag requires the third and fourth argument: mode and value of type mode_t and unsigned int, respectively. @code{O_EXCL} - If O_EXCL and O_CREAT are set, all call to sem_open() shall fail if the semaphore name exists @subheading STATUS CODES: @table @b @item EACCES Valid name specified but oflag permissions are denied, or the semaphore name specified does not exist and permission to create the named semaphore is denied. @item EEXIST O_CREAT and O_EXCL are set and the named semaphore already exists. @item EINTR The sem_open() operation was interrupted by a signal. @item EINVAL The sem_open() operation is not supported for the given name. @item EMFILE Too many semaphore descriptors or file descriptors in use by this process. @item ENAMETOOLONG The length of the name exceed PATH_MAX or name component is longer than NAME_MAX while POSIX_NO_TRUNC is in effect. @item ENOENT O_CREAT is not set and the named semaphore does not exist. @item ENOSPC There is insufficient space for the creation of a new named semaphore. @item ENOSYS The function sem_open() is not supported by this implementation. @end table @subheading DESCRIPTION: The sem_open() function establishes a connection between a specified semaphore and a process. After a call to sem_open with a specified semaphore name, a process can reference to semaphore by the associated name using the address returned by the call. The oflag arguments listed above control the state of the semaphore by determining if the semaphore is created or accessed by a call to sem_open(). @subheading NOTES: @c @c @c @page @subsection sem_close - Close a named semaphore @findex sem_close @cindex close a named semaphore @subheading CALLING SEQUENCE: @ifset is-C @example int sem_close( sem_t *sem_close ); @end example @end ifset @ifset is-Ada @end ifset @subheading STATUS CODES: @table @b @item EACCES The semaphore argument is not a valid semaphore descriptor. @item ENOSYS The function sem_close is not supported by this implementation. @end table @subheading DESCRIPTION: The sem_close() function is used to indicate that the calling process is finished using the named semaphore indicated by sem. The function sem_close deallocates any system resources that were previously allocated by a sem_open system call. If sem_close() completes successfully it returns a 1, otherwise a value of -1 is return and errno is set. @subheading NOTES: @c @c @c @page @subsection sem_unlink - Unlink a semaphore @findex sem_unlink @cindex unlink a semaphore @subheading CALLING SEQUENCE: @ifset is-C @example int sem_unlink( const char *name ); @end example @end ifset @ifset is-Ada @end ifset @subheading STATUS CODES: @table @b @item EACCESS Permission is denied to unlink a semaphore. @item ENAMETOOLONG The length of the strong name exceed NAME_MAX while POSIX_NO_TRUNC is in effect. @item ENOENT The name of the semaphore does not exist. @item ENOSPC There is insufficient space for the creation of a new named semaphore. @item ENOSYS The function sem_unlink is not supported by this implementation. @end table @subheading DESCRIPTION: The sem_unlink() function shall remove the semaphore name by the string name. If a process is currently accessing the name semaphore, the sem_unlink command has no effect. If one or more processes have the semaphore open when the sem_unlink function is called, the destruction of semaphores shall be postponed until all reference to semaphore are destroyed by calls to sem_close, _exit(), or exec. After all references have been destroyed, it returns immediately. If the termination is successful, the function shall return 0. Otherwise, a -1 is returned and the errno is set. @subheading NOTES: @c @c @c @page @subsection sem_wait - Wait on a Semaphore @findex sem_wait @cindex wait on a semaphore @subheading CALLING SEQUENCE: @ifset is-C @example int sem_wait( sem_t *sem ); @end example @end ifset @ifset is-Ada @end ifset @subheading STATUS CODES: @table @b @item EINVAL The "sem" argument does not refer to a valid semaphore @end table @subheading DESCRIPTION: This function attempts to lock a semaphore specified by @code{sem}. If the semaphore is available, then the semaphore is locked (i.e., the semaphore value is decremented). If the semaphore is unavailable (i.e., the semaphore value is zero), then the function will block until the semaphore becomes available. It will then successfully lock the semaphore. The semaphore remains locked until released by a @code{sem_post()} call. If the call is unsuccessful, then the function returns -1 and sets errno to the appropriate error code. @subheading NOTES: Multiprocessing is not supported in this implementation. @c @c @c @page @subsection sem_trywait - Non-blocking Wait on a Semaphore @findex sem_trywait @cindex non @subheading CALLING SEQUENCE: @ifset is-C @example int sem_trywait( sem_t *sem ); @end example @end ifset @ifset is-Ada @end ifset @subheading STATUS CODES: @table @b @item EAGAIN The semaphore is not available (i.e., the semaphore value is zero), so the semaphore could not be locked. @item EINVAL The @code{sem} argument does not refewr to a valid semaphore @end table @subheading DESCRIPTION: This function attempts to lock a semaphore specified by @code{sem}. If the semaphore is available, then the semaphore is locked (i.e., the semaphore value is decremented) and the function returns a value of 0. The semaphore remains locked until released by a @code{sem_post()} call. If the semaphore is unavailable (i.e., the semaphore value is zero), then the function will return a value of -1 immediately and set @code{errno} to EAGAIN. If the call is unsuccessful, then the function returns -1 and sets @code{errno} to the appropriate error code. @subheading NOTES: Multiprocessing is not supported in this implementation. @c @c @c @page @subsection sem_timedwait - Wait on a Semaphore for a Specified Time @findex sem_timedwait @cindex wait on a semaphore for a specified time @subheading CALLING SEQUENCE: @ifset is-C @example int sem_timedwait( sem_t *sem, const struct timespec *timeout ); @end example @end ifset @ifset is-Ada @end ifset @subheading STATUS CODES: @table @b @item EAGAIN The semaphore is not available (i.e., the semaphore value is zero), so the semaphore could not be locked. @item EINVAL The @code{sem} argument does not refewr to a valid semaphore @end table @subheading DESCRIPTION: This function attemtps to lock a semaphore specified by @code{sem}, and will wait for the semaphore for an interval specified by @code{timeout}. If the semaphore is available, then the semaphore is locked (i.e., the semaphore value is decremented) and the function returns a value of 0. The semaphore remains locked until released by a @code{sem_post()} call. If the semaphore is unavailable, then the function will wait for the semaphore to become available for the amount of time specified by @code{timeout}. If the semaphore does not become available within the interval specified by @code{timeout}, then the function returns -1 and sets @code{errno} to EAGAIN. If any other error occurs, the function returns -1 and sets @code{errno} to the appropriate error code. @subheading NOTES: Multiprocessing is not supported in this implementation. @c @c @c @page @subsection sem_post - Unlock a Semaphore @findex sem_post @cindex unlock a semaphore @subheading CALLING SEQUENCE: @ifset is-C @example int sem_post( sem_t *sem ); @end example @end ifset @ifset is-Ada @end ifset @subheading STATUS CODES: @table @b @item EINVAL The @code{sem} argument does not refer to a valid semaphore @end table @subheading DESCRIPTION: This function attempts to release the semaphore specified by @code{sem}. If other tasks are waiting on the semaphore, then one of those tasks (which one depends on the scheduler being used) is allowed to lock the semaphore and return from its @code{sem_wait()}, @code{sem_trywait()}, or @code{sem_timedwait()} call. If there are no other tasks waiting on the semaphore, then the semaphore value is simply incremented. @code{sem_post()} returns 0 upon successful completion. If an error occurs, the function returns -1 and sets @code{errno} to the appropriate error code. @subheading NOTES: Multiprocessing is not supported in this implementation. @c @c @c @page @subsection sem_getvalue - Get the value of a semaphore @findex sem_getvalue @cindex get the value of a semaphore @subheading CALLING SEQUENCE: @ifset is-C @example int sem_getvalue( sem_t *sem, int *sval ); @end example @end ifset @ifset is-Ada @end ifset @subheading STATUS CODES: @table @b @item EINVAL The "sem" argument does not refer to a valid semaphore @item ENOSYS The function sem_getvalue is not supported by this implementation @end table @subheading DESCRIPTION: The sem_getvalue functions sets the location referenced by the "sval" argument to the value of the semaphore without affecting the state of the semaphore. The updated value represents a semaphore value that occurred at some point during the call, but is not necessarily the actual value of the semaphore when it returns to the calling process. If "sem" is locked, the value returned by sem_getvalue will be zero or a negative number whose absolute value is the number of processes waiting for the semaphore at some point during the call. @subheading NOTES: If the functions completes successfully, it shall return a value of zero. Otherwise, it shall return a value of -1 and set "errno" to specify the error that occurred.