10. Semaphore Manager

10.1. 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:

• sem_init - Initialize an unnamed semaphore

• sem_destroy - Destroy an unnamed semaphore

• sem_open - Open a named semaphore

• sem_close - Close a named semaphore

• sem_unlink - Remove a named semaphore

• sem_wait - Lock a semaphore

• sem_trywait - Lock a semaphore

• sem_timedwait - Wait on a Semaphore for a Specified Time

• sem_post - Unlock a semaphore

• sem_getvalue - Get the value of a semeaphore

10.2. Background

10.2.1. 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.

10.2.2. “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:

typedef int sem_t;

10.2.3. Building a Semaphore Attribute Set

10.3. Operations

10.3.1. 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.

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10.4. 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.

10.4.1. sem_init - Initialize an unnamed semaphore

CALLING SEQUENCE:

int sem_init(
    sem_t        *sem,
    int           pshared,
    unsigned int  value
);

STATUS CODES:

EINVAL	The value argument exceeds SEM_VALUE_MAX
ENOSPC	A resource required to initialize the semaphore has been exhausted The limit on semaphores (SEM_VALUE_MAX) has been reached
ENOSYS	The function sem_init is not supported by this implementation
EPERM	The process lacks appropriate privileges to initialize the semaphore

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.

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.

10.4.2. sem_destroy - Destroy an unnamed semaphore

CALLING SEQUENCE:

int sem_destroy(
    sem_t *sem
);

STATUS CODES:

EINVAL	The value argument exceeds SEM_VALUE_MAX
ENOSYS	The function sem_init is not supported by this implementation
EBUSY	There are currently processes blocked on the semaphore

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.

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.

10.4.3. sem_open - Open a named semaphore

CALLING SEQUENCE:

int sem_open(
    const char *name,
    int         oflag
);

ARGUMENTS:

The following flag bit may be set in oflag:

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.
O_EXCL	If O_EXCL and O_CREAT are set, all call to sem_open() shall fail if the semaphore name exists

STATUS CODES:

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.
EEXIST	O_CREAT and O_EXCL are set and the named semaphore already exists.
EINTR	The sem_open() operation was interrupted by a signal.
EINVAL	The sem_open() operation is not supported for the given name.
EMFILE	Too many semaphore descriptors or file descriptors in use by this process.
ENAMETOOLONG	The length of the name exceed PATH_MAX or name component is longer than NAME_MAX while POSIX_NO_TRUNC is in effect.
ENOENT	O_CREAT is not set and the named semaphore does not exist.
ENOSPC	There is insufficient space for the creation of a new named semaphore.
ENOSYS	The function sem_open() is not supported by this implementation.

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().

NOTES:

10.4.4. sem_close - Close a named semaphore

CALLING SEQUENCE:

int sem_close(
    sem_t *sem_close
);

STATUS CODES:

EACCES	The semaphore argument is not a valid semaphore descriptor.
ENOSYS	The function sem_close is not supported by this implementation.

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.

NOTES:

10.4.5. sem_unlink - Unlink a semaphore

CALLING SEQUENCE:

int sem_unlink(
    const char *name
);

STATUS CODES:

EACCESS	Permission is denied to unlink a semaphore.
ENAMETOOLONG	The length of the strong name exceed NAME_MAX while POSIX_NO_TRUNC is in effect.
ENOENT	The name of the semaphore does not exist.
ENOSPC	There is insufficient space for the creation of a new named semaphore.
ENOSYS	The function sem_unlink is not supported by this implementation.

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.

NOTES:

10.4.6. sem_wait - Wait on a Semaphore

CALLING SEQUENCE:

int sem_wait(
    sem_t *sem
);

STATUS CODES:

EINVAL

The sem argument does not refer to a valid semaphore

DESCRIPTION:

This function attempts to lock a semaphore specified by 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 sem_post() call.

If the call is unsuccessful, then the function returns -1 and sets errno to the appropriate error code.

NOTES:

Multiprocessing is not supported in this implementation.

10.4.7. sem_trywait - Non-blocking Wait on a Semaphore

CALLING SEQUENCE:

int sem_trywait(
    sem_t *sem
);

STATUS CODES:

EAGAIN	The semaphore is not available (i.e., the semaphore value is zero), so the semaphore could not be locked.
EINVAL	The sem argument does not refewr to a valid semaphore

DESCRIPTION:

This function attempts to lock a semaphore specified by 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 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 errno to EAGAIN.

If the call is unsuccessful, then the function returns -1 and sets errno to the appropriate error code.

NOTES:

Multiprocessing is not supported in this implementation.

10.4.8. sem_timedwait - Wait on a Semaphore for a Specified Time

CALLING SEQUENCE:

int sem_timedwait(
    sem_t                 *sem,
    const struct timespec *abstime
);

STATUS CODES:

EINVAL	The sem argument does not refewr to a valid semaphore
EINVAL	The nanoseconds field of timeout is invalid.
ETIMEDOUT	The calling thread was unable to get the semaphore within the specified timeout period.

DESCRIPTION:

This function attemtps to lock a semaphore specified by sem, and will wait for the semaphore until the absolute time specified by abstime. 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 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 timeout.

If the semaphore does not become available within the interval specified by timeout, then the function returns -1 and sets errno to EAGAIN. If any other error occurs, the function returns -1 and sets errno to the appropriate error code.

NOTES:

Multiprocessing is not supported in this implementation.

10.4.9. sem_post - Unlock a Semaphore

CALLING SEQUENCE:

int sem_post(
    sem_t *sem
);

STATUS CODES:

EINVAL

The sem argument does not refer to a valid semaphore

DESCRIPTION:

This function attempts to release the semaphore specified by 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 sem_wait(), sem_trywait(), or sem_timedwait() call. If there are no other tasks waiting on the semaphore, then the semaphore value is simply incremented. sem_post() returns 0 upon successful completion.

If an error occurs, the function returns -1 and sets errno to the appropriate error code.

NOTES:

Multiprocessing is not supported in this implementation.

10.4.10. sem_getvalue - Get the value of a semaphore

CALLING SEQUENCE:

int sem_getvalue(
    sem_t *sem,
    int   *sval
);

STATUS CODES:

EINVAL	The sem argument does not refer to a valid semaphore
ENOSYS	The function sem_getvalue is not supported by this implementation

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.

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.