10. Semaphore Manager — RTEMS POSIX API Guide 5.4-rc2 (21st August 2026) documentation

# 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.

There is currently no text in this section.

## 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.