Introduction

The Real-Time Priority Inheritance Library (librtpi) is intended to bridge the gap between the glibc pthread implementation and a functionally correct priority inheritance for pthread locking primitives, such as pthread_mutex and pthread_condvar.

Specifically, priority based wakeup is required for correct operation, in contrast to the more time-based ordered wakeup groups in the glibc pthread condvar implementation.

A C library with C++ bindings are provided.

Approach

This library provides an API that is as close as possible to the POSIX pthread specification. Any changes or restrictions to this API are the result of ensuring priority based wakeup ordering and proper behavior in the context of real-time and priority inheritance requirements.

Wherever possible, the glibc pthread primitives and functions are used, and where necessary are wrapped or rewritten. When the glibc implementation is incompatible, a new mechanism is provided, e.g. the pthread_condvar specifically.

To encourage programming to the API, internal types specific to the implementation are opaque types, or private class members in the case of the C++ bindings (using the "Has a" vs. "Is a" inheritance model).

Source

• main: https://gitlab.com/linux-rt/librtpi
• backup: https://github.com/linux-rt/librtpi
• report issues at: https://gitlab.com/linux-rt/librtpi/-/issues

Build

$ autoreconf --install
$ ./configure
$ make
$ make check

License and Copyright

The Real-Time Priority Inheritance Library is licensed under the Lesser GNU Public License. The LGPL was chosen to make it possible to link with libc libraries, reuse code from libc, and still be as broadly usable as possible in industry.

Copyright © 2018 VMware, Inc. All Rights Reserved.
Copyright © 2019-2023 National Instruments Corp. All Rights Reserved.

C Specification

Source Files

• rtpi.h
• pi_mutex.c
• pi_cond.c

Packaged Collateral

• rtpi.h
• librtpi.a
• librtpi.so

Types

pi_mutex_t

New primitive modeled after the POSIX pthread_mutex_t guaranteed to use priority inheritance.

pi_cond_t

New primitive modeled after the POSIX pthread_cond_t, with the following modifications.

1. All wakeup events will wake the N highest priority waiters.
2. Waiters will be woken in priority FIFO order.
3. The associated mutex must be held for all condition variable operations: init, wait, timedwait, signal, broadcast, and destroy.
4. The associated mutex must be passed as a parameter to the signal and broadcast calls. The mutex is used to requeue woken waiters and avoid the "thundering herd" effect.

Functions

PI Mutex

The PI Mutex API models a subset of the Pthread Mutex API, written specifically for priority inheritance aware condition variables.

int pi_mutex_init(pi_mutex_t *mutex, uint32_t flags)

Initializes a PI aware mutex. Allows for the specification of process private or process shared flag.

Where flags are:

• RTPI_MUTEX_PSHARED

And future flags may include

• RTPI_MUTEX_ERRORCHECK
• RTPI_MUTEX_ROBUST

The following attributes are not supported:

type:

• PTHREAD_MUTEX_RECURSIVE
• PTHREAD_MUTEX_ERRORCHECK

robust:

• PTHREAD_MUTEX_ROBUST

protocol:

• PTHREAD_PRIO_NONE
• PTHREAD_PRIO_PROTECT

Returns 0 on success, otherwise an error number is returned.

int pi_mutex_destroy(pi_mutex_t *mutex)

int pi_mutex_lock(pi_mutex_t *mutex)

int pi_mutex_trylock(pi_mutex_t *mutex)

int pi_mutex_unlock(pi_mutex_t *mutex)

PI Condition

The PI Condition API represents a new implementation of a Non-POSIX PI aware condition variable.

int pi_cond_init(pi_cond_t *cond, uint32_t flags)

Where flags are:

• RTPI_COND_PSHARED
• RTPI_COND_CLOCK_REALTIME

int pi_cond_destroy(pi_cond_t *cond)

int pi_cond_wait(pi_cond_t *cond, pi_mutex_t *mutex)

int pi_cond_timedwait(pi_cond_t *cond, pi_mutex_t *mutex, const struct timespec *restrict abstime)

int pi_cond_signal(pi_cond_t *cond, pi_mutex_t *mutex)

int pi_cond_broadcast(pi_cond_t *cond, pi_mutex_t *mutex)

Initializers

DEFINE_PI_MUTEX(mutex, flags)

Defines and initializes a PI aware mutex.

DEFINE_PI_COND(condvar, flags)

Defines and initializes a PI aware conditional variable.

C++ Specification

Source files

• rtpi/mutex.hpp
• rtpi/condition_variable.hpp

Types

rtpi::mutex

Wrapper around the rtpi pi_mutex_t that is intended to work as a replacement for std::mutex.

rtpi::condition_variable

Wrapper around the rtpi pi_cond_t that is intended to work mostly as a drop-in replacement for std::condition_variable.

Notable differences from std::condition_variable:

• std::unique_lock<rtpi::mutex> is used for the wait methods instead of std::unique_lock<std::mutex>
• notify_one and notify_all require a std::unique_lock<rtpi::mutex> parameter

References

1. POSIX: The Open Group Base Specifications Issue 7, 2018 edition
2. Requeue-PI: Making Glibc Condvars PI-Aware
3. Bug 11588 - pthread condvars are not priority inheritance aware
4. Embedded Linux Conference 2020 presentation "librtpi: Conditional Variables for Real-Time Applications": slides, video