There are five conditions that proximity sensor disable during a phone call.
These conditions is default settings of Phone application in android.

You can find the source code in PhoneApp.java,

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2月 03 週四 201122:44
[Android] Sensor JNI Layer - Android 2.3 vs others

[Android 2.3]

/*
* Copyright (C) 2008 The Android Open Source Project
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
#ifndef ANDROID_SENSORS_INTERFACE_H
#define ANDROID_SENSORS_INTERFACE_H
#include <stdint.h>
#include <sys/cdefs.h>
#include <sys/types.h>
#include <hardware/hardware.h>
#include <cutils/native_handle.h>
__BEGIN_DECLS
/**
* The id of this module
*/
#define SENSORS_HARDWARE_MODULE_ID "sensors"
/**
* Name of the sensors device to open
*/
#define SENSORS_HARDWARE_POLL "poll"
/**
* Handles must be higher than SENSORS_HANDLE_BASE and must be unique.
* A Handle identifies a given sensors. The handle is used to activate
* and/or deactivate sensors.
* In this version of the API there can only be 256 handles.
*/
#define SENSORS_HANDLE_BASE 0
#define SENSORS_HANDLE_BITS 8
#define SENSORS_HANDLE_COUNT (1<<SENSORS_HANDLE_BITS)
/**
* Sensor types
*/
#define SENSOR_TYPE_ACCELEROMETER 1
#define SENSOR_TYPE_MAGNETIC_FIELD 2
#define SENSOR_TYPE_ORIENTATION 3
#define SENSOR_TYPE_GYROSCOPE 4
#define SENSOR_TYPE_LIGHT 5
#define SENSOR_TYPE_PRESSURE 6
#define SENSOR_TYPE_TEMPERATURE 7
#define SENSOR_TYPE_PROXIMITY 8
#define SENSOR_TYPE_GRAVITY 9
#define SENSOR_TYPE_LINEAR_ACCELERATION 10
#define SENSOR_TYPE_ROTATION_VECTOR 11
/**
* Values returned by the accelerometer in various locations in the universe.
* all values are in SI units (m/s^2)
*/
#define GRAVITY_SUN (275.0f)
#define GRAVITY_EARTH (9.80665f)
/** Maximum magnetic field on Earth's surface */
#define MAGNETIC_FIELD_EARTH_MAX (60.0f)
/** Minimum magnetic field on Earth's surface */
#define MAGNETIC_FIELD_EARTH_MIN (30.0f)
/**
* status of each sensor
*/
#define SENSOR_STATUS_UNRELIABLE 0
#define SENSOR_STATUS_ACCURACY_LOW 1
#define SENSOR_STATUS_ACCURACY_MEDIUM 2
#define SENSOR_STATUS_ACCURACY_HIGH 3
/**
* Definition of the axis
* ----------------------
*
* This API is relative to the screen of the device in its default orientation,
* that is, if the device can be used in portrait or landscape, this API
* is only relative to the NATURAL orientation of the screen. In other words,
* the axis are not swapped when the device's screen orientation changes.
* Higher level services /may/ perform this transformation.
*
* x<0 x>0
* ^
* |
* +-----------+--> y>0
* | |
* | |
* | |
* | | / z<0
* | | /
* | | /
* O-----------+/
* |[] [ ] []/
* +----------/+ y<0
* /
* /
* |/ z>0 (toward the sky)
*
* O: Origin (x=0,y=0,z=0)
*
*
* Orientation
* -----------
*
* All values are angles in degrees.
*
* Orientation sensors return sensor events for all 3 axes at a constant
* rate defined by setDelay().
*
* azimuth: angle between the magnetic north direction and the Y axis, around
* the Z axis (0<=azimuth<360).
* 0=North, 90=East, 180=South, 270=West
*
* pitch: Rotation around X axis (-180<=pitch<=180), with positive values when
* the z-axis moves toward the y-axis.
*
* roll: Rotation around Y axis (-90<=roll<=90), with positive values when
* the x-axis moves towards the z-axis.
*
* Note: For historical reasons the roll angle is positive in the clockwise
* direction (mathematically speaking, it should be positive in the
* counter-clockwise direction):
*
* Z
* ^
* (+roll) .--> |
* / |
* | | roll: rotation around Y axis
* X <-------(.)
* Y
* note that +Y == -roll
*
*
*
* Note: This definition is different from yaw, pitch and roll used in aviation
* where the X axis is along the long side of the plane (tail to nose).
*
*
* Acceleration
* ------------
*
* All values are in SI units (m/s^2) and measure the acceleration of the
* device minus the force of gravity.
*
* Acceleration sensors return sensor events for all 3 axes at a constant
* rate defined by setDelay().
*
* x: Acceleration minus Gx on the x-axis
* y: Acceleration minus Gy on the y-axis
* z: Acceleration minus Gz on the z-axis
*
* Examples:
* When the device lies flat on a table and is pushed on its left side
* toward the right, the x acceleration value is positive.
*
* When the device lies flat on a table, the acceleration value is +9.81,
* which correspond to the acceleration of the device (0 m/s^2) minus the
* force of gravity (-9.81 m/s^2).
*
* When the device lies flat on a table and is pushed toward the sky, the
* acceleration value is greater than +9.81, which correspond to the
* acceleration of the device (+A m/s^2) minus the force of
* gravity (-9.81 m/s^2).
*
*
* Magnetic Field
* --------------
*
* All values are in micro-Tesla (uT) and measure the ambient magnetic
* field in the X, Y and Z axis.
*
* Magnetic Field sensors return sensor events for all 3 axes at a constant
* rate defined by setDelay().
*
* Gyroscope
* ---------
* All values are in radians/second and measure the rate of rotation
* around the X, Y and Z axis. The coordinate system is the same as is
* used for the acceleration sensor. Rotation is positive in the
* counter-clockwise direction (right-hand rule). That is, an observer
* looking from some positive location on the x, y or z axis at a device
* positioned on the origin would report positive rotation if the device
* appeared to be rotating counter clockwise. Note that this is the
* standard mathematical definition of positive rotation and does not agree
* with the definition of roll given earlier.
* The range should at least be 17.45 rad/s (ie: ~1000 deg/s).
*
* Proximity
* ---------
*
* The distance value is measured in centimeters. Note that some proximity
* sensors only support a binary "close" or "far" measurement. In this case,
* the sensor should report its maxRange value in the "far" state and a value
* less than maxRange in the "near" state.
*
* Proximity sensors report a value only when it changes and each time the
* sensor is enabled. setDelay() is ignored.
*
* Light
* -----
*
* The light sensor value is returned in SI lux units.
*
* Light sensors report a value only when it changes and each time the
* sensor is enabled. setDelay() is ignored.
*
* Pressure
* --------
*
* The pressure sensor value is returned in hectopascal (hPa)
*
* Pressure sensors report events at a constant rate defined by setDelay().
*
* Gravity
* -------
* A gravity output indicates the direction of and magnitude of gravity in the devices's
* coordinates. On Earth, the magnitude is 9.8. Units are m/s^2. The coordinate system
* is the same as is used for the acceleration sensor.
*
* Linear Acceleration
* -------------------
* Indicates the linear acceleration of the device in device coordinates, not including gravity.
* This output is essentially Acceleration - Gravity. Units are m/s^2. The coordinate system is
* the same as is used for the acceleration sensor.
*
* Rotation Vector
* ---------------
* A rotation vector represents the orientation of the device as a combination
* of an angle and an axis, in which the device has rotated through an angle
* theta around an axis <x, y, z>. The three elements of the rotation vector
* are <x*sin(theta/2), y*sin(theta/2), z*sin(theta/2)>, such that the magnitude
* of the rotation vector is equal to sin(theta/2), and the direction of the
* rotation vector is equal to the direction of the axis of rotation. The three
* elements of the rotation vector are equal to the last three components of a
* unit quaternion <cos(theta/2), x*sin(theta/2), y*sin(theta/2), z*sin(theta/2)>.
* Elements of the rotation vector are unitless. The x, y, and z axis are defined
* in the same was as for the acceleration sensor.
*/
typedef struct {
   union {
   float v[3];
   struct {
   float x;
   float y;
   float z;
   };
   struct {
   float azimuth;
   float pitch;
   float roll;
   };
   };
   int8_t status;
   uint8_t reserved[3];
} sensors_vec_t;
/**
* Union of the various types of sensor data
* that can be returned.
*/
typedef struct sensors_event_t {
   /* must be sizeof(struct sensors_event_t) */
   int32_t version;
   /* sensor identifier */
   int32_t sensor;
   /* sensor type */
   int32_t type;
   /* reserved */
   int32_t reserved0;
   /* time is in nanosecond */
   int64_t timestamp;
   union {
   float data[16];
   /* acceleration values are in meter per second per second (m/s^2) */
   sensors_vec_t acceleration;
   /* magnetic vector values are in micro-Tesla (uT) */
   sensors_vec_t magnetic;
   /* orientation values are in degrees */
   sensors_vec_t orientation;
   /* gyroscope values are in rad/s */
   sensors_vec_t gyro;
   /* temperature is in degrees centigrade (Celsius) */
   float temperature;
   /* distance in centimeters */
   float distance;
   /* light in SI lux units */
   float light;
   /* pressure in hectopascal (hPa) */
   float pressure;
   };
   uint32_t reserved1[4];
} sensors_event_t;
struct sensor_t;
/**
* Every hardware module must have a data structure named HAL_MODULE_INFO_SYM
* and the fields of this data structure must begin with hw_module_t
* followed by module specific information.
*/
struct sensors_module_t {
   struct hw_module_t common;
   /**
   * Enumerate all available sensors. The list is returned in "list".
   * @return number of sensors in the list
   */
   int (*get_sensors_list)(struct sensors_module_t* module,
   struct sensor_t const** list);
};
struct sensor_t {
   /* name of this sensors */
   const char* name;
   /* vendor of the hardware part */
   const char* vendor;
   /* version of the hardware part + driver. The value of this field is
   * left to the implementation and doesn't have to be monotonically
   * increasing.
   */
   int version;
   /* handle that identifies this sensors. This handle is used to activate
   * and deactivate this sensor. The value of the handle must be 8 bits
   * in this version of the API.
   */
   int handle;
   /* this sensor's type. */
   int type;
   /* maximaum range of this sensor's value in SI units */
   float maxRange;
   /* smallest difference between two values reported by this sensor */
   float resolution;
   /* rough estimate of this sensor's power consumption in mA */
   float power;
   /* minimum delay allowed between events in microseconds. A value of zero
   * means that this sensor doesn't report events at a constant rate, but
   * rather only when a new data is available */
   int32_t minDelay;
   /* reserved fields, must be zero */
   void* reserved[8];
};
/**
* Every device data structure must begin with hw_device_t
* followed by module specific public methods and attributes.
*/
struct sensors_poll_device_t {
   struct hw_device_t common;
   /** Activate/deactivate one sensor.
   *
   * @param handle is the handle of the sensor to change.
   * @param enabled set to 1 to enable, or 0 to disable the sensor.
   *
   * @return 0 on success, negative errno code otherwise
   */
   int (*activate)(struct sensors_poll_device_t *dev,
   int handle, int enabled);
   /**
   * Set the delay between sensor events in nanoseconds for a given sensor.
   * It is an error to set a delay inferior to the value defined by
   * sensor_t::minDelay. If sensor_t::minDelay is zero, setDelay() is
   * ignored and returns 0.
   *
   * @return 0 if successful, < 0 on error
   */
   int (*setDelay)(struct sensors_poll_device_t *dev,
   int handle, int64_t ns);
   /**
   * Returns an array of sensor data.
   * This function must block until events are available.
   *
   * @return the number of events read on success, or -errno in case of an error.
   * This function should never return 0 (no event).
   *
   */
   int (*poll)(struct sensors_poll_device_t *dev,
   sensors_event_t* data, int count);
};
/** convenience API for opening and closing a device */
static inline int sensors_open(const struct hw_module_t* module,
   struct sensors_poll_device_t** device) {
   return module->methods->open(module,
   SENSORS_HARDWARE_POLL, (struct hw_device_t**)device);
}
static inline int sensors_close(struct sensors_poll_device_t* device) {
   return device->common.close(&device->common);
}
__END_DECLS
#include <hardware/sensors_deprecated.h>
#endif // ANDROID_SENSORS_INTERFACE_H

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12月 26 週日 201019:41
pthread introduction

Refer to https://computing.llnl.gov/tutorials/pthreads/

Privacy & Legal Notice

POSIX Threads Programming

Blaise Barney, Lawrence Livermore National Laboratory

UCRL-MI-133316

Abstract

Pthreads Overview

The Pthreads API

Compiling Threaded Programs

Thread Management

Mutex Variables

Condition Variables

LLNL Specific Information and Recommendations

Topics Not Covered

Pthread Library Routines Reference

References and More Information

Exercise

Abstract

In shared memory multiprocessor architectures, such as SMPs, threads can be used to implement parallelism. Historically, hardware vendors have implemented their own proprietary versions of threads, making portability a concern for software developers. For UNIX systems, a standardized C language threads programming interface has been specified by the IEEE POSIX 1003.1c standard. Implementations that adhere to this standard are referred to as POSIX threads, or Pthreads.

The tutorial begins with an introduction to concepts, motivations, and design considerations for using Pthreads. Each of the three major classes of routines in the Pthreads API are then covered: Thread Management, Mutex Variables, and Condition Variables. Example codes are used throughout to demonstrate how to use most of the Pthreads routines needed by a new Pthreads programmer. The tutorial concludes with a discussion of LLNL specifics and how to mix MPI with pthreads. A lab exercise, with numerous example codes (C Language) is also included.

Level/Prerequisites: Ideal for those who are new to parallel programming with threads. A basic understanding of parallel programming in C is assumed. For those who are unfamiliar with Parallel Programming in general, the material covered in EC3500: Introduction To Parallel Computing would be helpful.

Pthreads Overview

What is a Thread?

Technically, a thread is defined as an independent stream of instructions that can be scheduled to run as such by the operating system. But what does this mean?

To the software developer, the concept of a "procedure" that runs independently from its main program may best describe a thread.

To go one step further, imagine a main program (a.out) that contains a number of procedures. Then imagine all of these procedures being able to be scheduled to run simultaneously and/or independently by the operating system. That would describe a "multi-threaded" program.

How is this accomplished?

Before understanding a thread, one first needs to understand a UNIX process. A process is created by the operating system, and requires a fair amount of "overhead". Processes contain information about program resources and program execution state, including:
- Process ID, process group ID, user ID, and group ID
- Environment
- Working directory.
- Program instructions
- Registers
- Stack
- Heap
- File descriptors
- Signal actions
- Shared libraries
- Inter-process communication tools (such as message queues, pipes, semaphores, or shared memory).
UNIX PROCESS THREADS WITHIN A UNIX PROCESS

Threads use and exist within these process resources, yet are able to be scheduled by the operating system and run as independent entities largely because they duplicate only the bare essential resources that enable them to exist as executable code.

This independent flow of control is accomplished because a thread maintains its own:
- Stack pointer
- Registers
- Scheduling properties (such as policy or priority)
- Set of pending and blocked signals
- Thread specific data.

So, in summary, in the UNIX environment a thread:
- Exists within a process and uses the process resources
- Has its own independent flow of control as long as its parent process exists and the OS supports it
- Duplicates only the essential resources it needs to be independently schedulable
- May share the process resources with other threads that act equally independently (and dependently)
- Dies if the parent process dies - or something similar
- Is "lightweight" because most of the overhead has already been accomplished through the creation of its process.

Because threads within the same process share resources:
- Changes made by one thread to shared system resources (such as closing a file) will be seen by all other threads.
- Two pointers having the same value point to the same data.
- Reading and writing to the same memory locations is possible, and therefore requires explicit synchronization by the programmer.

Pthreads Overview

What are Pthreads?

Historically, hardware vendors have implemented their own proprietary versions of threads. These implementations differed substantially from each other making it difficult for programmers to develop portable threaded applications.

In order to take full advantage of the capabilities provided by threads, a standardized programming interface was required.
- For UNIX systems, this interface has been specified by the IEEE POSIX 1003.1c standard (1995).
- Implementations adhering to this standard are referred to as POSIX threads, or Pthreads.
- Most hardware vendors now offer Pthreads in addition to their proprietary API's.

The POSIX standard has continued to evolve and undergo revisions, including the Pthreads specification. The latest version is known as IEEE Std 1003.1, 2004 Edition.

Some useful links:
- POSIX FAQs: www.opengroup.org/austin/papers/posix_faq.html
- Download the Standard: www.unix.org/version3/ieee_std.html

Pthreads are defined as a set of C language programming types and procedure calls, implemented with a pthread.h header/include file and a thread library - though this library may be part of another library, such as libc, in some implementations.

Pthreads Overview

Why Pthreads?

The primary motivation for using Pthreads is to realize potential program performance gains.

When compared to the cost of creating and managing a process, a thread can be created with much less operating system overhead. Managing threads requires fewer system resources than managing processes.

For example, the following table compares timing results for the fork() subroutine and the pthread_create() subroutine. Timings reflect 50,000 process/thread creations, were performed with the time utility, and units are in seconds, no optimization flags.

Note: don't expect the sytem and user times to add up to real time, because these are SMP systems with multiple CPUs working on the problem at the same time. At best, these are approximations run on local machines, past and present.

Platform	`fork()`			`pthread_create()`
Platform	real	user	sys	real	user	sys
AMD 2.3 GHz Opteron (16cpus/node)	12.5	1.0	12.5	1.2	0.2	1.3
AMD 2.4 GHz Opteron (8cpus/node)	17.6	2.2	15.7	1.4	0.3	1.3
IBM 4.0 GHz POWER6 (8cpus/node)	9.5	0.6	8.8	1.6	0.1	0.4
IBM 1.9 GHz POWER5 p5-575 (8cpus/node)	64.2	30.7	27.6	1.7	0.6	1.1
IBM 1.5 GHz POWER4 (8cpus/node)	104.5	48.6	47.2	2.1	1.0	1.5
INTEL 2.4 GHz Xeon (2 cpus/node)	54.9	1.5	20.8	1.6	0.7	0.9
INTEL 1.4 GHz Itanium2 (4 cpus/node)	54.5	1.1	22.2	2.0	1.2	0.6

fork_vs_thread.txt

All threads within a process share the same address space. Inter-thread communication is more efficient and in many cases, easier to use than inter-process communication.

Threaded applications offer potential performance gains and practical advantages over non-threaded applications in several other ways:
- Overlapping CPU work with I/O: For example, a program may have sections where it is performing a long I/O operation. While one thread is waiting for an I/O system call to complete, CPU intensive work can be performed by other threads.
- Priority/real-time scheduling: tasks which are more important can be scheduled to supersede or interrupt lower priority tasks.
- Asynchronous event handling: tasks which service events of indeterminate frequency and duration can be interleaved. For example, a web server can both transfer data from previous requests and manage the arrival of new requests.

The primary motivation for considering the use of Pthreads on an SMP architecture is to achieve optimum performance. In particular, if an application is using MPI for on-node communications, there is a potential that performance could be greatly improved by using Pthreads for on-node data transfer instead.

For example:

MPI libraries usually implement on-node task communication via shared memory, which involves at least one memory copy operation (process to process).

For Pthreads there is no intermediate memory copy required because threads share the same address space within a single process. There is no data transfer, per se. It becomes more of a cache-to-CPU or memory-to-CPU bandwidth (worst case) situation. These speeds are much higher.

Some local comparisons are shown below:

Platform	MPI Shared Memory Bandwidth (GB/sec)	Pthreads Worst Case Memory-to-CPU Bandwidth (GB/sec)
AMD 2.3 GHz Opteron	1.8	5.3
AMD 2.4 GHz Opteron	1.2	5.3
IBM 1.9 GHz POWER5 p5-575	4.1	16
IBM 1.5 GHz POWER4	2.1	4
Intel 2.4 GHz Xeon	0.3	4.3
Intel 1.4 GHz Itanium 2	1.8	6.4

Pthreads Overview

Designing Threaded Programs

Parallel Programming:

On modern, multi-cpu machines, pthreads are ideally suited for parallel programming, and whatever applies to parallel programming in general, applies to parallel pthreads programs.

There are many considerations for designing parallel programs, such as:
- What type of parallel programming model to use?
- Problem partitioning
- Load balancing
- Communications
- Data dependencies
- Synchronization and race conditions
- Memory issues
- I/O issues
- Program complexity
- Programmer effort/costs/time
- ...

Covering these topics is beyond the scope of this tutorial, however interested readers can obtain a quick overview in the Introduction to Parallel Computing tutorial.

In general though, in order for a program to take advantage of Pthreads, it must be able to be organized into discrete, independent tasks which can execute concurrently. For example, if routine1 and routine2 can be interchanged, interleaved and/or overlapped in real time, they are candidates for threading.

Programs having the following characteristics may be well suited for pthreads:
- Work that can be executed, or data that can be operated on, by multiple tasks simultaneously
- Block for potentially long I/O waits
- Use many CPU cycles in some places but not others
- Must respond to asynchronous events
- Some work is more important than other work (priority interrupts)

Pthreads can also be used for serial applications, to emulate parallel execution. A perfect example is the typical web browser, which for most people, runs on a single cpu desktop/laptop machine. Many things can "appear" to be happening at the same time.

Several common models for threaded programs exist:
- Manager/worker: a single thread, the manager assigns work to other threads, the workers. Typically, the manager handles all input and parcels out work to the other tasks. At least two forms of the manager/worker model are common: static worker pool and dynamic worker pool.
- Pipeline: a task is broken into a series of suboperations, each of which is handled in series, but concurrently, by a different thread. An automobile assembly line best describes this model.
- Peer: similar to the manager/worker model, but after the main thread creates other threads, it participates in the work.

Shared Memory Model:

All threads have access to the same global, shared memory

Threads also have their own private data

Programmers are responsible for synchronizing access (protecting) globally shared data.

Thread-safeness:

Thread-safeness: in a nutshell, refers an application's ability to execute multiple threads simultaneously without "clobbering" shared data or creating "race" conditions.

For example, suppose that your application creates several threads, each of which makes a call to the same library routine:
- This library routine accesses/modifies a global structure or location in memory.
- As each thread calls this routine it is possible that they may try to modify this global structure/memory location at the same time.
- If the routine does not employ some sort of synchronization constructs to prevent data corruption, then it is not thread-safe.

The implication to users of external library routines is that if you aren't 100% certain the routine is thread-safe, then you take your chances with problems that could arise.

Recommendation: Be careful if your application uses libraries or other objects that don't explicitly guarantee thread-safeness. When in doubt, assume that they are not thread-safe until proven otherwise. This can be done by "serializing" the calls to the uncertain routine, etc.

The Pthreads API

The original Pthreads API was defined in the ANSI/IEEE POSIX 1003.1 - 1995 standard. The POSIX standard has continued to evolve and undergo revisions, including the Pthreads specification. The latest version is known as IEEE Std 1003.1, 2004 Edition.

Copies of the standard can be purchased from IEEE or downloaded for free from www.unix.org/version3/ieee_std.html.

The subroutines which comprise the Pthreads API can be informally grouped into four major groups:
1. Thread management: Routines that work directly on threads - creating, detaching, joining, etc. They also include functions to set/query thread attributes (joinable, scheduling etc.)
2. Mutexes: Routines that deal with synchronization, called a "mutex", which is an abbreviation for "mutual exclusion". Mutex functions provide for creating, destroying, locking and unlocking mutexes. These are supplemented by mutex attribute functions that set or modify attributes associated with mutexes.
3. Condition variables: Routines that address communications between threads that share a mutex. Based upon programmer specified conditions. This group includes functions to create, destroy, wait and signal based upon specified variable values. Functions to set/query condition variable attributes are also included.
4. Synchronization: Routines that manage read/write locks and barriers.

Naming conventions: All identifiers in the threads library begin with pthread_. Some examples are shown below.

Routine Prefix	Functional Group
pthread_	Threads themselves and miscellaneous subroutines
pthread_attr_	Thread attributes objects
pthread_mutex_	Mutexes
pthread_mutexattr_	Mutex attributes objects.
pthread_cond_	Condition variables
pthread_condattr_	Condition attributes objects
pthread_key_	Thread-specific data keys
pthread_rwlock_	Read/write locks
pthread_barrier_	Synchronization barriers

The concept of opaque objects pervades the design of the API. The basic calls work to create or modify opaque objects - the opaque objects can be modified by calls to attribute functions, which deal with opaque attributes.

The Pthreads API contains around 100 subroutines. This tutorial will focus on a subset of these - specifically, those which are most likely to be immediately useful to the beginning Pthreads programmer.

For portability, the pthread.h header file should be included in each source file using the Pthreads library.

The current POSIX standard is defined only for the C language. Fortran programmers can use wrappers around C function calls. Some Fortran compilers (like IBM AIX Fortran) may provide a Fortram pthreads API.

A number of excellent books about Pthreads are available. Several of these are listed in the References section of this tutorial.

Compiling Threaded Programs

Several examples of compile commands used for pthreads codes are listed in the table below.

Compiler / Platform	Compiler Command	Description
IBM AIX	`xlc_r / cc_r`	C (ANSI / non-ANSI)
	`xlC_r`	C++
	`xlf_r -qnosave xlf90_r -qnosave`	Fortran - using IBM's Pthreads API (non-portable)
INTEL Linux	`icc -pthread`	C
INTEL Linux	`icpc -pthread`	C++
PathScale Linux	`pathcc -pthread`	C
PathScale Linux	`pathCC -pthread`	C++
PGI Linux	`pgcc -lpthread`	C
PGI Linux	`pgCC -lpthread`	C++
GNU Linux, AIX	`gcc -pthread`	GNU C
GNU Linux, AIX	`g++ -pthread`	GNU C++

Thread Management

Creating and Terminating Threads

Routines:

pthread_create (thread,attr,start_routine,arg) pthread_exit (status) pthread_attr_init (attr)

pthread_attr_destroy (attr)

Creating Threads:

Initially, your main() program comprises a single, default thread. All other threads must be explicitly created by the programmer.

pthread_create creates a new thread and makes it executable. This routine can be called any number of times from anywhere within your code.

pthread_create arguments:
- thread: An opaque, unique identifier for the new thread returned by the subroutine.
- attr: An opaque attribute object that may be used to set thread attributes. You can specify a thread attributes object, or NULL for the default values.
- start_routine: the C routine that the thread will execute once it is created.
- arg: A single argument that may be passed to start_routine. It must be passed by reference as a pointer cast of type void. NULL may be used if no argument is to be passed.

The maximum number of threads that may be created by a process is implementation dependent.

Once created, threads are peers, and may create other threads. There is no implied hierarchy or dependency between threads.

Question: After a thread has been created, how do you know when it will be scheduled to run by the operating system?

Thread Attributes:

By default, a thread is created with certain attributes. Some of these attributes can be changed by the programmer via the thread attribute object.

pthread_attr_init and pthread_attr_destroy are used to initialize/destroy the thread attribute object.

Other routines are then used to query/set specific attributes in the thread attribute object.

Some of these attributes will be discussed later.

Terminating Threads:

There are several ways in which a Pthread may be terminated:
- The thread returns from its starting routine (the main routine for the initial thread).
- The thread makes a call to the pthread_exit subroutine (covered below).
- The thread is canceled by another thread via the pthread_cancel routine (not covered here).
- The entire process is terminated due to a call to either the exec or exit subroutines.

pthread_exit is used to explicitly exit a thread. Typically, the pthread_exit() routine is called after a thread has completed its work and is no longer required to exist.

If main() finishes before the threads it has created, and exits with pthread_exit(), the other threads will continue to execute. Otherwise, they will be automatically terminated when main() finishes.

The programmer may optionally specify a termination status, which is stored as a void pointer for any thread that may join the calling thread.

Cleanup: the pthread_exit() routine does not close files; any files opened inside the thread will remain open after the thread is terminated.

Discussion: In subroutines that execute to completion normally, you can often dispense with calling pthread_exit() - unless, of course, you want to pass a return code back. However, in main(), there is a definite problem if main() completes before the threads it spawned. If you don't callpthread_exit() explicitly, when main() completes, the process (and all threads) will be terminated. By calling pthread_exit() in main(), the process and all of its threads will be kept alive even though all of the code in main() has been executed.

Example: Pthread Creation and Termination

This simple example code creates 5 threads with the pthread_create() routine. Each thread prints a "Hello World!" message, and then terminates with a call to pthread_exit().

Example Code - Pthread Creation and Termination

#include <pthread.h>
#include <stdio.h>
#define NUM_THREADS 5
void *PrintHello(void *threadid)
{
long tid;
tid = (long)threadid;
printf("Hello World! It's me, thread #%ld!\n", tid);
pthread_exit(NULL);
}
int main (int argc, char *argv[])
{
pthread_t threads[NUM_THREADS];
int rc;
long t;
for(t=0; t<NUM_THREADS; t++){
printf("In main: creating thread %ld\n", t);
rc = pthread_create(&threads[t], NULL, PrintHello, (void *)t);
if (rc){
printf("ERROR; return code from pthread_create() is %d\n", rc);
exit(-1);
}
}
pthread_exit(NULL);
}

Thread Management

Passing Arguments to Threads

The pthread_create() routine permits the programmer to pass one argument to the thread start routine. For cases where multiple arguments must be passed, this limitation is easily overcome by creating a structure which contains all of the arguments, and then passing a pointer to that structure in thepthread_create() routine.

All arguments must be passed by reference and cast to (void *).

Question: How can you safely pass data to newly created threads, given their non-deterministic start-up and scheduling?

Example 1 - Thread Argument Passing

long *taskids[NUM_THREADS];
for(t=0; t<NUM_THREADS; t++)
{
taskids[t] = (long *) malloc(sizeof(long));
*taskids[t] = t;
printf("Creating thread %ld\n", t);
rc = pthread_create(&threads[t], NULL, PrintHello, (void *) taskids[t]);
...
}

Example 2 - Thread Argument Passing

struct thread_data{
int thread_id;
int sum;
char *message;
};
struct thread_data thread_data_array[NUM_THREADS];
void *PrintHello(void *threadarg)
{
struct thread_data *my_data;
...
my_data = (struct thread_data *) threadarg;
taskid = my_data->thread_id;
sum = my_data->sum;
hello_msg = my_data->message;
...
}
int main (int argc, char *argv[])
{
...
thread_data_array[t].thread_id = t;
thread_data_array[t].sum = sum;
thread_data_array[t].message = messages[t];
rc = pthread_create(&threads[t], NULL, PrintHello, 
(void *) &thread_data_array[t]);
...
}

Example 3 - Thread Argument Passing (Incorrect)

address

t

int rc;
long t;
for(t=0; t<NUM_THREADS; t++) 
{
printf("Creating thread %ld\n", t);
rc = pthread_create(&threads[t], NULL, PrintHello, (void *) &t);
...
}

Thread Management

Joining and Detaching Threads

Routines:

pthread_join (threadid,status) pthread_detach (threadid) pthread_attr_setdetachstate (attr,detachstate)

pthread_attr_getdetachstate (attr,detachstate)

Joining:

"Joining" is one way to accomplish synchronization between threads. For example:

The pthread_join() subroutine blocks the calling thread until the specified threadid thread terminates.

The programmer is able to obtain the target thread's termination return status if it was specified in the target thread's call to pthread_exit().

A joining thread can match one pthread_join() call. It is a logical error to attempt multiple joins on the same thread.

Two other synchronization methods, mutexes and condition variables, will be discussed later.

Joinable or Not?

When a thread is created, one of its attributes defines whether it is joinable or detached. Only threads that are created as joinable can be joined. If a thread is created as detached, it can never be joined.

The final draft of the POSIX standard specifies that threads should be created as joinable.

To explicitly create a thread as joinable or detached, the attr argument in the pthread_create() routine is used. The typical 4 step process is:
1. Declare a pthread attribute variable of the pthread_attr_t data type
2. Initialize the attribute variable with pthread_attr_init()
3. Set the attribute detached status with pthread_attr_setdetachstate()
4. When done, free library resources used by the attribute with pthread_attr_destroy()

Detaching:

The pthread_detach() routine can be used to explicitly detach a thread even though it was created as joinable.

There is no converse routine.

Recommendations:

If a thread requires joining, consider explicitly creating it as joinable. This provides portability as not all implementations may create threads as joinable by default.

If you know in advance that a thread will never need to join with another thread, consider creating it in a detached state. Some system resources may be able to be freed.

Example: Pthread Joining

Example Code - Pthread Joining

#include <pthread.h>
#include <stdio.h>
#include <stdlib.h>
#include <math.h>
#define NUM_THREADS 4
void *BusyWork(void *t)
{
int i;
long tid;
double result=0.0;
tid = (long)t;
printf("Thread %ld starting...\n",tid);
for (i=0; i<1000000; i++)
{
result = result + sin(i) * tan(i);
}
printf("Thread %ld done. Result = %e\n",tid, result);
pthread_exit((void*) t);
}
int main (int argc, char *argv[])
{
pthread_t thread[NUM_THREADS];
pthread_attr_t attr;
int rc;
long t;
void *status;
/* Initialize and set thread detached attribute */
pthread_attr_init(&attr);
pthread_attr_setdetachstate(&attr, PTHREAD_CREATE_JOINABLE);
for(t=0; t<NUM_THREADS; t++) {
printf("Main: creating thread %ld\n", t);
rc = pthread_create(&thread[t], &attr, BusyWork, (void *)t); 
if (rc) {
printf("ERROR; return code from pthread_create() 
is %d\n", rc);
exit(-1);
}
}
/* Free attribute and wait for the other threads */
pthread_attr_destroy(&attr);
for(t=0; t<NUM_THREADS; t++) {
rc = pthread_join(thread[t], &status);
if (rc) {
printf("ERROR; return code from pthread_join() 
is %d\n", rc);
exit(-1);
}
printf("Main: completed join with thread %ld having a status 
of %ld\n",t,(long)status);
}
printf("Main: program completed. Exiting.\n");
pthread_exit(NULL);
}

Thread Management

Stack Management

Routines:

pthread_attr_getstacksize (attr, stacksize) pthread_attr_setstacksize (attr, stacksize) pthread_attr_getstackaddr (attr, stackaddr)

pthread_attr_setstackaddr (attr, stackaddr)

Preventing Stack Problems:

The POSIX standard does not dictate the size of a thread's stack. This is implementation dependent and varies.

Exceeding the default stack limit is often very easy to do, with the usual results: program termination and/or corrupted data.

Safe and portable programs do not depend upon the default stack limit, but instead, explicitly allocate enough stack for each thread by using the pthread_attr_setstacksize routine.

The pthread_attr_getstackaddr and pthread_attr_setstackaddr routines can be used by applications in an environment where the stack for a thread must be placed in some particular region of memory.

Some Practical Examples at LC:

Default thread stack size varies greatly. The maximum size that can be obtained also varies greatly, and may depend upon the number of threads per node.

Node
Architecture #CPUs Memory (GB) Default Size
(bytes)
AMD Opteron 8 16 2,097,152
Intel IA64 4 8 33,554,432
Intel IA32 2 4 2,097,152
IBM Power5 8 32 196,608
IBM Power4 8 16 196,608
IBM Power3 16 16 98,304

Node Architecture	#CPUs	Memory (GB)	Default Size (bytes)
AMD Opteron	8	16	2,097,152
Intel IA64	4	8	33,554,432
Intel IA32	2	4	2,097,152
IBM Power5	8	32	196,608
IBM Power4	8	16	196,608
IBM Power3	16	16	98,304

Example: Stack Management

Example Code - Stack Management

#include <pthread.h>
#include <stdio.h>
#define NTHREADS 4
#define N 1000
#define MEGEXTRA 1000000
pthread_attr_t attr;
void *dowork(void *threadid)
{
double A[N][N];
int i,j;
long tid;
size_t mystacksize;
tid = (long)threadid;
pthread_attr_getstacksize (&attr, &mystacksize);
printf("Thread %ld: stack size = %li bytes \n", tid, mystacksize);
for (i=0; i<N; i++)
for (j=0; j<N; j++)
A[i][j] = ((i*j)/3.452) + (N-i);
pthread_exit(NULL);
}
int main(int argc, char *argv[])
{
pthread_t threads[NTHREADS];
size_t stacksize;
int rc;
long t;
pthread_attr_init(&attr);
pthread_attr_getstacksize (&attr, &stacksize);
printf("Default stack size = %li\n", stacksize);
stacksize = sizeof(double)*N*N+MEGEXTRA;
printf("Amount of stack needed per thread = %li\n",stacksize);
pthread_attr_setstacksize (&attr, stacksize);
printf("Creating threads with stack size = %li bytes\n",stacksize);
for(t=0; t<NTHREADS; t++){
rc = pthread_create(&threads[t], &attr, dowork, (void *)t);
if (rc){
printf("ERROR; return code from pthread_create() is %d\n", rc);
exit(-1);
}
}
printf("Created %ld threads.\n", t);
pthread_exit(NULL);
}

Thread Management

Miscellaneous Routines

pthread_self ()

pthread_equal (thread1,thread2)

pthread_self returns the unique, system assigned thread ID of the calling thread.

pthread_equal compares two thread IDs. If the two IDs are different 0 is returned, otherwise a non-zero value is returned.

Note that for both of these routines, the thread identifier objects are opaque and can not be easily inspected. Because thread IDs are opaque objects, the C language equivalence operator == should not be used to compare two thread IDs against each other, or to compare a single thread ID against another value.

pthread_once (once_control, init_routine)

pthread_once executes the init_routine exactly once in a process. The first call to this routine by any thread in the process executes the given init_routine, without parameters. Any subsequent call will have no effect.

The init_routine routine is typically an initialization routine.

The once_control parameter is a synchronization control structure that requires initialization prior to calling pthread_once. For example:
pthread_once_t once_control = PTHREAD_ONCE_INIT;

Mutex Variables

Overview

Mutex is an abbreviation for "mutual exclusion". Mutex variables are one of the primary means of implementing thread synchronization and for protecting shared data when multiple writes occur.

A mutex variable acts like a "lock" protecting access to a shared data resource. The basic concept of a mutex as used in Pthreads is that only one thread can lock (or own) a mutex variable at any given time. Thus, even if several threads try to lock a mutex only one thread will be successful. No other thread can own that mutex until the owning thread unlocks that mutex. Threads must "take turns" accessing protected data.

Mutexes can be used to prevent "race" conditions. An example of a race condition involving a bank transaction is shown below:

Thread 1 Thread 2 Balance
Read balance: $1000 $1000
Read balance: $1000 $1000
Deposit $200 $1000
Deposit $200 $1000
Update balance $1000+$200 $1200
Update balance $1000+$200 $1200

In the above example, a mutex should be used to lock the "Balance" while a thread is using this shared data resource.

Very often the action performed by a thread owning a mutex is the updating of global variables. This is a safe way to ensure that when several threads update the same variable, the final value is the same as what it would be if only one thread performed the update. The variables being updated belong to a "critical section".

A typical sequence in the use of a mutex is as follows:
- Create and initialize a mutex variable
- Several threads attempt to lock the mutex
- Only one succeeds and that thread owns the mutex
- The owner thread performs some set of actions
- The owner unlocks the mutex
- Another thread acquires the mutex and repeats the process
- Finally the mutex is destroyed

When several threads compete for a mutex, the losers block at that call - an unblocking call is available with "trylock" instead of the "lock" call.

When protecting shared data, it is the programmer's responsibility to make sure every thread that needs to use a mutex does so. For example, if 4 threads are updating the same data, but only one uses a mutex, the data can still be corrupted.

Thread 1	Thread 2	Balance
Read balance: $1000		$1000
	Read balance: $1000	$1000
	Deposit $200	$1000
Deposit $200		$1000
Update balance $1000+$200		$1200
	Update balance $1000+$200	$1200

Mutex Variables

Creating and Destroying Mutexes

Routines:

pthread_mutex_init (mutex,attr) pthread_mutex_destroy (mutex) pthread_mutexattr_init (attr)

pthread_mutexattr_destroy (attr)

Usage:

Mutex variables must be declared with type pthread_mutex_t, and must be initialized before they can be used. There are two ways to initialize a mutex variable:
1. Statically, when it is declared. For example:
  pthread_mutex_t mymutex = PTHREAD_MUTEX_INITIALIZER;
2. Dynamically, with the pthread_mutex_init() routine. This method permits setting mutex object attributes, attr.
The mutex is initially unlocked.

The attr object is used to establish properties for the mutex object, and must be of type pthread_mutexattr_t if used (may be specified as NULL to accept defaults). The Pthreads standard defines three optional mutex attributes:
- Protocol: Specifies the protocol used to prevent priority inversions for a mutex.
- Prioceiling: Specifies the priority ceiling of a mutex.
- Process-shared: Specifies the process sharing of a mutex.
Note that not all implementations may provide the three optional mutex attributes.

The pthread_mutexattr_init() and pthread_mutexattr_destroy() routines are used to create and destroy mutex attribute objects respectively.

pthread_mutex_destroy() should be used to free a mutex object which is no longer needed.

Mutex Variables

Locking and Unlocking Mutexes

Routines:

pthread_mutex_lock (mutex) pthread_mutex_trylock (mutex)

pthread_mutex_unlock (mutex)

Usage:

The pthread_mutex_lock() routine is used by a thread to acquire a lock on the specified mutex variable. If the mutex is already locked by another thread, this call will block the calling thread until the mutex is unlocked.

pthread_mutex_trylock() will attempt to lock a mutex. However, if the mutex is already locked, the routine will return immediately with a "busy" error code. This routine may be useful in preventing deadlock conditions, as in a priority-inversion situation.

pthread_mutex_unlock() will unlock a mutex if called by the owning thread. Calling this routine is required after a thread has completed its use of protected data if other threads are to acquire the mutex for their work with the protected data. An error will be returned if:
- If the mutex was already unlocked
- If the mutex is owned by another thread

There is nothing "magical" about mutexes...in fact they are akin to a "gentlemen's agreement" between participating threads. It is up to the code writer to insure that the necessary threads all make the the mutex lock and unlock calls correctly. The following scenario demonstrates a logical error:
```
 Thread 1 Thread 2 Thread 3
Lock Lock 
A = 2 A = A+1 A = A*B
Unlock Unlock 
```

Question: When more than one thread is waiting for a locked mutex, which thread will be granted the lock first after it is released?

Example: Using Mutexes

Example Code - Using Mutexes

#include <pthread.h>
#include <stdio.h>
#include <stdlib.h>
/* 
The following structure contains the necessary information 
to allow the function "dotprod" to access its input data and 
place its output into the structure. 
*/
typedef struct 
{
double *a;
double *b;
double sum; 
int veclen; 
} DOTDATA;
/* Define globally accessible variables and a mutex */
#define NUMTHRDS 4
#define VECLEN 100
DOTDATA dotstr; 
pthread_t callThd[NUMTHRDS];
pthread_mutex_t mutexsum;
/*
The function dotprod is activated when the thread is created.
All input to this routine is obtained from a structure 
of type DOTDATA and all output from this function is written into
this structure. The benefit of this approach is apparent for the 
multi-threaded program: when a thread is created we pass a single
argument to the activated function - typically this argument
is a thread number. All the other information required by the 
function is accessed from the globally accessible structure. 
*/
void *dotprod(void *arg)
{
/* Define and use local variables for convenience */
int i, start, end, len ;
long offset;
double mysum, *x, *y;
offset = (long)arg;
len = dotstr.veclen;
start = offset*len;
end = start + len;
x = dotstr.a;
y = dotstr.b;
/*
Perform the dot product and assign result
to the appropriate variable in the structure. 
*/
mysum = 0;
for (i=start; i<end ; i++) 
{
mysum += (x[i] * y[i]);
}
/*
Lock a mutex prior to updating the value in the shared
structure, and unlock it upon updating.
*/
pthread_mutex_lock (&mutexsum);
dotstr.sum += mysum;
pthread_mutex_unlock (&mutexsum);
pthread_exit((void*) 0);
}
/* 
The main program creates threads which do all the work and then 
print out result upon completion. Before creating the threads,
the input data is created. Since all threads update a shared structure, 
we need a mutex for mutual exclusion. The main thread needs to wait for
all threads to complete, it waits for each one of the threads. We specify
a thread attribute value that allow the main thread to join with the
threads it creates. Note also that we free up handles when they are
no longer needed.
*/
int main (int argc, char *argv[])
{
long i;
double *a, *b;
void *status;
pthread_attr_t attr;
/* Assign storage and initialize values */
a = (double*) malloc (NUMTHRDS*VECLEN*sizeof(double));
b = (double*) malloc (NUMTHRDS*VECLEN*sizeof(double));
for (i=0; i<VECLEN*NUMTHRDS; i++)
{
a[i]=1.0;
b[i]=a[i];
}
dotstr.veclen = VECLEN; 
dotstr.a = a; 
dotstr.b = b; 
dotstr.sum=0;
pthread_mutex_init(&mutexsum, NULL);
/* Create threads to perform the dotproduct */
pthread_attr_init(&attr);
pthread_attr_setdetachstate(&attr, PTHREAD_CREATE_JOINABLE);
for(i=0; i<NUMTHRDS; i++)
{
/* 
Each thread works on a different set of data.
The offset is specified by 'i'. The size of
the data for each thread is indicated by VECLEN.
*/
pthread_create(&callThd[i], &attr, dotprod, (void *)i);
}
pthread_attr_destroy(&attr);
/* Wait on the other threads */
for(i=0; i<NUMTHRDS; i++)
{
pthread_join(callThd[i], &status);
}
/* After joining, print out the results and cleanup */
printf ("Sum = %f \n", dotstr.sum);
free (a);
free (b);
pthread_mutex_destroy(&mutexsum);
pthread_exit(NULL);
}

Serial version

Pthreads version

Condition Variables

Overview

Condition variables provide yet another way for threads to synchronize. While mutexes implement synchronization by controlling thread access to data, condition variables allow threads to synchronize based upon the actual value of data.

Without condition variables, the programmer would need to have threads continually polling (possibly in a critical section), to check if the condition is met. This can be very resource consuming since the thread would be continuously busy in this activity. A condition variable is a way to achieve the same goal without polling.

A condition variable is always used in conjunction with a mutex lock.

A representative sequence for using condition variables is shown below.

Main Thread Declare and initialize global data/variables which require synchronization (such as "count") Declare and initialize a condition variable object Declare and initialize an associated mutex Create threads A and B to do work
Thread A Do work up to the point where a certain condition must occur (such as "count" must reach a specified value) Lock associated mutex and check value of a global variable Call `pthread_cond_wait()` to perform a blocking wait for signal from Thread-B. Note that a call to `pthread_cond_wait()`automatically and atomically unlocks the associated mutex variable so that it can be used by Thread-B. When signalled, wake up. Mutex is automatically and atomically locked. Explicitly unlock mutex Continue	Thread B Do work Lock associated mutex Change the value of the global variable that Thread-A is waiting upon. Check value of the global Thread-A wait variable. If it fulfills the desired condition, signal Thread-A. Unlock mutex. Continue
Main Thread Join / Continue

Condition Variables

Creating and Destroying Condition Variables

Routines:

pthread_cond_init (condition,attr) pthread_cond_destroy (condition) pthread_condattr_init (attr)

pthread_condattr_destroy (attr)

Usage:

Condition variables must be declared with type pthread_cond_t, and must be initialized before they can be used. There are two ways to initialize a condition variable:
1. Statically, when it is declared. For example:
  pthread_cond_t myconvar = PTHREAD_COND_INITIALIZER;
2. Dynamically, with the pthread_cond_init() routine. The ID of the created condition variable is returned to the calling thread through the condition parameter. This method permits setting condition variable object attributes, attr.

The optional attr object is used to set condition variable attributes. There is only one attribute defined for condition variables: process-shared, which allows the condition variable to be seen by threads in other processes. The attribute object, if used, must be of type pthread_condattr_t (may be specified as NULL to accept defaults).
Note that not all implementations may provide the process-shared attribute.

The pthread_condattr_init() and pthread_condattr_destroy() routines are used to create and destroy condition variable attribute objects.

pthread_cond_destroy() should be used to free a condition variable that is no longer needed.

Condition Variables

Waiting and Signaling on Condition Variables

Routines:

pthread_cond_wait (condition,mutex) pthread_cond_signal (condition)

pthread_cond_broadcast (condition)

Usage:

pthread_cond_wait() blocks the calling thread until the specified condition is signalled. This routine should be called while mutex is locked, and it will automatically release the mutex while it waits. After signal is received and thread is awakened, mutex will be automatically locked for use by the thread. The programmer is then responsible for unlocking mutex when the thread is finished with it.

The pthread_cond_signal() routine is used to signal (or wake up) another thread which is waiting on the condition variable. It should be called after mutex is locked, and must unlock mutex in order for pthread_cond_wait() routine to complete.

The pthread_cond_broadcast() routine should be used instead of pthread_cond_signal() if more than one thread is in a blocking wait state.

It is a logical error to call pthread_cond_signal() before calling pthread_cond_wait().

Proper locking and unlocking of the associated mutex variable is essential when using these routines. For example:

Failing to lock the mutex before calling pthread_cond_wait() may cause it NOT to block.

Failing to unlock the mutex after calling pthread_cond_signal() may not allow a matching pthread_cond_wait() routine to complete (it will remain blocked).

Example: Using Condition Variables

Example Code - Using Condition Variables

#include <pthread.h>
#include <stdio.h>
#include <stdlib.h>
#define NUM_THREADS 3
#define TCOUNT 10
#define COUNT_LIMIT 12
int count = 0;
int thread_ids[3] = {0,1,2};
pthread_mutex_t count_mutex;
pthread_cond_t count_threshold_cv;
void *inc_count(void *t) 
{
int i;
long my_id = (long)t;
for (i=0; i<TCOUNT; i++) {
pthread_mutex_lock(&count_mutex);
count++;
/* 
Check the value of count and signal waiting thread when condition is
reached. Note that this occurs while mutex is locked. 
*/
if (count == COUNT_LIMIT) {
pthread_cond_signal(&count_threshold_cv);
printf("inc_count(): thread %ld, count = %d Threshold reached.\n", 
my_id, count);
}
printf("inc_count(): thread %ld, count = %d, unlocking mutex\n", 
my_id, count);
pthread_mutex_unlock(&count_mutex);
/* Do some "work" so threads can alternate on mutex lock */
sleep(1);
}
pthread_exit(NULL);
}
void *watch_count(void *t) 
{
long my_id = (long)t;
printf("Starting watch_count(): thread %ld\n", my_id);
/*
Lock mutex and wait for signal. Note that the pthread_cond_wait 
routine will automatically and atomically unlock mutex while it waits. 
Also, note that if COUNT_LIMIT is reached before this routine is run by
the waiting thread, the loop will be skipped to prevent pthread_cond_wait
from never returning. 
*/
pthread_mutex_lock(&count_mutex);
while (count<COUNT_LIMIT) {
pthread_cond_wait(&count_threshold_cv, &count_mutex);
printf("watch_count(): thread %ld Condition signal received.\n", my_id);
count += 125;
printf("watch_count(): thread %ld count now = %d.\n", my_id, count);
}
pthread_mutex_unlock(&count_mutex);
pthread_exit(NULL);
}
int main (int argc, char *argv[])
{
int i, rc;
long t1=1, t2=2, t3=3;
pthread_t threads[3];
pthread_attr_t attr;
/* Initialize mutex and condition variable objects */
pthread_mutex_init(&count_mutex, NULL);
pthread_cond_init (&count_threshold_cv, NULL);
/* For portability, explicitly create threads in a joinable state */
pthread_attr_init(&attr);
pthread_attr_setdetachstate(&attr, PTHREAD_CREATE_JOINABLE);
pthread_create(&threads[0], &attr, watch_count, (void *)t1);
pthread_create(&threads[1], &attr, inc_count, (void *)t2);
pthread_create(&threads[2], &attr, inc_count, (void *)t3);
/* Wait for all threads to complete */
for (i=0; i<NUM_THREADS; i++) {
pthread_join(threads[i], NULL);
}
printf ("Main(): Waited on %d threads. Done.\n", NUM_THREADS);
/* Clean up and exit */
pthread_attr_destroy(&attr);
pthread_mutex_destroy(&count_mutex);
pthread_cond_destroy(&count_threshold_cv);
pthread_exit(NULL);
}

LLNL Specific Information and Recommendations

This section describes details specific to Livermore Computing's systems.

Implementations:

All LC production systems include a Pthreads implementation that follows draft 10 (final) of the POSIX standard. This is the preferred implementation.

Implementations differ in the maximum number of threads that a process may create. They also differ in the default amount of thread stack space.

Compiling:

LC maintains a number of compilers, and usually several different versions of each - see the LC's Supported Compilers web page.

The compiler commands described in the Compiling Threaded Programs section apply to LC systems.

Additionally, all LC IBM compilers are aliased to their thread-safe command. For example, xlc really uses xlc_r. This is only true for LC IBM systems.

Mixing MPI with Pthreads:

Programs that contain both MPI and Pthreads are common and easy to develop on all LC systems.

Design:
- Each MPI process typically creates and then manages N threads, where N makes the best use of the available CPUs/node.
- Finding the best value for N will vary with the platform and your application's characteristics.
- For IBM SP systems with two communication adapters per node, it may prove more efficient to use two (or more) MPI tasks per node.
- In general, there may be problems if multiple threads make MPI calls. The program may fail or behave unexpectedly. If MPI calls must be made from within a thread, they should be made only by one thread.

Compiling:
- Use the appropriate MPI compile command for the platform and language of choice
- Be sure to include the required flag as in the table above (-pthread or -qnosave)
- MPICH is not thread safe

An example code that uses both MPI and Pthreads is available below. The serial, threads-only, MPI-only and MPI-with-threads versions demonstrate one possible progression.
- Serial
- Pthreads only
- MPI only
- MPI with pthreads
- makefile (for IBM SP)

Topics Not Covered

Several features of the Pthreads API are not covered in this tutorial. These are listed below. See the Pthread Library Routines Reference section for more information.

Thread Scheduling
- Implementations will differ on how threads are scheduled to run. In most cases, the default mechanism is adequate.
- The Pthreads API provides routines to explicitly set thread scheduling policies and priorities which may override the default mechanisms.
- The API does not require implementations to support these features.

Keys: Thread-Specific Data
- As threads call and return from different routines, the local data on a thread's stack comes and goes.
- To preserve stack data you can usually pass it as an argument from one routine to the next, or else store the data in a global variable associated with a thread.
- Pthreads provides another, possibly more convenient and versatile, way of accomplishing this through keys.

Mutex Protocol Attributes and Mutex Priority Management for the handling of "priority inversion" problems.

Condition Variable Sharing - across processes

Thread Cancellation

Threads and Signals

Synchronization constructs - barriers and locks

Pthread Library Routines Reference

pthread_atfork
pthread_attr_destroy
pthread_attr_getdetachstate
pthread_attr_getguardsize
pthread_attr_getinheritsched
pthread_attr_getschedparam
pthread_attr_getschedpolicy
pthread_attr_getscope
pthread_attr_getstack
pthread_attr_getstackaddr
pthread_attr_getstacksize
pthread_attr_init
pthread_attr_setdetachstate
pthread_attr_setguardsize
pthread_attr_setinheritsched
pthread_attr_setschedparam
pthread_attr_setschedpolicy
pthread_attr_setscope
pthread_attr_setstack
pthread_attr_setstackaddr
pthread_attr_setstacksize
pthread_barrier_destroy
pthread_barrier_init
pthread_barrier_wait
pthread_barrierattr_destroy
pthread_barrierattr_getpshared
pthread_barrierattr_init
pthread_barrierattr_setpshared
pthread_cancel
pthread_cleanup_pop
pthread_cleanup_push
pthread_cond_broadcast
pthread_cond_destroy
pthread_cond_init
pthread_cond_signal
pthread_cond_timedwait
pthread_cond_wait
pthread_condattr_destroy
pthread_condattr_getclock
pthread_condattr_getpshared
pthread_condattr_init
pthread_condattr_setclock
pthread_condattr_setpshared
pthread_create
pthread_detach
pthread_equal
pthread_exit
pthread_getconcurrency
pthread_getcpuclockid
pthread_getschedparam
pthread_getspecific
pthread_join
pthread_key_create
pthread_key_delete
pthread_kill
pthread_mutex_destroy
pthread_mutex_getprioceiling
pthread_mutex_init
pthread_mutex_lock
pthread_mutex_setprioceiling
pthread_mutex_timedlock
pthread_mutex_trylock
pthread_mutex_unlock
pthread_mutexattr_destroy
pthread_mutexattr_getprioceiling
pthread_mutexattr_getprotocol
pthread_mutexattr_getpshared
pthread_mutexattr_gettype
pthread_mutexattr_init
pthread_mutexattr_setprioceiling
pthread_mutexattr_setprotocol
pthread_mutexattr_setpshared
pthread_mutexattr_settype
pthread_once
pthread_rwlock_destroy
pthread_rwlock_init
pthread_rwlock_rdlock
pthread_rwlock_timedrdlock
pthread_rwlock_timedwrlock
pthread_rwlock_tryrdlock
pthread_rwlock_trywrlock
pthread_rwlock_unlock
pthread_rwlock_wrlock
pthread_rwlockattr_destroy
pthread_rwlockattr_getpshared
pthread_rwlockattr_init
pthread_rwlockattr_setpshared
pthread_self
pthread_setcancelstate
pthread_setcanceltype
pthread_setconcurrency
pthread_setschedparam
pthread_setschedprio
pthread_setspecific
pthread_sigmask
pthread_spin_destroy
pthread_spin_init
pthread_spin_lock
pthread_spin_trylock
pthread_spin_unlock
pthread_testcancel

This completes the tutorial.

Please complete the online evaluation form - unless you are doing the exercise, in which case please complete it at the end of the exercise.

Where would you like to go now?

Exercise

Agenda

Back to the top

References and More Information

Author: Blaise Barney, Livermore Computing.

POSIX Standard: www.unix.org/version3/ieee_std.html

"Pthreads Programming". B. Nichols et al. O'Reilly and Associates.

"Threads Primer". B. Lewis and D. Berg. Prentice Hall

"Programming With POSIX Threads". D. Butenhof. Addison Wesley
www.awl.com/cseng/titles/0-201-63392-2

"Programming With Threads". S. Kleiman et al. Prentice Hall

https://computing.llnl.gov/tutorials/pthreads/
Last Modified: 10/14/2010 15:00:48 blaiseb@llnl.gov
UCRL-MI-133316

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pthread introduction

POSIX Threads Programming

Table of Contents

What is a Thread?

What are Pthreads?

Why Pthreads?

Designing Threaded Programs

Creating and Terminating Threads

Example: Pthread Creation and Termination

Passing Arguments to Threads

Joining and Detaching Threads

Example: Pthread Joining

Stack Management

Example: Stack Management

Miscellaneous Routines

Overview

Creating and Destroying Mutexes

Locking and Unlocking Mutexes

Example: Using Mutexes

Overview

Creating and Destroying Condition Variables

Waiting and Signaling on Condition Variables

Example: Using Condition Variables

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UNIX PROCESS	THREADS WITHIN A UNIX PROCESS