Multi-threaded programming with NSPR

Larry Hardiman

Overview

• What is NSPR?

• Why program using threads?

• What you need to program with threads

• NSPR’s thread abstraction

• Thread synchronization using locks

• Thread synch using condition variables

• Techniques for high performance threads

What is NSPR?

• Original NSPR 1.0 was NS Java base

• NSPR 2.0 began in early 1996

• NSPR 2 is platform abstraction

• No GUI

• Threading, thread sync

• File and Socket I/O

• Time, IPC, ...

Why program using threads?

• Simplify program design

• Improve performance when calling blocking functions

• Utilize all processors on multi-processor systems

Thread Programming Basics

• Thread Management

• Thread private data

• Thread synchronization

Thread Management

• PR_CreateThread()

• PR_Interrupt()

• PR_JoinThread()

• PR_SetThreadPriority()

• PR_GetThreadPriority()

Creating a thread

1 #include “prthread.h”

2 PRThread *3 PR_CreateThread(4 PRThreadType type, /* user or system */5 (void)(start)(void *arg), /* function */6 void *arg; /*argument to start function */7 PRThreadPriority priority, /* low, norm … */8 PRThreadScope scope, /* local, global */9 PRThreadState state, /* joinable? */10 PRInt32 stacksize );

Thread Private Data

Think of “thread private data” as global variables, but for exclusive use for a single thread.

Void (*PR_ThreadPrivateDTOR)(void*);PRStatus PR_NewThreadPrivateIndex( int *ndx, (dtor));PRStatus PR_SetThreadPrivate( ndx, *priv );void* PR_GetThreadPrivate( ndx );

See: mozilla/nsprpub/pr/tests/tpd.c

Thread sync using PRLock

1 #include “prlock.h”

2 PRLock *lock;

3 lock = PR_NewLock();4 PR_Lock(lock);5 /* operate on your data … */6 PR_Unlock(lock);7 PR_DestroyLock(lock);

Using Condition Variables

1 PRIntn data = 0; /* Thread A */2 PRLock *lock = PR_NewLock();3 PRCondVar *cvar = PR_NewCondVar(lock);

4 PR_Lock(lock);5 while( data == 0 )6 PR_WaitCondVar( cvar, timeout ); /* lock released while blocked. */ /* re-locked at fall through */7 data = 0; /* satisfy Thread B’s condition */8 PR_NotifyCondVar( cvar );9 PR_Unlock( lock );

Using Condition Variables

/* Thread B */

4 PR_Lock(lock);5 while( data == 1 )6 PR_WaitCondVar( cvar, timeout ); /* lock released while blocked. */ /* re-locked at fall through */7 data = 1;8 PR_NotifyCondVar( cvar );9 PR_Unlock( lock );

What Not to Do

1 PRLock *lock;2 PRCondVar *cv;

3 PR_Lock( lock );4 PR_WaitCondVar( cv, PR_INTERVAL_NO_TIMEOUT ); /* implies state in *cv */5 /* do something */6 PR_Unlock( lock );

There is no state in a PRCondVar.

… More What Not to Do

1 PRBool dataIsAvailable = PR_FALSE;

2 PR_Lock( lock );3 if ( !dataIsAvailable )4 PR_WaitCondVar( cv, PR_INTERVAL_NO_TIMEOUT );

/* Bad! dataIsAvailable may change during wait */5 /* work on the available data */6 PR_Unlock( lock );

The lock is released during PR_WaitCondvar(). Other threads can change dataIsAvailable. Use the while() loop;test, wait, test again.

Condvar Example

A hacked cvar.c goes here.

High Performance Threads

• … you are protecting data, not code

• partition assets so that you protect the smallest piece possible

• hold exclusive control as short a time as possible

Bibliography

• David R. Butenhof, "Programming with POSIX Threads", Addison-Wesley, 1997

• Steve Kleiman, et.al, “Programming with Threads”, Prentice Hall, 1996

• NSPR group, “NSPR Reference”, Ch. 1, 3, 5, 6. http://www.mozilla.org/projects/nspr/reference/html/index.html

• UseNet newsgroup: comp.programming.threads.