Thread-Specific Data

Thread-Specific Data
in Pthreads

Prof. David Bernstein
James Madison University

Computer Science Department

bernstdh@jmu.edu

Review

Threads:
- A mechanism that permits a single process to perform multiple tasks concurrently
Shared Variables:
- Threads can "share" variables in the initialized data, uninitialized data, and heap segments

Overview

Purpose:
- Allows a function to maintain a separate copy of a variable for each thread that calls the function
What a Function Requires of the Library:
- The ability to allocate a block of memory the first time a thread calls the function
- The ability to obtain the address of the memory without maintaining a pointer in an automatic variable
- The ability to free the memory when the thread terminates (which may happen in any function)

Create a Key: pthread_key_create()

int pthread_key_create(pthread_key_t *key, void (*destructor)(void *)

Purpose:

Create a thread-specific key (and install a destructor)

Details:

`key`	A key that can be used to obtain thread-specific data
`destructor`	The function to call to free the thread-specific memory (or NULL if no destructor is needed)
Return	0 on success; a positive error number on error

#include <pthread.h>

Note: destructor() will be invoked automatically by the system when the thread terminates (and passed the address of the thread-specific memory for this key).

Set a Value: pthread_setspecific()

int pthread_setspecific(pthread_key_t key, const void *value)

Purpose:

Save a value in thread-specific memory

Details:

`key`	The key used to identify the thread-specific data
`value`	The value to save (which is normally a block of memory that has been previously allocated by the caller)
Return	0 on success; a positive error number on error

#include <pthread.h>

Get a Value: pthread_getspecific()

void *pthread_getspecific(pthread_key_t key)

Purpose:

Retrieve a value from thread-specific memory

Details:

`key`	The key used to identify the thread-specific data
Return	A pointer to the value on success; NULL is there is no thread-specific data associated with the key

#include <pthread.h>

A Very, Very Problematic Motivating Example

A Driver with a Single Thread

unixexamples/threadspecificdata/driver.c

A Very, Very Problematic Motivating Example (cont.)

unixexamples/threadspecificdata/grader_local.c

A Very Problematic Motivating Example

A Driver with Multiple Threads

unixexamples/threadspecificdata/threaded_driver.c

A Very Problematic Motivating Example (cont.)

unixexamples/threadspecificdata/grader_static.c

A Problematic Motivating Example

unixexamples/threadspecificdata/grader_file.c

An Example Using Thread-Specific Data

unixexamples/threadspecificdata/grader_specific.c

There's Always More to Learn