Multi-Threaded Programming With POSIX Threads
Table Of Contents:
- Before We Start...
- What Is a Thread? Why Use Threads?
- Creating And Destroying Threads
- Synchronizing Threads With Mutexes
- What Is A Mutex?
- Creating And Initializing A Mutex
- Locking And Unlocking A Mutex
- Destroying A Mutex
- Using A Mutex - A Complete Example
- Starvation And Deadlock Situations
- Refined Synchronization - Condition Variables
- What Is A Condition Variable?
- Creating And Initializing A Condition Variable
- Signaling A Condition Variable
- Waiting On A Condition Variable
- Destroying A Condition Variable
- A Real Condition For A Condition Variable
- Using A Condition Variable - A Complete Example
- "Private" thread data - Thread-Specific Data
- Overview Of Thread-Specific Data Support
- Allocating Thread-Specific Data Block
- Accessing Thread-Specific Data
- Deleting Thread-Specific Data Block
- A Complete Example
- Thread Cancellation And Termination
- Canceling A Thread
- Setting Thread Cancellation State
- Cancellation Points
- Setting Thread Cleanup Functions
- Synchronizing On Threads Exiting
- Detaching A Thread
- Threads Cancellation - A Complete Example
- Using Threads For Responsive User Interface Programming
- User Interaction - A Complete Example
- Using 3rd-Party Libraries In A Multi-Threaded Application
- Using A Threads-Aware Debugger
Before We Start...
This tutorial is an attempt to help you become familiar with multi-threaded
programming with the POSIX threads (pthreads) library, and attempts to show
how its features can be used in "real-life" programs. It explains the
different tools defined by the library, shows how to use them, and then gives
an example of using them to solve programming problems. There is an implicit
assumption that the user has some theoretical familiarity with paralell
programming (or multi-processing) concepts. Users without such background
might find the concepts harder to grasp. A seperate tutorial will be prepared
to explain the theoreticl background and terms to those who are familiar only
with normal "serial" programming.
I would assume that users which are familiar with asynchronous programming
models, such as those used in windowing environments (X, Motif), will find it
easier to grasp the concepts of multi-threaded programming.
When talking about POSIX threads, one cannot avoid the question "Which draft
of the POSIX threads standard shall be used?". As this threads standard has been
revised over a period of several years, one will find that implementations
adhering to different drafts of the standard have a different set of functions,
different default values, and different nuances. Since this tutorial was
written using a Linux system with the kernel-level LinuxThreads library, v0.5,
programmers with access to other systems, using different versions of pthreads,
should refer to their system's manuals in case of incompatibilities. Also, since
some of the example programs are using blocking system calls, they won't
work with user-level threading libraries (refer to our
parallel programming theory tutorial for
more information).
Having said that,
i'd try to check the example programs on other systems as well (Solaris 2.5
comes to mind), to make it more "cross-platform".
What Is a Thread? Why Use Threads
A thread is a semi-process, that has its own stack, and executes a given
piece of code. Unlike a real process, the thread normally shares its memory
with other threads (where as for processes we usually have a different memory
area for each one of them). A Thread Group is a set of threads all executing
inside the same process. They all share the same memory, and thus can access
the same global variables, same heap memory, same set of file descriptors,
etc. All these threads execute in parallel (i.e. using time slices, or if
the system has several processors, then really in parallel).
The advantage of using a thread group instead of a normal serial program
is that several operations may be carried out in parallel, and thus events
can be handled immediately as they arrive (for example, if we have one thread
handling a user interface, and another thread handling database queries,
we can execute a heavy query requested by the user, and still respond to
user input while the query is executed).
The advantage of using a thread group over using a process group is that
context switching between threads is much faster than context switching
between processes (context switching means that the system switches from
running one thread or process, to running another thread or process).
Also, communications between two threads is usually faster and easier to
implement than communications between two processes.
On the other hand, because threads in a group all use the same memory space,
if one of them corrupts the contents of its memory, other threads might
suffer as well. With processes, the operating system normally protects
processes from one another, and thus if one corrupts its own memory space,
other processes won't suffer. Another advantage of using processes is that
they can run on different machines, while all the threads have to run
on the same machine (at least normally).
Creating And Destroying Threads
When a multi-threaded program starts executing, it has one thread running,
which executes the main() function of the program. This is already a
full-fledged thread, with its own thread ID. In order to create a new thread,
the program should use the pthread_create() function.
Here is how to use it:
#include <stdio.h> /* standard I/O routines */
#include <pthread.h> /* pthread functions and data structures */
/* function to be executed by the new thread */
void*
do_loop(void* data)
{
int i; /* counter, to print numbers */
int j; /* counter, for delay */
int me = *((int*)data); /* thread identifying number */
for (i=0; i<10; i++) {
for (j=0; j<500000; j++) /* delay loop */
;
printf("'%d' - Got '%d'\n", me, i);
}
/* terminate the thread */
pthread_exit(NULL);
}
/* like any C program, program's execution begins in main */
int
main(int argc, char* argv[])
{
int thr_id; /* thread ID for the newly created thread */
pthread_t p_thread; /* thread's structure */
int a = 1; /* thread 1 identifying number */
int b = 2; /* thread 2 identifying number */
/* create a new thread that will execute 'do_loop()' */
thr_id = pthread_create(&p_thread, NULL, do_loop, (void*)&a);
/* run 'do_loop()' in the main thread as well */
do_loop((void*)&b);
/* NOT REACHED */
return 0;
}
A few notes should be mentioned about this program:
- Note that the main program is also a thread, so it executes the
do_loop() function in parallel to the thread it creates.
-
pthread_create() gets 4 parameters. The first parameter is
used by pthread_create() to supply the program with
information about the thread. The second parameter is used to set some
attributes for the new thread. In our case we supplied a NULL pointer to
tell pthread_create() to use the default values. The third
parameter is the name of the function that the thread will start
executing. The forth parameter is an argument to pass to this function.
Note the cast to a 'void*'. It is not required by ANSI-C syntax, but is
placed here for clarification.
- The delay loop inside the function is used only to demonstrate that the
threads are executing in parallel. Use a larger delay value if your CPU
runs too fast, and you see all the printouts of one thread before the
other.
- The call to
pthread_exit() Causes the current thread
to exit and free any thread-specific resources it is taking. There is no
need to use this call at the end of the thread's top function, since when
it returns, the thread would exit automatically anyway. This function is
useful if we want to exit a thread in the middle of its execution.
In order to compile a multi-threaded program using gcc ,
we need to link it with the pthreads library. Assuming you have this library
already installed on your system, here is how to compile our first program:
gcc pthread_create.c -o pthread_create -lpthread
Note that for some of the programs later on this tutorial, one may need to
add a '-D_GNU_SOURCE' flag to this compile line, to get the source compiled.
The source code for this program may be found in the
pthread_create.c file.
Synchronizing Threads With Mutexes
One of the basic problems when running several threads that use the same
memory space, is making sure they don't "step on each other's toes". By this
we refer to the problem of using a data structure from two different threads.
For instance, consider the case where two threads try to update two variables.
One tries to set both to 0, and the other tries to set both to 1. If both
threads would try to do that at the same time, we might get with a situation
where one variable contains 1, and one contains 0. This is because a
context-switch (we already know what this is by now, right?) might occur after
the first tread zeroed out the first variable, then the second thread would
set both variables to 1, and when the first thread resumes operation, it will
zero out the second variable, thus getting the first variable set to '1',
and the second set to '0'.
What Is A Mutex?
A basic mechanism supplied by the pthreads library to solve this problem,
is called a mutex. A mutex is a lock that guarantees three things:
- Atomicity - Locking a mutex is an atomic operation, meaning that the
operating system (or threads library) assures you that if you locked a
mutex, no other thread succeeded in locking this mutex at the same time.
- Singularity - If a thread managed to lock a mutex, it is assured
that no other thread will be able to lock the thread until the original
thread releases the lock.
- Non-Busy Wait - If a thread attempts to lock a thread that was
locked by a second thread, the first thread will be suspended (and will
not consume any CPU resources) until the lock is freed by the second
thread. At this time, the first thread will wake up and continue execution,
having the mutex locked by it.
From these three points we can see how a mutex can be used to assure exclusive
access to variables (or in general critical code sections). Here is some
pseudo-code that updates the two variables we were talking about in the
previous section, and can be used by the first thread:
lock mutex 'X1'.
set first variable to '0'.
set second variable to '0'.
unlock mutex 'X1'.
Meanwhile, the second thread will do something like this:
lock mutex 'X1'.
set first variable to '1'.
set second variable to '1'.
unlock mutex 'X1'.
Assuming both threads use the same mutex, we are assured that after they both
ran through this code, either both variables are set to '0', or both are set
to '1'. You'd note this requires some work from the programmer - If a third
thread was to access these variables via some code that does not use this
mutex, it still might mess up the variable's contents. Thus, it is important
to enclose all the code that accesses these variables in a small set of
functions, and always use only these functions to access these variables.
In order to create a mutex, we first need to declare a variable of type
pthread_mutex_t , and then initialize it. The simplest way it
by assigning it the PTHREAD_MUTEX_INITIALIZER constant. So
we'll use a code that looks something like this:
pthread_mutex_t a_mutex = PTHREAD_MUTEX_INITIALIZER;
One note should be made here: This type of initialization creates a mutex
called 'fast mutex'. This means that if a thread locks the mutex and then
tries to lock it again, it'll get stuck - it will be in a deadlock.
There is another type of mutex, called 'recursive mutex', which allows the
thread that locked it, to lock it several more times, without getting blocked
(but other threads that try to lock the mutex now will get blocked). If the
thread then unlocks the mutex, it'll still be locked, until it is unlocked
the same amount of times as it was locked. This is similar to the way modern
door locks work - if you turned it twice clockwise to lock it, you need to turn
it twice counter-clockwise to unlock it. This kind of mutex can be created
by assigning the constant PTHREAD_RECURSIVE_MUTEX_INITIALIZER_NP
to a mutex variable.
Locking And Unlocking A Mutex
In order to lock a mutex, we may use the function
pthread_mutex_lock() . This function attempts to lock the mutex,
or block the thread if the mutex is already locked by another thread. In this
case, when the mutex is unlocked by the first process, the function will return
with the mutex locked by our process. Here is how to lock a mutex (assuming it
was initialized earlier):
int rc = pthread_mutex_lock(&a_mutex);
if (rc) { /* an error has occurred */
perror("pthread_mutex_lock");
pthread_exit(NULL);
}
/* mutex is now locked - do your stuff. */
.
.
After the thread did what it had to (change variables or data structures,
handle file, or whatever it intended to do), it should free the mutex,
using the pthread_mutex_unlock() function, like this:
rc = pthread_mutex_unlock(&a_mutex);
if (rc) {
perror("pthread_mutex_unlock");
pthread_exit(NULL);
}
Destroying A Mutex
After we finished using a mutex, we should destroy it. Finished using means
no thread needs it at all. If only one thread finished with the mutex,
it should leave it alive, for the other threads that might still need to use
it. Once all finished using it, the last one can destroy it using the
pthread_mutex_destroy() function:
rc = pthread_mutex_destroy(&a_mutex);
After this call, this variable (a_mutex) may not be used as a mutex any more,
unless it is initialized again. Thus, if one destroys a mutex too early,
and another thread tries to lock or unlock it, that thread will get a
EINVAL error code from the lock or unlock function.
Using A Mutex - A Complete Example
After we have seen the full life cycle of a mutex, lets see an example
program that uses a mutex. The program introduces two employees competing
for the "employee of the day" title, and the glory that comes with it.
To simulate that in a rapid pace, the program employs 3 threads: one that
promotes Danny to "employee of the day", one that promotes Moshe to
that situation, and a third thread that makes sure that the employee
of the day's contents is consistent (i.e. contains exactly the data of
one employee).
Two copies of the program are supplied. One that uses a mutex, and one that
does not. Try them both, to see the differences, and be convinced that mutexes
are essential in a multi-threaded environment.
The programs themselves are in the files accompanying this tutorial.
The one that uses a mutex is
employee-with-mutex.c. The one that does
not use a mutex is
employee-without-mutex.c. Read the
comments inside the source files to get a better understanding of how they
work.
Starvation And Deadlock Situations
Again we should remember that pthread_mutex_lock() might block
for a non-determined duration, in case of the mutex being already locked.
If it remains locked forever, it is said that our poor thread is "starved" -
it was trying to acquire a resource, but never got it. It is up to the
programmer to ensure that such starvation won't occur. The pthread library
does not help us with that.
The pthread library might, however, figure out a "deadlock". A deadlock is
a situation in which a set of threads are all waiting for resources taken by
other threads, all in the same set. Naturally, if all threads are blocked
waiting for a mutex, none of them will ever come back to life again. The
pthread library keeps track of such situations, and thus would fail the last
thread trying to call pthread_mutex_lock() , with an error
of type EDEADLK . The programmer should check for such a value,
and take steps to solve the deadlock somehow.
As we've seen before with mutexes, they allow for simple coordination -
exclusive access to a resource. However, we often need to be able to
make real synchronization between threads:
- In a server, one thread reads requests from clients, and dispatches
them to several threads for handling. These threads need to be notified
when there is data to process, otherwise they should wait without
consuming CPU time.
- In a GUI (Graphical User Interface) Application, one thread reads user
input, another handles graphical output, and a third thread sends
requests to a server and handles its replies. The server-handling
thread needs to be able to notify the graphics-drawing thread when a reply
from the server arrived, so it will immediately show it to the user.
The user-input thread needs to be always responsive to the user, for
example, to allow her to cancel long operations currently executed by
the server-handling thread.
All these examples require the ability to send notifications between threads.
This is where condition variables are brought into the picture.
What Is A Condition Variable?
A condition variable is a mechanism that allows threads to wait (without
wasting CPU cycles) for some even to occur. Several threads may wait on
a condition variable, until some other thread signals this condition variable
(thus sending a notification). At this time, one of the threads waiting on this
condition variable wakes up, and can act on the event. It is possible to also
wake up all threads waiting on this condition variable by using a broadcast
method on this variable.
Note that a condition variable does not provide locking. Thus, a mutex is
used along with the condition variable, to provide the necessary locking
when accessing this condition variable.
Creating And Initializing A Condition Variable
Creation of a condition variable requires defining a variable of type
pthread_cond_t , and initializing it properly. Initialization
may be done with either a simple use of a macro named
PTHREAD_COND_INITIALIZER or the usage of the
pthread_cond_init() function. We will show the first form
here:
pthread_cond_t got_request = PTHREAD_COND_INITIALIZER;
This defines a condition variable named 'got_request', and initializes it.
Note: since the PTHREAD_COND_INITIALIZER is actually a
structure initializer, it may be used to initialize a condition variable
only when it is declared. In order to initialize it during runtime, one
must use the pthread_cond_init() function.
Signaling A Condition Variable
In order to signal a condition variable, one should either the
pthread_cond_signal() function (to wake up a only one
thread waiting on this variable), or the pthread_cond_broadcast()
function (to wake up all threads waiting on this variable). Here is
an example using signal, assuming 'got_request' is a properly
initialized condition variable:
int rc = pthread_cond_signal(&got_request);
Or by using the broadcast function:
int rc = pthread_cond_broadcast(&got_request);
When either function returns, 'rc' is set to 0 on success, and to a
non-zero value on failure. In such a case (failure), the return value denotes
the error that occured (EINVAL denotes that the given parameter
is not a condition variable. ENOMEM denotes that the system
has run out of memory.
Note: success of a signaling operation does not mean any thread
was awakened - it might be that no thread was waiting on the condition variable,
and thus the signaling does nothing (i.e. the signal is lost).
It is also not remembered for
future use - if after the signaling function returns another thread starts
waiting on this condition variable, a further signal is required to wake it up.
Waiting On A Condition Variable
If one thread signals the condition variable, other threads would probably
want to wait for this signal. They may do so using one of two functions,
pthread_cond_wait() or pthread_cond_timedwait() .
Each of these functions takes a condition variable, and a mutex (which should
be locked before calling the wait function), unlocks the mutex,
and waits until the condition variable is signaled, suspending
the thread's execution. If this signaling causes the thread to awake (see
discussion of pthread_cond_signal() earlier), the mutex is
automagically locked again by the wait funciton, and the wait function
returns.
The only difference between these two functions is that
pthread_cond_timedwait() allows the programmer to specify a
timeout for the waiting, after which the function always returns, with a
proper error value (ETIMEDOUT) to notify that condition variable was NOT
signaled before the timeout passed. The pthread_cond_wait()
would wait indefinitely if it was never signaled.
Here is how to use these two functions. We make the assumption that
'got_request' is a properly initialized condition variable, and that
'request_mutex' is a properly initialized mutex. First, we try
the pthread_cond_wait() function:
/* first, lock the mutex */
int rc = pthread_mutex_lock(&request_mutex);
if (rc) { /* an error has occurred */
perror("pthread_mutex_lock");
pthread_exit(NULL);
}
/* mutex is now locked - wait on the condition variable. */
/* During the execution of pthread_cond_wait, the mutex is unlocked. */
rc = pthread_cond_wait(&got_request, &request_mutex);
if (rc == 0) { /* we were awakened due to the cond. variable being signaled */
/* The mutex is now locked again by pthread_cond_wait() */
/* do your stuff... */
.
}
/* finally, unlock the mutex */
pthread_mutex_unlock(&request_mutex);
Now an example using the pthread_cond_timedwait() function:
#include <sys/time.h> /* struct timeval definition */
#include <unistd.h> /* declaration of gettimeofday() */
struct timeval now; /* time when we started waiting */
struct timespec timeout; /* timeout value for the wait function */
int done; /* are we done waiting? */
/* first, lock the mutex */
int rc = pthread_mutex_lock(&a_mutex);
if (rc) { /* an error has occurred */
perror("pthread_mutex_lock");
pthread_exit(NULL);
}
/* mutex is now locked */
/* get current time */
gettimeofday(&now);
/* prepare timeout value. */
/* Note that we need an absolute time. */
timeout.tv_sec = now.tv_sec + 5
timeout.tv_nsec = now.tv_usec * 1000; /* timeval uses micro-seconds. */
/* timespec uses nano-seconds. */
/* 1 micro-second = 1000 nano-seconds. */
/* wait on the condition variable. */
/* we use a loop, since a Unix signal might stop the wait before the timeout */
done = 0;
while (!done) {
/* remember that pthread_cond_timedwait() unlocks the mutex on entrance */
rc = pthread_cond_timedwait(&got_request, &request_mutex, &timeout);
switch(rc) {
case 0: /* we were awakened due to the cond. variable being signaled */
/* the mutex was now locked again by pthread_cond_timedwait. */
/* do your stuff here... */
.
.
done = 0;
break;
default: /* some error occurred (e.g. we got a Unix signal) */
if (errno == ETIMEDOUT) { /* our time is up */
done = 0;
}
break; /* break this switch, but re-do the while loop. */
}
}
/* finally, unlock the mutex */
pthread_mutex_unlock(&request_mutex);
As you can see, the timed wait version is way more complex, and thus
better be wrapped up by some function, rather than being re-coded in every
necessary location.
Note: it might be that a condition variable that has 2 or more threads
waiting on it is signaled many times, and yet one of the threads waiting on
it never awakened. This is because we are not guaranteed which of the waiting
threads is awakened when the variable is signaled. It might be that the awakened
thread quickly comes back to waiting on the condition variables, and gets
awakened again when the variable is signaled again, and so on. The situation
for the un-awakened thread is called 'starvation'. It is up to the programmer
to make sure this situation does not occur if it implies bad behavior. Yet,
in our server example from before, this situation might indicate requests are
coming in a very slow pace, and thus perhaps we have too many threads waiting
to service requests. In this case, this situation is actually good, as it means
every request is handled immediately when it arrives.
Note 2: when the mutex is being broadcast (using pthread_cond_broadcast),
this does not mean all threads are running together. Each of them tries to
lock the mutex again before returning from their wait function, and thus they'll
start running one by one, each one locking the mutex, doing their work, and
freeing the mutex before the next thread gets its chance to run.
Destroying A Condition Variable
After we are done using a condition variable, we should destroy it, to free
any system resources it might be using. This can be done using the
pthread_cond_destroy() . In order for this to work, there should
be no threads waiting on this condition variable. Here is how to use
this function, again, assuming 'got_request' is a pre-initialized condition
variable:
int rc = pthread_cond_destroy(&got_request);
if (rc == EBUSY) { /* some thread is still waiting on this condition variable */
/* handle this case here... */
.
.
}
What if some thread is still waiting on this variable? depending on the case,
it might imply some flaw in the usage of this variable, or just lack of proper
thread cleanup code. It is probably good to alert the programmer, at least
during debug phase of the program, of such a case. It might mean nothing,
but it might be significant.
A Real Condition For A Condition Variable
A note should be taken about condition variables - they are usually pointless
without some real condition checking combined with them. To make this clear,
lets consider the server example we introduced earlier. Assume that we use
the 'got_request' condition variable to signal that a new request has arrived
that needs handling, and is held in some requests queue. If we had threads
waiting on the condition variable when this variable is signaled, we are
assured that one of these threads will awake and handle this request.
However, what if all threads are busy handling previous requests, when a new
one arrives? the signaling of the condition variable will do nothing (since
all threads are busy doing other things, NOT waiting on the condition variable
now), and after all threads finish handling their current request, they come
back to wait on the variable, which won't necessarily be signaled again
(for example, if no new requests arrive). Thus, there is at least one
request pending, while all handling threads are blocked, waiting for a signal.
In order to overcome this problem, we may set some integer variable to
denote the number of pending requests, and have each thread check the value
of this variable before waiting on the variable. If this variable's value
is positive, some request is pending, and the thread should go and handle
it, instead of going to sleep. Further more, a thread that handled a request,
should reduce the value of this variable by one, to make the count correct.
Lets see how this affects the waiting code we have seen above.
/* number of pending requests, initially none */
int num_requests = 0;
.
.
/* first, lock the mutex */
int rc = pthread_mutex_lock(&request_mutex);
if (rc) { /* an error has occurred */
perror("pthread_mutex_lock");
pthread_exit(NULL);
}
/* mutex is now locked - wait on the condition variable */
/* if there are no requests to be handled. */
rc = 0;
if (num_requests == 0)
rc = pthread_cond_wait(&got_request, &request_mutex);
if (num_requests > 0 && rc == 0) { /* we have a request pending */
/* unlock mutex - so other threads would be able to handle */
/* other reqeusts waiting in the queue paralelly. */
rc = pthread_mutex_unlock(&request_mutex);
/* do your stuff... */
.
.
/* decrease count of pending requests */
num_requests--;
/* and lock the mutex again - to remain symmetrical,. */
rc = pthread_mutex_lock(&request_mutex);
}
}
/* finally, unlock the mutex */
pthread_mutex_unlock(&request_mutex);
Using A Condition Variable - A Complete Example
As an example for the actual usage of condition variables, we will show
a program that simulates the server we have described earlier - one thread,
the receiver, gets client requests. It inserts the requests to a linked list,
and a hoard of threads, the handlers, are handling these requests.
For simplicity, in our simulation, the receiver thread creates requests
and does not read them from real clients.
The program source is available in the file
thread-pool-server.c, and contains
many comments. Please read the source file first, and then read the
following clarifying notes.
- The 'main' function first launches the handler threads, and then
performs the chord of the receiver thread, via its main loop.
- A single mutex is used both to protect the condition variable,
and to protect the linked list of waiting requests. This simplifies
the design. As an exercise, you may think how to divide these roles
into two mutexes.
- The mutex itself MUST be a recursive mutex. In order to see why,
look at the code of the 'handle_requests_loop' function. You will
notice that it first locks the mutex, and afterwards calls the
'get_request' function, which locks the mutex again. If we used
a non-recursive mutex, we'd get locked indefinitely in the mutex locking
operation of the 'get_request' function.
You may argue that we could remove the mutex locking in the 'get_request'
function, and thus remove the double-locking problem, but this is
a flawed design - in a larger program, we might call the 'get_request'
function from other places in the code, and we'll need to check for
proper locking of the mutex in each of them.
- As a rule, when using recursive mutexes, we should try to make sure that
each lock operation is accompanied by a matching unlock operation in the
same function. Otherwise, it will be very hard to make sure that after
locking the mutex several times, it is being unlocked the same number
of times, and deadlocks would occur.
- The implicit unlocking and re-locking of the mutex on the call to
the
pthread_cond_wait() function is confusing at first.
It is best to add a comment regarding this behavior in the code,
or else someone that reads this code might accidentally add a further
mutex lock.
- When a handler thread handles a request - it should free the mutex,
to avoid blocking all the other handler threads. After it finished
handling the request, it should lock the mutex again, and check if
there are more requests to handle.
"Private" thread data - Thread-Specific Data
In "normal", single-thread programs, we sometimes find the need to use a global
variable. Ok, so good old teach' told us it is bad practice to have global
variables, but they sometimes do come handy. Especially if they are static
variables - meaning, they are recognized only on the scope of a single file.
In multi-threaded programs, we also might find a need for such variables.
We should note, however, that the same variable is accessible from all the
threads, so we need to protect access to it using a mutex, which is extra
overhead. Further more, we sometimes need to have a variable that is
'global', but only for a specific thread. Or the same 'global' variable
should have different values in different threads. For example, consider
a program that needs to have one globally accessible linked list in each
thread, but note the same list. Further, we want the same code to be
executed by all threads. In this case, the global pointer to the start of the
list should be point to a different address in each thread.
In order to have such a pointer, we need a mechanism that enables the same
global variable to have a different location in memory. This is what
the thread-specific data mechanism is used for.
Overview Of Thread-Specific Data Support
In the thread-specific data (TSD) mechanism, we have notions of keys and values.
Each key has a name, and pointer to some memory area. Keys with the same name
in two separate threads always point to different memory locations - this
is handled by the library functions that allocate memory blocks to be
accessed via these keys. We have a function to create a key (invoked once per
key name for the whole process), a function to allocate memory (invoked
separately in each thread), and functions to de-allocate this memory for
a specific thread, and a function to destroy the key, again, process-wide.
we also have functions to access the data pointed to by a key, either
setting its value, or returning the value it points to.
Allocating Thread-Specific Data Block
The pthread_key_create() function is used to allocate
a new key. This key now becomes valid for all threads in our process.
When a key is created, the value it points to defaults to NULL. Later
on each thread may change its copy of the value as it wishes. Here is
how to use this function:
/* rc is used to contain return values of pthread functions */
int rc;
/* define a variable to hold the key, once created. */
pthread_key_t list_key;
/* cleanup_list is a function that can clean up some data */
/* it is specific to our program, not to TSD */
extern void* cleanup_list(void*);
/* create the key, supplying a function that'll be invoked when it's deleted. */
rc = pthread_key_create(&list_key, cleanup_list);
Some notes:
- After
pthread_key_create() returns, the variable 'list_key'
points to the newly created key.
- The function pointer passed as second parameter to
pthread_key_create() , will be automatically invoked by the
pthread library when our thread exits, with a pointer to the key's value
as its parameter. We may supply a NULL pointer as the function pointer,
and then no function will be invoked for key. Note that the function will
be invoked once in each thread, even thought we created this key only
once, in one thread.
If we created several keys, their associated destructor functions will
be called in an arbitrary order, regardless of the order of keys creation.
- If the
pthread_key_create() function succeeds, it returns 0.
Otherwise, it returns some error code.
- There is a limit of
PTHREAD_KEYS_MAX keys that may exist
in our process at any given time. An attempt to create a key after
PTHREAD_KEYS_MAX exits, will cause a return value of
EAGAIN from the pthread_key_create() function.
Accessing Thread-Specific Data
After we have created a key, we may access its value using two pthread
functions: pthread_getspecific() and
pthread_setspecific() . The first is used to get the value of a
given key, and the second is used to set the data of a given key. A key's value
is simply a void pointer (void*), so we can store in it anything that we want.
Lets see how to use these functions. We assume that 'a_key' is a properly
initialized variable of type pthread_key_t that contains a
previously created key:
/* this variable will be used to store return codes of pthread functions */
int rc;
/* define a variable into which we'll store some data */
/* for example, and integer. */
int* p_num = (int*)malloc(sizeof(int));
if (!p_num) {
fprintf(stderr, "malloc: out of memory\n";
exit(1);
}
/* initialize our variable to some value */
(*p_num) = 4;
/* now lets store this value in our TSD key. */
/* note that we don't store 'p_num' in our key. */
/* we store the value that p_num points to. */
rc = pthread_setspecific(a_key, (void*)p_num);
.
.
/* and somewhere later in our code... */
.
.
/* get the value of key 'a_key' and print it. */
{
int* p_keyval = (int*)pthread_getspecific(a_key);
if (p_keyval != NULL) {
printf("value of 'a_key' is: %d\n", *p_keyval);
}
}
Note that if we set the value of the key in one thread, and try to get it
in another thread, we will get a NULL, since this value is distinct for
each thread.
Note also that there are two cases where pthread_getspecific()
might return NULL:
- The key supplied as a parameter is invalid (e.g. its key wasn't created).
- The value of this key is NULL. This means it either wasn't initialized,
or was set to NULL explicitly by a previous call to
pthread_setspecific() .
Deleting Thread-Specific Data Block
The pthread_key_delete() function may be used to delete keys.
But do not be confused by this function's name: it does not delete memory
associated with this key, nor does it call the destructor function defined
during the key's creation. Thus, you still need to do memory cleanup on
your own if you need to free this memory during runtime. However, since
usage of global variables (and thus also thread-specific data), you usually
don't need to free this memory until the thread terminate, in which case
the pthread library will invoke your destructor functions anyway.
Using this function is simple. Assuming list_key is a
pthread_key_t variable pointing to a properly created key, use
this function like this:
int rc = pthread_key_delete(key);
the function will return 0 on success, or EINVAL if the supplied variable
does not point to a valid TSD key.
A Complete Example
None yet. Give me a while to think of one...... sorry. All i can
think of right now is 'global variables are evil'. I'll try to find a good
example for the future. If you have a good example, please let me know.
Thread Cancellation And Termination
As we create threads, we need to think about terminating them as well.
There are several issues involved here. We need to be able to
terminate threads cleanly. Unlike processes, where a very ugly method of
using signals is used, the folks that designed the pthreads library were
a little more thoughtful. So they supplied us with a whole system of
canceling a thread, cleaning up after a thread, and so on. We will discuss
these methods here.
Canceling A Thread
When we want to terminate a thread, we can use the pthread_cancel
function. This function gets a thread ID as a parameter, and sends a
cancellation request to this thread. What this thread does with this
request depends on its state. It might act on it immediately, it might
act on it when it gets to a cancellation point (discussed below), or
it might completely ignore it. We'll see later how to set the state of
a thread and define how it acts on cancellation requests. Lets first see
how to use the cancel function. We assume that 'thr_id' is a variable
of type pthread_id containing the ID of a running thread:
pthread_cancel(thr_id);
The pthread_cancel() function returns 0, so we cannot know
if it succeeded or not.
Setting Thread Cancellation State
A thread's cancel state may be modified using several methods. The first
is by using the pthread_setcancelstate() function. This function
defines whether the thread will accept cancellation requests or not. The
function takes two arguments. One that sets the new cancel state, and one
into which the previous cancel state is stored by the function. Here is
how it is used:
int old_cancel_state;
pthread_setcancelstate(PTHREAD_CANCEL_DISABLE, &old_cancel_state);
This will disable canceling this thread. We can also enable canceling
the thread like this:
int old_cancel_state;
pthread_setcancelstate(PTHREAD_CANCEL_ENABLE, &old_cancel_state);
Note that you may supply a NULL pointer as the second parameter, and then
you won't get the old cancel state.
A similar function, named pthread_setcanceltype() is used
to define how a thread responds to a cancellation request, assuming
it is in the 'ENABLED' cancel state. One option is to handle the request
immediately (asynchronously). The other is to defer the request until
a cancellation point. To set the first option (asynchronous cancellation),
do something like:
int old_cancel_type;
pthread_setcanceltype(PTHREAD_CANCEL_ASYNCHRONOUS, &old_cancel_type);
And to set the second option (deferred cancellation):
int old_cancel_type;
pthread_setcanceltype(PTHREAD_CANCEL_DEFERRED, &old_cancel_type);
Note that you may supply a NULL pointer as the second parameter, and then
you won't get the old cancel type.
You might wonder - "What if i never set the cancellation state or type
of a thread?". Well, in such a case, the pthread_create()
function automatically sets the thread to enabled deferred cancellation,
that is, PTHREAD_CANCEL_ENABLE for the cancel mode, and
PTHREAD_CANCEL_DEFERRED for the cancel type.
Cancellation Points
As we've seen, a thread might be in a state where it does not handle
cancel requests immediately, but rather defers them until it reaches
a cancellation point. So what are these cancellation points?
In general, any function that might suspend the execution of a thread
for a long time, should be a cancellation point. In practice, this
depends on the specific implementation, and how conformant it is to
the relevant POSIX standard (and which version of the standard it
conforms to...). The following set of pthread functions serve as
cancellation points:
-
pthread_join()
-
pthread_cond_wait()
-
pthread_cond_timedwait()
-
pthread_testcancel()
-
sem_wait()
-
sigwait()
This means that if a thread executes any of these functions, it'll check
for deferred cancel requests. If there is one, it will execute the cancellation
sequence, and terminate. Out of these functions,
pthread_testcancel() is unique - it's only purpose is to test
whether a cancellation request is pending for this thread. If there is,
it executes the cancellation sequence. If not, it returns immediately. This
function may be used in a thread that does a lot of processing without
getting into a "natural" cancellation state.
Note: In real conformant implementations of the pthreads standard, normal
system calls that cause the process to block, such as read() ,
select() , wait() and so on, are also cancellation
points. The same goes for standard C library functions that use these
system calls (the various printf functions, for example).
Setting Thread Cleanup Functions
One of the features the pthreads library supplies is the ability for
a thread to clean up after itself, before it exits. This is done by
specifying one or more functions that will be called automatically
by the pthreads library when the thread exits, either due to its
own will (e.g. calling pthread_exit() ), or due to it being
canceled.
Two functions are supplied for this purpose. The
pthread_cleanup_push() function is used to add a cleanup function
to the set of cleanup functions for the current thread. The
pthread_cleanup_pop() function removes the last function added
with pthread_cleanup_push() . When the thread terminates, its
cleanup functions are called in the reverse order of their registration. So
the the last one to be registered is the first one to be called.
When the cleanup functions are called, each one is supplied with one parameter,
that was supplied as the second parameter to the
pthread_cleanup_push() function call. Lets see how these functions
may be used. In our example we'll see how these functions may be used to
clean up some memory that our thread allocates when it starts running.
/* first, here is the cleanup function we want to register. */
/* it gets a pointer to the allocated memory, and simply frees it. */
void
cleanup_after_malloc(void* allocated_memory)
{
if (allocated_memory)
free(allocated_memory);
}
/* and here is our thread's function. */
/* we use the same function we used in our */
/* thread-pool server. */
void*
handle_requests_loop(void* data)
{
.
.
/* this variable will be used later. please read on... */
int old_cancel_type;
/* allocate some memory to hold the start time of this thread. */
/* assume MAX_TIME_LEN is a previously defined macro. */
char* start_time = (char*)malloc(MAX_TIME_LEN);
/* push our cleanup handler. */
pthread_cleanup_push(cleanup_after_malloc, (void*)start_time);
.
.
/* here we start the thread's main loop, and do whatever is desired.. */
.
.
.
/* and finally, we unregister the cleanup handler. our method may seem */
/* awkward, but please read the comments below for an explanation. */
/* put the thread in deferred cancellation mode. */
pthread_setcanceltype(PTHREAD_CANCEL_DEFERRED, &old_cancel_type);
/* supplying '1' means to execute the cleanup handler */
/* prior to unregistering it. supplying '0' would */
/* have meant not to execute it. */
pthread_cleanup_pop(1);
/* restore the thread's previous cancellation mode. */
pthread_setcanceltype(old_cancel_type, NULL);
}
As we can see, we allocated some memory here, and registered a cleanup handler
that will free this memory when our thread exits. After the execution of
the main loop of our thread, we unregistered the cleanup handler. This must
be done in the same function that registered the cleanup handler, and in the
same nesting level, since both pthread_cleanup_pop()
and pthread_cleanup_pop() functions are actually macros
that add a '{' symbol and a '}' symbol, respectively.
As to the reason that we used that complex piece of code to unregister
the cleanup handler, this is done to assure that our thread won't get
canceled in the middle of the execution of our cleanup handler.
This could have happened if our thread was in asynchronous cancellation
mode. Thus, we made sure it was in deferred cancellation mode, then
unregistered the cleanup handler, and finally restored whatever cancellation
mode our thread was in previously. Note that we still assume the thread cannot
be canceled in the execution of pthread_cleanup_pop() itself -
this is true, since pthread_cleanup_pop() is not a cancellation
point.
Synchronizing On Threads Exiting
Sometimes it is desired for a thread to wait for the end of execution of
another thread. This can be done using the pthread_join()
function. It receives two parameters: a variable of type pthread_t ,
denoting the thread to be joined, and an address of a void*
variable, into which the exit code of the thread will be placed (or
PTHREAD_CANCELED if the joined thread was canceled).
The pthread_join() function suspends the execution of the
calling thread until the joined thread is terminated.
For example, consider our earlier thread pool server.
Looking back at the code, you'll see that we used an odd sleep()
call before terminating the process. We did this since the main thread
had no idea when the other threads finished processing all pending
requests. We could have solved it by making the main thread run a loop
of checking if no more requests are pending, but that would be a busy loop.
A cleaner way of implementing this, is by adding three changes to the code:
- Tell the handler threads when we are done creating requests, by setting
some flag.
- Make the threads check, whenever the requests queue is empty, whether
or not new requests are supposed to be generated. If not, then the
thread should exit.
- Make the main thread wait for the end of execution of each of the threads
it spawned.
The first 2 changes are rather easy. We create a global variable named
'done_creating_requests' and set it to '0' initially. Each thread checks
the contents of this variable every time before it intends to go to wait
on the condition variable (i.e. the requests queue is empty).
The main thread is modified to set this variable to '1' after it finished
generating all requests. Then the condition variable is being broadcast,
in case any of the threads is waiting on it, to make sure all threads
go and check the 'done_creating_requests' flag.
The last change is done using a pthread_join() loop:
call pthread_join() once for each handler thread. This way,
we know that only after all handler threads have exited, this loop
is finished, and then we may safely terminate the process. If we didn't
use this loop, we might terminate the process while one of the handler
threads is still handling a request.
The modified program is available in the file named
thread-pool-server-with-join.c.
Look for the word 'CHANGE' (in capital letters) to see the locations
of the three changes.
Detaching A Thread
We have seen how threads can be joined using the pthread_join()
function. In fact, threads that are in a 'join-able' state, must be
joined by other threads, or else their memory resources will not be fully
cleaned out. This is similar to what happens with processes whose parents
didn't clean up after them (also called 'orphan' or 'zombie' processes).
If we have a thread that we wish would exit whenever it wants without
the need to join it, we should put it in the detached state. This can
be done either with appropriate flags to the pthread_create()
function, or by using the pthread_detach() function. We'll
consider the second option in our tutorial.
The pthread_detach() function gets one parameter, of type
pthread_t , that denotes the thread we wish to put in the detached
state. For example, we can create a thread and immediately detach it
with a code similar to this:
pthread_t a_thread; /* store the thread's structure here */
int rc; /* return value for pthread functions. */
extern void* thread_loop(void*); /* declare the thread's main function. */
/* create the new thread. */
rc = pthread_create(&a_thread, NULL, thread_loop, NULL);
/* and if that succeeded, detach the newly created thread. */
if (rc == 0) {
rc = pthread_detach(a_thread);
}
Of-course, if we wish to have a thread in the detached state immediately,
using the first option (setting the detached state directly when calling
pthread_create() is more efficient.
Threads Cancellation - A Complete Example
Our next example is much larger than the previous examples. It demonstrates
how one could write a multi-threaded program in C, in a more or less clean
manner. We take our previous thread-pool server, and enhance it in two
ways. First, we add the ability to tune the number of handler threads
based on the requests load. New threads are created if the requests queue
becomes too large, and after the queue becomes shorter again, extra threads
are canceled.
Second, we fix up the termination of the server when there are no more new
requests to handle. Instead of the ugly sleep we used in our first example,
this time the main thread waits for all threads to finish handling their
last requests, by joining each of them using pthread_join() .
The code is now being split to 4 separate files, as follows:
- requests_queue.c
- This file contains functions to manipulate a requests queue. We took
the
add_request() and get_request() functions
and put them here, along with a data structure that contains all the
variables previously defined as globals - pointer to queue's head,
counter of requests, and even pointers to the queue's mutex and
condition variable. This way, all the manipulation of the data is done
in a single file, and all its functions receive a pointer to a
'requests_queue' structure.
- handler_thread.c
- this contains the functions executed by each handler thread - a function
that runs the main loop (an enhanced version of the
'handle_requests_loop()' function, and a few local functions explained
below). We also define a data structure to collect all the data we want
to pass to each thread. We pass a pointer to such a structure as a
parameter to the thread's function in the
pthread_create()
call, instead of using a bunch of ugly globals: the thread's ID, a pointer
to the requests queue structure, and pointers to the mutex and condition
variable to be used.
-
handler_threads_pool.c -
here we define an abstraction of a thread pool. We have a function
to create a thread, a function to delete (cancel) a thread, and a function
to delete all active handler threads, called during program termination.
we define here a structure similar to that used to hold the requests
queue, and thus the functions are similar. However, because we only
access this pool from one thread, the main thread, we don't need to
protect it using a mutex. This saves some overhead caused by mutexes.
the overhead is small, but for a busy server, it might begin to become
noticeable.
- main.c -
and finally, the main function to rule them all, and in the system
bind them. This function creates a requests queue, creates a threads
pool, creates few handler threads, and then starts generating requests.
After adding a request to the queue, it checks the queue size and the
number of active handler threads, and adjusts the number of threads
to the size of the queue. We use a simple
water-marks algorithm here,
but as you can see from the code, it can be easily be replaced by
a more sophisticated algorithm. In our water-marks algorithm
implementation, when the high water-mark is reached, we start creating new
handler threads, to empty the queue faster. Later, when the low water-mark
is reached, we start canceling the extra threads, until we are left with
the original number of handler threads.
After rewriting the program in a more manageable manner, we added code that
uses the newly learned pthreads functions, as follows:
- Each handler thread created puts itself in the deferred cancellation mode.
This makes sure that when it gets canceled, it can finish handling
its current request, before terminating.
- Each handler thread also registers a cleanup function, to unlock
the mutex when it terminates. This is done, since a thread is most likely
to get canceled when calling
pthread_cond_wait() , which
is a cancellation point. Since the function is called with the mutex
locked, it might cause the thread to exit and cause all other threads
to 'hang' on the mutex. Thus, unlocking the mutex in a cleanup
handler (registered with the pthread_cleanup_push() function)
is the proper solution.
- Finally, the main thread is set to clean up properly, and not brutally,
as we did before. When it wishes to terminate, it calls the
'delete_handler_threads_pool()' function, which calls
pthread_join for each remaining handler thread. This way,
the function returns only after all handler threads finished handling
their last request.
Please refer to the source code for
the full details. Reading the header files first will make it easier to
understand the design. To compile the program, just switch to the
thread-pool-server-changes directory, and type 'gmake'.
Exercise: our last program contains some possible race condition during
its termination process. Can you see what this race is all about? Can you offer
a complete solution to this problem? (hint - think of what happens to threads
deleted using 'delete_handler_thread()').
Exercise 2: the way we implement the water-marks algorithm might come up too
slow on creation of new threads. Try thinking of a different algorithm that
will shorten the average time a request stays on the queue until it gets
handled. Add some code to measure this time, and experiment until you find
your "optimal pool algorithm". Note - Time should be measured in very small
units (using the getrusage system call), and several runs of
each algorithm should be made, to get more accurate measurements.
Using Threads For Responsive User Interface Programming
One area in which threads can be very helpful is in user-interface programs.
These programs are usually centered around a loop of reading user input,
processing it, and showing the results of the processing. The processing part
may sometimes take a while to complete, and the user is made to wait during
this operation. By placing such long operations in a seperate thread, while
having another thread to read user input, the program can be more responsive.
It may allow the user to cancel the operation in the middle.
In graphical programs the problem is more severe, since the application
should always be ready for a message from the windowing system telling it
to repaint part of its window. If it's too busy executing some other
task, its window will remain blank, which is rather ugly. In such a case,
it is a good idea to have one thread handle the message loop of the windowing
systm and always ready to get such repain requests (as well as user input).
When ever this thread sees a need to do an operation that might take a long
time to complete (say, more than 0.2 seconds in the worse case), it will
delegate the job to a seperate thread.
In order to structure things better, we may use a third thread, to control
and synchronize the user-input and task-performing threads. If the user-input
thread gets any user input, it will ask the controlling thread to handle the
operation. If the task-performing thread finishes its operation, it will ask
the controlling thread to show the results to the user.
User Interaction - A Complete Example
As an example, we will write a simple character-mode program that counts
the number of lines in a file, while allowing the user to cancel the operation
in the middle.
Our main thread will launch one thread to perform the
line counting, and a second thread to check for user input. After that,
the main thread waits on a condition variable. When any of the threads finishes
its operation, it signals this condition variable, in order to let the main
thread check what happened. A global variable is used to flag whether or not
a cancel request was made by the user. It is initialized to '0', but if
the user-input thread receives a cancellation request (the user pressing 'e'),
it sets this flag to '1', signals the condition variable, and terminates.
The line-counting thread will signal the condition variable only after it
finished its computation.
Before you go read the program, we should explain the use of the
system() function and the 'stty' Unix command. The
system() function spawns a shell in which it executes the Unix
command given as a parameter. The stty Unix command is used to
change terminal mode settings. We use it to switch the terminal from
its default, line-buffered mode, to a character mode (also known as raw mode),
so the call to getchar() in the user-input thread will return
immediatly after the user presses any key. If we hadn't done so, the system will
buffer all input to the program until the user presses the ENTER key. Finally,
since this raw mode is not very useful (to say the least) once the program
terminates and we get the shell prompt again, the user-input thread registers
a cleanup function that restores the normal terminal mode, i.e. line-buffered.
For more info, please refer to stty's manual page.
The program's source can be found in the file
line-count.c.
The name of the file whose lines it reads is hardcoded to
'very_large_data_file'. You should create a file with this name in the
program's directory (large enough for the operation to take enough time).
Alternatively, you may un-compress the file 'very_large_data_file.Z' found
in this directory, using the command:
uncompress very_large_data_file.Z
note that this will create a 5MB(!) file named 'very_large_data_file', so make
sure you have enough free disk-space before performing this operation.
Using 3rd-Party Libraries In A Multi-Threaded Application
One more point, and a very important one, should be taken by programmers
employeeing multi-threading in their programs. Since a multi-threaded program
might have the same function executed by different threads at the same time,
one must make sure that any function that might be invoked from more than one
thread at a time, is MT-safe (Multi-Thread Safe). This means that any access
to data structures and other shared resources is protected using mutexes.
It may be possibe to use a non-MT-safe library in a multi-threaded programs in
two ways:
- Use this library only from a single thread. This way we are assured
that no function from the library is executed simultanouasly from two
seperate threads. The problem here is that it might limit your whole
design, and might force you to add more communications between threads,
if another thread needs to somehow use a function from this library.
- Use mutexes to protect function calls to the library. This means
that a single mutex is used by any thread invoking any function in this
library. The mutex is locked, the function is invoked, and then the mutex
is unlocked. The problem with this solution is that the locking is not
done in a fine granularity - even if two functions from the library do not
interfere with each other, they still cannot be invoked at the same
time by seperate threads. The second thread will be blocked on the mutex
until the first thread finishes the function call. You might call for using
seperate mutexes for unrelated functions, but usually you've no idea how
the library really works and thus cannot know which functions access the
same set of resources. More than that, even if you do know that, a new
version of the library might behave differently, forcing you to modify
your whole locking system.
As you can see, non-MT-safe libraries need special attention, so it is best
to find MT-safe libraries with a similar functionality, if possible.
Using A Threads-Aware Debugger
One last thing to note - when debugging a multi-threaded application, one
needs to use a debugger that "sees" the threads in the program. Most
up-to-date debuggers that come with commercial development environments
are thread-aware. As for Linux, gdb as is shiped with most (all?) distributions
seems to be not thread-aware. There is a project, called 'SmartGDB', that
added thread support to gdb, as well as a graphical user interface (which
is almost a must when debugging multi-threaded applications). However,
it may be used to debug only multi-threaded applications that use
the various user-level thread libraries. Debugging LinuxThreads with SmartGDB
requires applying some kernel patches, that
are currently available only for Linux kernels from the 2.1.X series. More
information about this tool may be found at
http://hegel.ittc.ukans.edu/projects/smartgdb/.
There is also some information about availability of patches to the 2.0.32
kernel and gdb 4.17. This information may be found on the
LinuxThreads homepage.
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