I am making a multi-threaded C program which involves the sharing of a global dynamic integer array between two threads. One thread will keep adding elements to it & the other will independently scan the array & free the scanned elements.
can any one suggest me the way how can I do that because what I am doing is creating deadlock
Please also can any one provide the code for it or a way to resolve this deadlock with full explanation
For the threads I would use pthread. Compile it with -pthread.
#include <pthread.h>
int *array;
// return and argument should be `void *` for pthread
void *addfunction(void *p) {
// add to array
}
// same with this thread
void *scanfunction(void *p) {
// scan in array
}
int main(void) {
// pthread_t variable needed for pthread
pthread_t addfunction_t, scanfunction_t; // names are not important but use the same for pthread_create() and pthread_join()
// start the threads
pthread_create(&addfunction_t, NULL, addfunction, NULL); // the third argument is the function you want to call in this case addfunction()
pthread_create(&scanfunction_t, NULL, scanfunction, NULL); // same for scanfunction()
// wait until the threads are finish leave out to continue while threads are running
pthread_join(addfunction_t, NULL);
pthread_join(scanfunction_t, NULL);
// code after pthread_join will executed if threads aren't running anymore
}
Here is a good example/tutorial for pthread: *klick*
In cases like this, you need to look at the frequency and loading generated by each operation on the array. For instance, if the array is being scanned continually, but only added to once an hour, its worth while finding a really slow, latency-ridden write mechanism that eliminates the need for read locks. Locking up every access with a mutex would be very unsatisfactory in such a case.
Without details of the 'scan' operation, especially duration and frequency, it's not possible to suggest a thread communication strategy for good performance.
Anohter thing ee don't know are consequences of failure - it may not matter if a new addition is queued up for a while before actually being inserted, or it may.
If you want a 'Computer Science 101' answer with, quite possibly, very poor performance, lock up every access to the array with a mutex.
http://www.liblfds.org
Release 6 contains a lock-free queue.
Compiles out of the box for Windows and Linux.
Related
For an assignment, I need to use sched_yield() to synchronize threads. I understand a mutex lock/conditional variables would be much more effective, but I am not allowed to use those.
The only functions we are allowed to use are sched_yield(), pthread_create(), and pthread_join(). We cannot use mutexes, locks, semaphores, or any type of shared variable.
I know sched_yield() is supposed to relinquish access to the thread so another thread can run. So it should move the thread it executes on to the back of the running queue.
The code below is supposed to print 'abc' in order and then the newline after all three threads have executed. I looped sched_yield() in functions b() and c() because it wasn't working as I expected, but I'm pretty sure all that is doing is delaying the printing because a function is running so many times, not because sched_yield() is working.
The server it needs to run on has 16 CPUs. I saw somewhere that sched_yield() may immediately assign the thread to a new CPU.
Essentially I'm unsure of how, using only sched_yield(), to synchronize these threads given everything I could find and troubleshoot with online.
#include <stdio.h>
#include <pthread.h>
#include <stdlib.h>
#include <sched.h>
void* a(void*);
void* b(void*);
void* c(void*);
int main( void ){
pthread_t a_id, b_id, c_id;
pthread_create(&a_id, NULL, a, NULL);
pthread_create(&b_id, NULL, b, NULL);
pthread_create(&c_id, NULL, c, NULL);
pthread_join(a_id, NULL);
pthread_join(b_id, NULL);
pthread_join(c_id, NULL);
printf("\n");
return 0;
}
void* a(void* ret){
printf("a");
return ret;
}
void* b(void* ret){
for(int i = 0; i < 10; i++){
sched_yield();
}
printf("b");
return ret;
}
void* c(void* ret){
for(int i = 0; i < 100; i++){
sched_yield();
}
printf("c");
return ret;
}
There's 4 cases:
a) the scheduler doesn't use multiplexing (e.g. doesn't use "round robin" but uses "highest priority thread that can run does run", or "earliest deadline first", or ...) and sched_yield() does nothing.
b) the scheduler does use multiplexing in theory, but you have more CPUs than threads so the multiplexing doesn't actually happen, and sched_yield() does nothing. Note: With 16 CPUs and 2 threads, this is likely what you'd get for "default scheduling policy" on an OS like Linux - the sched_yield() just does a "Hrm, no other thread I could use this CPU for, so I guess the calling thread can keep using the same CPU!").
c) the scheduler does use multiplexing and there's more threads than CPUs, but to improve performance (avoid task switches) the scheduler designer decided that sched_yield() does nothing.
d) sched_yield() does cause a task switch (yielding the CPU to some other task), but that is not enough to do any kind of synchronization on its own (e.g. you'd need an atomic variable or something for the actual synchronization - maybe like "while( atomic_variable_not_set_by_other_thread ) { sched_yield(); }). Note that with an atomic variable (introduced in C11) it'd work without sched_yield() - the sched_yield() (if it does anything) merely makes busy waiting less awful/wasteful.
Essentially I'm unsure of how, using only sched_yield(), to
synchronize these threads given everything I could find and
troubleshoot with online.
That would be because sched_yield() is not well suited to the task. As I wrote in comments, sched_yield() is about scheduling, not synchronization. There is a relationship between the two, in the sense that synchronization events affect which threads are eligible to run, but that goes in the wrong direction for your needs.
You are probably looking at the problem from the wrong end. You need each of your threads to wait to execute until it is their turn, and for them to do that, they need some mechanism to convey information among them about whose turn it is. There are several alternatives for that, but if "only sched_yield()" is taken to mean that no library functions other than sched_yield() may be used for that purpose then a shared variable seems the expected choice. The starting point should therefore be how you could use a shared variable to make the threads take turns in the appropriate order.
Flawed starting point
Here is a naive approach that might spring immediately to mind:
/* FLAWED */
void *b(void *data){
char *whose_turn = data;
while (*whose_turn != 'b') {
// nothing?
}
printf("b");
*whose_turn = 'c';
return NULL;
}
That is, the thread executes a busy loop, monitoring the shared variable to await it taking a value signifying that the thread should proceed. When it has done its work, the thread modifies the variable to indicate that the next thread may proceed. But there are several problems with that, among them:
Supposing that at least one other thread writes to the object designated by *whose_turn, the program contains a data race, and therefore its behavior is undefined. As a practical matter, a thread that once entered the body of the loop in that function might loop infinitely, notwithstanding any action by other threads.
Without making additional assumptions about thread scheduling, such as a fairness policy, it is not safe to assume that the thread that will make the needed modification to the shared variable will be scheduled in bounded time.
While a thread is executing the loop in that function, it prevents any other thread from executing on the same core, yet it cannot make progress until some other thread takes action. To the extent that we can assume preemptive thread scheduling, this is an efficiency issue and contributory to (2). However, if we assume neither preemptive thread scheduling nor the threads being scheduled each on a separate core then this is an invitation to deadlock.
Possible improvements
The conventional and most appropriate way to do that in a pthreads program is with the use of a mutex and condition variable. Properly implemented, that resolves the data race (issue 1) and it ensures that other threads get a chance to run (issue 3). If that leaves no other threads eligible to run besides the one that will modify the shared variable then it also addresses issue 2, to the extent that the scheduler is assumed to grant any CPU to the process at all.
But you are forbidden to do that, so what else is available? Well, you could make the shared variable _Atomic. That would resolve the data race, and in practice it would likely be sufficient for the wanted thread ordering. In principle, however, it does not resolve issue 3, and as a practical matter, it does not use sched_yield(). Also, all that busy-looping is wasteful.
But wait! You have a clue in that you are told to use sched_yield(). What could that do for you? Suppose you insert a call to sched_yield() in the body of the busy loop:
/* (A bit) better */
void* b(void *data){
char *whose_turn = data;
while (*whose_turn != 'b') {
sched_yield();
}
printf("b");
*whose_turn = 'c';
return NULL;
}
That resolves issues 2 and 3, explicitly affording the possibility for other threads to run and putting the calling thread at the tail of the scheduler's thread list. Formally, it does not resolve issue 1 because sched_yield() has no documented effect on memory ordering, but in practice, I don't think it can be implemented without a (full) memory barrier. If you are allowed to use atomic objects then combining an atomic shared variable with sched_yield() would tick all three boxes. Even then, however, there would still be a bunch of wasteful busy-looping.
Final remarks
Note well that pthread_join() is a synchronization function, thus, as I understand the task, you may not use it to ensure that the main thread's output is printed last.
Note also that I have not spoken to how the main() function would need to be modified to support the approach I have suggested. Changes would be needed for that, and they are left as an exercise.
I am trying to write a program that will continuously take reading from a sensor that will monitor water level. Then after every (10-15 mins) it will need to take soil moisture readings from other sensors. I have never used POSIX pthreads before. This is what I have so far as a concept of how it may work.
It seems to be working the way I want it to, but is it a correct way to implement this. Is there anything else I need to do?
void *soilMoisture(void *vargp)
{
sleep(10);
funct();
return NULL;
}
int main()
{
pthread_t pt;
int k=1;
pthread_create(&pt, NULL, soilMoisture, NULL);
while(k>0)
{
printf("This is the main thread (Measuring the water level) : %d\n", k);
sleep(1);
}
return 0;
}
void funct()
{
printf("******(Measuring soil moisture after sleeping for 10SEC)***********\n");
pthread_t ptk;
pthread_create(&ptk, NULL, soilMoisture, NULL);
}
It is not clear why you create a new thread every 10 seconds rather than just letting the original continue. Since the original thread exits, you aren't directly accumulating threads, but you aren't waiting for any of them, so there are some resources unreleased. You also aren't error checking, so you won't know when anything does go wrong; monitoring will simply stop.
You will eventually run out of space, one way or another. You have a couple of options.
Don't create a new thread every 10 seconds. Leave the thread running by making a loop in the soilMoisture() function and do away with funct() — or at least the pthread_create() call in it.
If you must create new threads, make them detached. You'll need to create a non-default pthread_attr_t using the functions outlined and linked to in When pthread_attr_t is not NULL.
There are a myriad issues you've not yet dealt with, notably synchronization between the two threads. If you don't have any such synchronization, you'd be better off with two separate programs — the Unix mantra of "each program does one job but does it well" still applies. You'd have one program to do the soil moisture reading, and the other to do the water level reading. You'll need to decide whether data is stored in a database or otherwise logged, and for how log such data is kept. You'll need to think about rotating logs. What should happen if sensors go off-line? How can you restart threads or processes? How can you detect when threads or processes lock up unexpectedly or exit unexpectedly? Etc.
I assume the discrepancy between 10-15 minutes mentioned in the question and 10 seconds in the code is strictly for practical testing rather than a misunderstanding of the POSIX sleep() function.
I'm creating a function that searches through a directory, prints out files, and when it runs into a folder, a new thread is created to run through that folder and do the same thing.
It makes sense to me to use recursion then as follows:
pthread_t tid[500];
int i = 0;
void *search(void *dir)
{
struct dirent *dp;
DIR *df;
df = opendir(dir)
char curFile[100];
while ((dp = readdir(df)) != NULL)
{
sprintf(curFile, "%s/%s",dir,dp->d_name);
if(isADirectory(curFile))
{
pthread_create(&tid[i], NULL, &search, &curFile);
i++;
}
else
{
printf("%s\n", curFile);
}
}
pthread_join(&tid[i])
return 0;
}
When I do this, however, the function ends up trying to access directories that don't actually exist. Initially I had pthread_join() directly after pthread_create(), which worked, but I don't know if you can count that as multithreading since each thread waits for its worker thread to exit before doing anything.
Is the recursive aspect of this problem even possible, or is it necessary for a new thread to call a different function other than itself?
I haven't dealt with multithreading in a while but if memory serves threads share resources. Which means (in your example) every new thread you make accesses the same variable "i". Now if those threads only read variable "i" there would be no problem whatsoever (every thread keeps reading ... i = 2 wohoo :D).
But issues arise when threads share resources that are being read and written on.
i = 2
i++
// there are many threads running this code
// and "i" is shared among them, are you sure i = 3?
Read, write on shared resources problem is solved with thread synchronization. I recommend reading/googling upon it since it's a pretty unique topic to be solved in one question.
P.S. I pointed out variable "i" in your code but there may be more such resources since your code doesn't display any attempt at thread synchronization.
Consider your while loop. Inside it you have:
sprintf(curFile, "%s/%s",dir,dp->d_name);
and
pthread_create(&tid[i], NULL, &search, &curFile);
So, you mutate the contents of curFile inside the loop, and you also create a thread which you are trying to pass the current contents of curFile. This is a spectacular race hazard - there is no guarantee that the new thread will see the intended contents of curFile, since it may have changed in the meantime. You need to duplicate the string and pass the new thread a copy which won't be mutated by the calling thread. The thread is therefore also going to have be responsible for deallocating the copy, which means either that the search method do exactly that or that you have a second method.
You have another race condition in using i and tid in all threads. As I have suggested in the comment on your question, I think these variables should be method local.
In general I suggest that you read on thread safety and learn about data race hazards before you attempt to use threads. It is usually best to avoid the use of threads unless you really need the extra performance.
I am new to C and wanted to know about race conditions. I found this on the internet and it asked to find the race condition, and a solution to it.
My analysis is that the race condition is in the create-thread() method has the race condition, specifically in the if-else statement. So when the method is being accessed another thread could be created or removed during the check-and-act and the thread_amt would be off.
In order to not have the race condition, then lock the if-else using mutex, semaphores, etc?
Can anyone correct me if I am wrong, and could possibly show me how to implement mutex?
#define MAXT 255
int threads_amt = 0;
int create-thread() // create a new thread
{
int tid;
if (threads_amt == MAXT) return -1;
else
{
threads_amt++;
return tid;
}
}
void release-thread()
{
/* release thread resources */
--threads_amt;
}
Yeah, the race condition in this case happens because you have no guarantee that the checking and the manipulation of threads_amt are going to happen with no interruption/execution of another thread.
Three solutions off the top of my head:
1) Force mutual exclusion to that part of code using a binary semaphore (or mutex) to protect the if-else part.
2) Use a semaphore with initial value MAXT, and then, upon calling create_thread (mind, you can't use hyphens in function names!), use "wait()" (depending on the type of semaphore, it could have different names (such as sem_wait())). After that, create the thread. When calling release_thread(), simply use "signal()" (sem_post(), when using semaphore.h).
3) This is more of an "hardware" solution: you could assume that you are given an atomic function that performs the entire if-else part, and therefore avoids any race condition problem.
Of these solutions, the "easiest" one (based on the code you already have) is the first one.
Let's use semaphore.h's semaphores:
#define MAXT 255
// Global semaphore
sem_t s;
int threads_amt = 0;
int main () {
...
sem_init (&s, 0, 1); // init semaphore (initial value = 1)
...
}
int create_thread() // create a new thread
{
int tid;
sem_wait(&s);
if (threads_amt == MAXT) {
sem_post(&s); // the semaphore is now available
return -1;
}
else
{
threads_amt++;
sem_post(&s); // the semaphore is now available
return tid;
}
}
void release_thread()
{
/* release thread resources */
sem_wait(&s);
--threads_amt;
sem_post(&s);
}
This should work just fine.
I hope it's clear. If it's not, I suggest that you study how semaphores work (use the web, or buy some Operating System book). Also, you mentioned that you are new to C: IMHO you should start with something easier than this: semaphores aren't exactly the next thing you want to learn after the 'hello world' ;-)
The race condition is not in the if() statements.
It is with access to the variable threads_amt that is potentially changed and accessed at the same time in multiple threads.
Essentially, any thread that modifies the variable must have exclusive access to avoid a race condition. That means all code which modifies the variable or reads its value must be synchronised (e.g. grab a mutex first, release after). Readers don't necessarily need exclusive access (e.g. two threads reading at the same time won't necessarily affect each other) but writers do (so avoid reading a value while trying to change it in another thread) - such considerations can be opportunities to use synchronisation methods other than a mutex - for example, semaphores.
To use a mutex, it is necessary to create it first (e.g. during project startup). Then grab it when needed, and remember to release it when done. Every function should minimise the time that it holds the mutex, since other threads trying to grab the mutex will be forced to wait.
The trick is to make the grabbing and releasing of the mutex unconditional, wherever it occurs (i.e. avoid a function that grabs the mutex, being able to return without releasing it). That depends on how you structure each function.
The actual code for implementing depends on which threading library you're using (so you need to read the documentation) but the concepts are the same. All threading libraries have functions for creating, grabbing (or entering), and releasing mutexes, semaphores, etc etc.
I have a function that is called millions of times, and the work done by this function is multithreaded. Here is the function:
void functionCalledSoManyTimes()
{
for (int i = 0; i < NUM_OF_THREADS; i++)
{
pthread_create(&threads[i], &attr, thread_work_function, (void *)&thread_data[i]);
}
// wait
}
I'm creating the threads each time the function is called, and I give each thread its data struct (that's been set once at the beginning of the algorithm) to use in the thread_work_function. The thread_work_functionsimply processes a series of arrays, and the thread_data struct contains pointers to those arrays and the indices that each thread is responsible for.
Although multithreading the algorithm in this way did improve the performance by more than 20%, my profiling shows that the repetitive calls to pthread_create are causing a significant overhead.
My question is: Is there a way to achieve my goal without calling pthread_create each time the function is called?
Problem Solved.
Thank you guys, I really appreciate your help! I've written a solution here using your tips.
Just start a fixed set of threads and use an inter-thread communication system (ring buffer, for instance) to pass the data to process.
Solving the problem gracefully is not so easy. You can use static storage for a thread pool, but then what happens if functionCalledSoManyTimes itself can be called from multiple threads? It's not a good design.
What I would do to handle this sort of situation is create a thread-local storage key with pthread_key_create on the first call (using pthread_once), and store your thread-pool there with pthread_setspecific the first time functionCalledSoManyTimes gets called in a given thread. You can provide a destructor function to pthread_key_create which will get called when the the thread exists, and this function can then be responsible for signaling the worker threads in the thread pool to terminate themselves (via pthread_cancel or some other mechanism).