I m doing thread programming and trying to implement MonteCarlo technique for calculating Pi value in it. I compiled the code and I have no error but when I execute I get no output for it. Kindly correct me if there's any mistake.
Here's my code:
#include <stdio.h>
#include <stdlib.h>
#include <time.h>
#include <pthread.h>
#define frand() ((double) rand() / (RAND_MAX))
#define MAX_LEN 1353
const size_t N = 4;
float circlePoints=0;
void* point_counter(void *param){
float xcord;
float ycord;
while(MAX_LEN){
xcord=frand();
ycord=frand();
float cord = (xcord*xcord) + (ycord*ycord);
if(cord <= 1){
circlePoints++;}
}
}
int main()
{
printf("out");
size_t i;
pthread_t thread[N];
srand(time(NULL));
for( i=0;i <4;++i){
printf("in creating thread");
pthread_create( &thread[i], NULL, &point_counter, NULL);
}
for(i=0;i <4;++i){
printf("in joining thread");
pthread_join( thread[i], NULL );
}
for( i=0;i <4;++i){
printf("in last thread");
float pi = 4.0 * (float)circlePoints /MAX_LEN;
printf("pi is %2.4f: \n", pi);
}
return 0;
}
You're hitting an infinite loop here:
while(MAX_LEN){
Since MAX_LEN is and remains non-zero.
As to why you see no output before that, see Why does printf not flush after the call unless a newline is in the format string?
You have an infinite loop in your thread function:
while(MAX_LEN){
...
}
So all the threads you create never come out that loop.
Also, circlePoints is modified by all the threads which will lead to race condition ( what's a race condition? ) and likely render the value incorrect. You should use a mutex lock to avoid it.
while(any_non_zero_number_which does_not_update)
{
infinite loop //not good unless you intend it that way
}
Related
Consider the following program,
static long count = 0;
void thread()
{
printf("%d\n",++count);
}
int main()
{
pthread_t t;
sigset_t set;
int i,limit = 30000;
struct rlimit rlim;
getrlimit(RLIMIT_NPROC, &rlim);
rlim.rlim_cur = rlim.rlim_max;
setrlimit(RLIMIT_NPROC, &rlim);
for(i=0; i<limit; i++) {
if(pthread_create(&t,NULL,(void *(*)(void*))thread, NULL) != 0) {
printf("thread creation failed\n");
return -1;
}
}
sigemptyset(&set);
sigsuspend(&set);
return 0;
}
This program is expected to print 1 to 30000. But it some times prints 29945, 29999, 29959, etc. Why this is happening?
Because count isn't atomic, so you have a race condition both in the increment and in the subsequent print.
The instruction you need is atomic_fetch_add, to increment the counter and avoid the race condition. The example on cppreference illustrates the exact problem you laid out.
Your example can be made to work with just a minor adjustment:
#include <stdio.h>
#include <signal.h>
#include <sys/resource.h>
#include <pthread.h>
#include <stdatomic.h>
static atomic_long count = 1;
void * thread(void *data)
{
printf("%ld\n", atomic_fetch_add(&count, 1));
return NULL;
}
int main()
{
pthread_t t;
sigset_t set;
int i,limit = 30000;
struct rlimit rlim;
getrlimit(RLIMIT_NPROC, &rlim);
rlim.rlim_cur = rlim.rlim_max;
setrlimit(RLIMIT_NPROC, &rlim);
for(i=0; i<limit; i++) {
if(pthread_create(&t, NULL, thread, NULL) != 0) {
printf("thread creation failed\n");
return -1;
}
}
sigemptyset(&set);
sigsuspend(&set);
return 0;
}
I made a handful of other changes, such as fixing the thread function signature and using the correct printf format for printing longs. But the atomic issue is why you weren't printing all the numbers you expected.
Why this is happening?
Because you have a data race (undefined behavior).
In particular, this statement:
printf("%d\n",++count);
modifies a global (shared) variable without any locking. Since the ++ does not atomically increment it, it's quite possible for multiple threads to read the same value (say 1234), increment it, and store the updated value in parallel, resulting in 1235 being printed repeatedly (two or more times), and one or more of the increments being lost.
A typical solution is to either use mutex to avoid the data race, or (rarely) an atomic variable (which guarantees atomic increment). Beware: atomic variables are quite hard to get right. You are not ready to use them yet.
On a 64-bit architecture pc, the next program should return the result 1.350948.
But it is not thread safe and every time I run it gives (obviously) a different result.
#include <stdlib.h>
#include <stdio.h>
#include <math.h>
#include <pthread.h>
const unsigned int ndiv = 1000;
double res = 0;
struct xval{
double x;
};
// Integrate exp(x^2 + y^2) over the unit circle on the
// first quadrant.
void* sum_function(void*);
void* sum_function(void* args){
unsigned int j;
double y = 0;
double localres = 0;
double x = ((struct xval*)args)->x;
for(j = 0; (x*x)+(y*y) < 1; y = (++j)*(1/(double)ndiv)){
localres += exp((x*x)+(y*y));
}
// Globla variable:
res += (localres/(double)(ndiv*ndiv));
// This is not thread safe!
// mutex? futex? lock? semaphore? other?
}
int main(void){
unsigned int i;
double x = 0;
pthread_t thr[ndiv];
struct xval* xvarray;
if((xvarray = calloc(ndiv, sizeof(struct xval))) == NULL){
exit(EXIT_FAILURE);
}
for(i = 0; x < 1; x = (++i)*(1/(double)ndiv)){
xvarray[i].x = x;
pthread_create(&thr[i], NULL, &sum_function, &xvarray[i]);
// Should check return value.
}
for(i = 0; i < ndiv; i++){
pthread_join(thr[i], NULL);
// If
// pthread_join(thr[i], &retval);
// res += *((double*)retval) <-?
// there would be no problem.
}
printf("The integral of exp(x^2 + y^2) over the unit circle on\n\
the first quadrant is: %f\n", res);
return 0;
}
How can it be thread safe?
NOTE: I know that 1000 threads is not a good way to solve this problem, but I really really want to know how to write thread-safe c programs.
Compile the above program with
gcc ./integral0.c -lpthread -lm -o integral
pthread_mutex_lock(&my_mutex);
// code to make thread safe
pthread_mutex_unlock(&my_mutex);
Declare my_mutex either as a global variable like pthread_mutex_t my_mutex;. Or initialize in code using pthread_mutex_t my_mutex; pthread_mutex_init(&my_mutex, NULL);. Also don't forget to include #include <pthread.h> and link your program with -lpthread when compiling.
The question (in a comment in the code):
// mutex? futex? lock? semaphore? other?
Answer: mutex.
See pthread_mutex_init, pthread_mutex_lock, and pthread_mutex_unlock.
Closed. This question needs details or clarity. It is not currently accepting answers.
Want to improve this question? Add details and clarify the problem by editing this post.
Closed 9 years ago.
Improve this question
I have started learning C in Uni, and now I'm stuck on posix threads. I have a program that has a single thread, 2 threads and 4 threads as an example from lecture. I need your help to extend this program from 4 to 8/16/32 and how it will perform a difference or not?
Thank you in advance.
Here is the code for 4 thread programm:
/****************************************************************************
This program finds groups of three numbers that when multiplied together
equal 98931313. Compile with:
cc -o factorise4 factorise4.c -lrt -pthread
Kevan Buckley, University of Wolverhampton, October 2012
*****************************************************************************/
#include <stdlib.h>
#include <stdio.h>
#include <ctype.h>
#include <errno.h>
#include <sys/stat.h>
#include <string.h>
#include <time.h>
#include <pthread.h>
#include <math.h>
#define goal 98931313
typedef struct arguments {
int start;
int n;
} arguments_t;
void factorise(int n) {
pthread_t t1, t2, t3, t4;
//1st pthread
arguments_t t1_arguments;
t1_arguments.start = 0;
t1_arguments.n = n;
//2nd pthread
arguments_t t2_arguments;
t2_arguments.start = 250;
t2_arguments.n = n;
//3rd pthread
arguments_t t3_arguments;
t3_arguments.start = 500;
t3_arguments.n = n;
//4th pthread
arguments_t t4_arguments;
t4_arguments.start = 750;
t4_arguments.n = n;
void *find_factors();
//creating threads
pthread_create(&t1, NULL, find_factors, &t1_arguments);
pthread_create(&t2, NULL, find_factors, &t2_arguments);
pthread_create(&t3, NULL, find_factors, &t3_arguments);
pthread_create(&t4, NULL, find_factors, &t4_arguments);
pthread_join(t1, NULL);
pthread_join(t2, NULL);
pthread_join(t3, NULL);
pthread_join(t4, NULL);
}
//Using 3 loops, 1 loop represents one value that we need to find, and go throught it until 98931313 not will be find.
void *find_factors(arguments_t *args){
int a, b, c;
for(a=args->start;a<args->start+250;a++){
for(b=0;b<1000;b++){
for(c=0;c<1000;c++){
if(a*b*c == args->n){
printf("solution is %d, %d, %d\n", a, b, c);// Printing out the answer
}
}
}
}
}
// Calculate the difference between two times.
long long int time_difference(struct timespec *start, struct timespec *finish, long long int *difference) {
long long int ds = finish->tv_sec - start->tv_sec;
long long int dn = finish->tv_nsec - start->tv_nsec;
if(dn < 0 ) {
ds--;
dn += 1000000000;
}
*difference = ds * 1000000000 + dn;
return !(*difference > 0);
}
//Prints elapsed time
int main() {
struct timespec start, finish;
long long int time_elapsed;
clock_gettime(CLOCK_MONOTONIC, &start);
factorise(goal); //This is our goal = 98931313
clock_gettime(CLOCK_MONOTONIC, &finish);
time_difference(&start, &finish, &time_elapsed);
printf("Time elaipsed was %lldns or %0.9lfs\n", time_elapsed, (time_elapsed/1.0e9));
return 0;
}
I'll give you a hint:
If you call a function twice manually, you can put its results into two separate variables:
int y0 = f(0);
int y1 = f(1);
You as well can put them into one array:
int y[2];
y[0] = f(0);
y[1] = f(1);
Or into a memory area on heap (obtained via malloc()):
int * y = malloc(2 * sizeof(*y));
y[0] = f(0);
y[1] = f(1);
In the latter two cases, you can replace the two function calls with
for (i = 0; i < 2; i++) {
y[i] = f(i);
}
Another hint:
For a changed number of threads, you will as well have to change your parameter set.
And another hint:
Thread creation, in your case, can be put into a function:
void facthread_create(pthread_t * thread, int start, int n)
{
arguments_t arguments;
arguments.start = start;
arguments.n = n;
void *find_factors();
//creating thread
pthread_create(thread, NULL, find_factors, &arguments);
}
But - there is a caveat: we have a race condition here. As soon as the thread starts, we can return and the stack space occupied by arguments is freed. So we use an improved version here which is useful for cooperation:
We add a field to arguments_t:
typedef struct arguments {
char used;
int start;
int n;
} arguments_t;
We set used to 0:
void facthread_create(pthread_t * thread, int start, int n)
{
arguments_t arguments;
arguments.start = start;
arguments.n = n;
arguments.used = 0;
void *find_factors();
//creating thread
pthread_create(thread, NULL, find_factors, &arguments);
while (!arguments.used); // wait until thread has "really" started
}
Set used to 1 once the data has safely copied:
void *find_factors(arguments_t *args){
arguments_t my_args = *args; // is this valid? Don't remember... If not, do it element-wise...
*args.used = 1; // Inform the caller that it is safe to continue
int a, b, c;
for(a=my_args.start;a<my_args.start+250;a++){
...
You should get a command line parameter (maybe -t for threads). Then instead of calling factorise from main, have a for loop which does the thread create with the parameter which is calculated from the loop number. Something like:
for (int i = 0; i < threads; i++) {
arguments.start = 250 * i;
arguments.n = n;
pthread_start(...)
}
Note that you should allocate the argument structs before the for loop for clarity.
Let me know if you need more help.
Here is some more help:
0) get the number of threads and the skip (in your case 250) from the command line.
1) create a control stuct which contains the args for the thread, the thread id, etc.
2) using the args, allocate the control struct and fill it in.
3) do a for loop to spawn off the treads.
4) do another for loop to wait for the threads to complete.
For some extra complexity, you could introduce a global variable which any thread could set to signal the other threads that the work is done and they should exit. But don't do this until you get the simple case correct.
If you post some updated code, I will help you some more.
I just learn the bare bone of making a thread inside a program using process.h in C programming. And now, my problem is how to stop a specific thread.
Here is my code:
#include <stdio.h>
#include <windows.h>
#include <process.h>
void mimicCounter( void * );
int main()
{
int i;
printf( "Now in the main() function.\n" );
_beginthread( mimicCounter, 0, (void*)12 );
for(i = 2; i <= 10; i++){
Sleep(500);
printf("%d\n",i);
}
system("PAUSE");
printf("\n");
}
void mimicCounter( void *arg )
{
int i;
printf( "The mimicCounter() function was passed %d\n", (INT_PTR)arg ) ;
for(i = 1; i <= 10; i++){
Sleep(500);
printf("%d\n",i);
}
}
I just want to stop the thread that I have created (the mimicCounter function) when it reaches i = 5, (yeah I know I set it to 10 but this is for ending a thread demo).
Thank you so much :)
The _endthread and _endthreadex functions terminate a thread created by _beginthread or _beginthreadex, respectively. You can call _endthread or _endthreadex explicitly to terminate a thread; however, _endthread or _endthreadex is called automatically when the thread returns from the routine passed as a parameter to _beginthread or _beginthreadex. Terminating a thread with a call to endthread or _endthreadex helps to ensure proper recovery of resources allocated for the thread.
From http://msdn.microsoft.com/en-us/library/aa246804(v=vs.60).aspx
is there anyway to duplicate some static variable each time a thread make access to them?
I post a simple example:
Module testF.c
#define <stdio.h>
#include <windows.h>
#include <process.h>
#include "testF.h"
#define MAX_THREADS 10
int *var;
void testF( void *arg ){
int a,N,i;
a = (INT_PTR)arg;
N = (int)(10000/(int)(a+1));
var = (int*) malloc(N*sizeof(int));
for(i = 0; i<N; i++)
var[i] = (int)a;
_endthread();
}
...
And in another module main.c,
...
#include "testF.h"
int main(void){
HANDLE hth[MAX_THREADS];
DWORD dwExitCode;
int i;
for(i = 0; i<MAX_THREADS; i++)
hth[i] = (HANDLE)_beginthread( testF, 0, (void*)i );
WaitForMultipleObjects(MAX_THREADS, hth, TRUE, INFINITE);
for(i = 0; i<MAX_THREADS; i++){
GetExitCodeThread( hth[i], &dwExitCode );
printf( "thread 1 exited with code %u\n", dwExitCode );
CloseHandle( hth[i] );
}
}
In the this example the variable i would like to duplicate is *var.
I've seen that functions like rand() give always the same result if called from different thread, so I think that there should be a way to do it.
Thread-local storage.
Functions like rand() generally use thread local storage to maintain intercall state on a per-thread basis. This is what you would need to do instead of using a language construct like static. Your other option is to supply the variable into the thread start function, instead of using a global or static.