How to "multithread" C code

Asked 2010-10-11T16:19:13.067

I have a number crunching application written in C. It is kind of a main loop that for each value calls, for increasing values of "i", a function that performs some calculations. I read about multithreading, and I am considering learning a bit about it, in C. I wonder if somehow general code like mine could be automatically multithreaded and how.

Thanks

P.D. To get an idea about my code, let's say that it is something like this:

main(...)
for(i=0;i<=ntimes;i++)get_result(x[i],y[i],result[i]);

...

void get_result(float x,float y,float result){
  result=sqrt(log (x) + log (y) + cos (exp (x + y));
(and some more similar mathematical operations)
}

c multithreading

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2 Answers

One alternative to multithread your code would be using pthreads ( provides more precise control than OpenMP ).

Assuming x, y & result are global variable arrays,

#include <pthread.h>

...

void *get_result(void *param)  // param is a dummy pointer
{
...
}

int main()
{
...
pthread_t *tid = malloc( ntimes * sizeof(pthread_t) );

for( i=0; i<ntimes; i++ ) 
    pthread_create( &tid[i], NULL, get_result, NULL );

... // do some tasks unrelated to result    

for( i=0; i<ntimes; i++ ) 
    pthread_join( tid[i], NULL );
...
}

(Compile your code with gcc prog.c -lpthread)

answered 2010-10-16T17:25:31.290

If an iteration in loop is independent of the ones before it, then there's a very simple approach: try multi-processing, rather than multi-threading.

Say you have 2 cores and ntimes is 100, then 100/2=50, so create 2 versions of the program where the first iterates from 0 to 49, the other from 50 to 99. Run them both, your cores should be kept quite busy.

This is a very simplistic approach, yet you don't have to mess with thread creation, synchronization, etc

answered 2010-10-18T15:32:50.883

How to "multithread" C code

2 Answers

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