Question

I am writting an application that has multiple threads in Linux environment using C or python. I am using pthread for that. But How number of threads should be accepted via command line.

Answer 1

In C you handle command line arguments by having main take two arguments,

int main(int argc, char** argv)

in which argc is the number of command line arguments (including the program itself) and argv is a pointer to a memory location where argc-1 pointers to strings with the actual arguments are located. Example:

int main(int argc, char** argv)
{
  printf("The program was executed as %s.\n", argv[0]);
  printf("The arguments were:\n");
  for (int i = 1; i < argc; i++)
    printf("%s\n", argv[i]);
  return 0;
}

Let’s now assume that your program takes a single command line argument, an integer telling you how many threads to spawn. The integer is given as a string, so we have to convert it using atoi:

if (argc != 2)
{
  printf("Need exactly one argument!\n");
  return 1;
}
int num_threads = atoi(argv[1]); // Convert first argument to integer.
if (num_threads < 1)
{
  printf("I'll spawn no less than 1 thread!\n");
  return 2;
}

Now what you do is simply create an array of thread handles,

pthread_t* threads = malloc(num_threads*sizeof(pthread_t));

and use it to store the thread handles as you start num_threads number of threads using pthread_create.

If you are not familiar with pthreads at all, I recommend this short tutorial.