add option to define num_steps by the command line

chapters/chapter14/pi/pi_spin/main.c

@@ -5,7 +5,7 @@
 #include <sys/time.h>
 
 volatile int SEM = 1;
-int num_steps = 10000000;
+long long num_steps = 10000000;
 double step;
 double sum;
 
@@ -47,6 +47,13 @@ int main(int argc, char** argv)
 	thread_param thr_arg[MAX_THREADS];
 	struct timeval start, end;
 
+	if (argc > 1)
+		num_steps = atoll(argv[1]);
+	if (num_steps < 100)
+		num_steps = 1000000;
+
+	printf("\nnum_steps = %lld\n", num_steps);
+
 	step = 1.0 / (double)num_steps;
 	sum = 0.0;