I've noticed that a few of my classmates have actually tried asking questions about this same assignment on StackOverflow over the past few days so I'm going to shamelessly copy paste (only) the context of one question that was deleted (but still cached on Google with no answers) to save time. I apologize in advance for that.
Context
I am trying to write a C program that measures the data throughput (MBytes/sec) of the L2 cache of my system. To perform the measurement I have to write a program that copies an array A to an array B, repeated multiple times, and measure the throughput.
Consider at least two scenarios:
Using memcpy() from string.h to copy the arrays, initialize both arrays with some values (e.g. random numbers using rand()), and repeat at least 100 times, otherwise you do not see a difference.
The array size and number of repeats should be input parameters. One of the array sizes should be half of my L2 cache size.
Question
So based on that context of the assignment I have a good idea of what I need to do because it pretty much tells me straight out. The problem is that we were given some template code to work with and I'm having trouble deciphering parts of it. I would really appreciate it if someone would help me to just figure out what is going on.
The code is:
/* do not add other includes */
#include <stdio.h>
#include <stdlib.h>
#include <sys/time.h>
#include <time.h>
#include <string.h>
double getTime(){
struct timeval t;
double sec, msec;
while (gettimeofday(&t, NULL) != 0);
sec = t.tv_sec;
msec = t.tv_usec;
sec = sec + msec/1000000.0;
return sec;
}
/* for task 1 only */
void usage(void)
{
fprintf(stderr, "bandwith [--no_iterations iterations] [--array_size size]\n");
exit(1);
}
int main (int argc, char *argv[])
{
double t1, t2;
/* variables for task 1 */
unsigned int size = 1024;
unsigned int N = 100;
unsigned int i;
/* declare variables; examples, adjust for task */
int *A;
int *B;
/* parameter parsing task 1 */
for(i=1; i<(unsigned)argc; i++) {
if (strcmp(argv[i], "--no_iterations") == 0) {
i++;
if (i < argc)
sscanf(argv[i], "%u", &N);
else
usage();
} else if (strcmp(argv[i], "--array_size") == 0) {
i++;
if (i < argc)
sscanf(argv[i], "%u", &size);
else
usage();
} else usage();
}
/* allocate memory for arrays; examples, adjust for task */
A = malloc (size*size * sizeof (int));
B = malloc (size*size * sizeof (int));
/* initialise arrray elements */
t1 = getTime();
/* code to be measured goes here */
t2 = getTime();
/* output; examples, adjust for task */
printf("time: %6.2f secs\n",t2 - t1);
/* free memory; examples, adjust for task */
free(B);
free(A);
return 0;
}
My questions are:
size variable value of 1024 would result in 1MB * sizeof(int) (I think at least). In this case would the array size I should record be 1MB or 1MB * sizeof(int)?size variable value would the array size always be the size variable squared? Or would the array size be considered to be just the size variable? It seems very unintuitive to malloc size*size instead of just size unless there's something I'm missing about all this.These are the only hurdles in my understanding of this assignment. Any help would be much appreciated.
The usage function tells you what arguments are supposed to be passed to the program on the command-line.
It leads the calling the usage() function when an invalid argument is passed to the program.
Otherwise, it sets the number of iterations to the variable N to the value of the argument no_iterations (default value of 100), and it sets the size of the array to the variable size to the value of the argument array_size (default value of 1024).
If your size is supposed to be 1 MB, then that is probably what the size should be.
If you want to make it sure the size is a factor of the size of the data type, then you can do:
if (size % sizeof(int) != 0)
{
size = ((int)(size / sizeof(int))) * sizeof(int);
}
You probably just want to allocate size bytes. Unless you are supposed to be working with matrices, rather than just arrays. In that case, it would be size * size bytes.
I guess so.