Calculate time in C or CPP program

      There are many methods to calculate time taken by part of execution or whole program in gcc as mention below.

       Out of this time() function gives the accuracy up-to seconds. But generally we require accuracy in micro or nano seconds to check the little difference. So we are going to see the time calculation using clock_gettime() and gettimeofday() function for well known application matrix multiplication.
      The programs matrix_mul_gettimeofday.c given below show the time calculated using gettimeofday() function

#include"stdio.h"
#include"sys/time.h"
#include"stdlib.h"
#define N 4000
#define X 300
#define Y 200

int main()
{
 int sum = 0,i,j,k;
 int **mat1; //declared mat1[X][N]
 int **mat2; //declare mat2[N][Y]
  int **mat_res; //resultant matrix become mat_res[X][Y]
  struct timeval tim;
  double t1,t2,result;
  
  /*----------------------------------------------------*/
 //create array of pointers(Rows)
  mat1 =(int **) malloc(X*sizeof(int*));
  mat2 =(int **)malloc(N*sizeof(int*));
  mat_res=(int **)malloc(X*sizeof(int*));
  /*----------------------------------------------------*/
  
  /*--------------------------------------------------------------------------------*/
 //allocate memory for each Row pointer
  for(i=0; i < X; i++)
  {
   mat1[i] = (int *) malloc(N * sizeof(int));
   mat_res[i] = (int *) malloc(Y * sizeof(int));
  }
  
  for(i = 0; i < N; i++)
  mat2[i] = (int *) malloc(Y * sizeof(int));
  /*--------------------------------------------------------------------------------*/
  
  for(i = 0; i < X; i++)
 {
  for(j = 0; j < N; j++)
  {
   mat1[i][j] = 1; 
  }
 }
 
 for(i = 0; i < N; i++)
 {
  for(j = 0; j < Y; j++)
  {
   mat2[i][j] = 2;
  }
 }
 //------------------calculate Start time------------------------
 gettimeofday(&tim, NULL);
 t1 = tim.tv_sec + (tim.tv_usec/1000000.0);
 //--------------------------------------------------------------
 
  for(i = 0; i < X; i++)
  {
   for(j = 0; j < Y;j++)
   {
    sum=0;
    for(k = 0; k < N; k++)
    {
     sum = sum + mat1[i][k] * mat2[k][j];
    }
    mat_res[i][j] = sum;
   }
  }
 //---------------calculate End time-----------------------------
  gettimeofday(&tim, NULL);
 t2 = tim.tv_sec + (tim.tv_usec/1000000.0);
 //--------------------------------------------------------------

 // result = End_time - Start_time
 result = t2 - t1;
  for(i = 0; i < X; i++)
  {
   for(j = 0; j < Y; j++)
   {
    printf("%d\t",mat_res[i][j]);
   }
   printf("\n");
  }
  printf("time taken =%f\n",result);
  return 0;
}

Compile:
gcc matrix_mul_gettimeofday.c -o exec
Run:
./exec

The program matrix_mul_clock_gettime.c given below will show how to calculate time using clock_gettime() function

#include"stdio.h"
#include"time.h"
#include"math.h"
#include"stdlib.h"
#define N 4000
#define X 300
#define Y 200

int main()
{
 int sum = 0,i,j,k;
 int **mat1; //declared mat1[X][N]
 int **mat2; //declare mat2[N][Y]
  int **mat_res; //resultant matrix become mat_res[X][Y]
  struct timespec start,stop;
  double t1=0,t2=0,result=0;
  
  /*----------------------------------------------------*/
 //create array of pointers(Rows)
  mat1 =(int **) malloc(X*sizeof(int*));
  mat2 =(int **)malloc(N*sizeof(int*));
  mat_res=(int **)malloc(X*sizeof(int*));
  /*----------------------------------------------------*/
  
  /*--------------------------------------------------------------------------------*/
 //allocate memory for each Row pointer
  for(i=0; i < X; i++)
  {
   mat1[i]=(int *)malloc(N*sizeof(int));
   mat_res[i]=(int *)malloc(Y*sizeof(int));
  }
  
  for(i = 0; i < N; i++)
  mat2[i]=(int *)malloc(Y*sizeof(int));
  /*--------------------------------------------------------------------------------*/
  
  for(i = 0; i < X; i++)
 {
  for(j = 0; j < N; j++)
  {
   mat1[i][j] = 1; 
  }
 }
 
 for(i = 0; i < N; i++)
 {
  for(j = 0; j < Y; j++)
  {
   mat2[i][j] = 2;
  }
 }
 //------------------calculate Starting time----------------------
 clock_gettime(CLOCK_REALTIME,&start);
 t1 = start.tv_sec + (start.tv_nsec/pow(10,9));
 //--------------------------------------------------------------
 
  for(i = 0; i < X; i++)
  {
   for(j = 0; j < Y;j++)
   {
    sum=0;
    for(k = 0; k < N; k++)
    {
     sum = sum + mat1[i][k] * mat2[k][j];
    }
    mat_res[i][j] = sum;
   }
  }
 //---------------calculate End time-------------------------
  clock_gettime(CLOCK_REALTIME,&stop);
 t2 = stop.tv_sec + (stop.tv_nsec/pow(10,9));
 //--------------------------------------------------------------
 // result = End_time - Start_time
 result = t2 - t1;
  for(i = 0; i < X; i++)
  {
   for(j = 0; j < Y; j++)
   {
    printf("%d\t",mat_res[i][j]);
   }
   printf("\n");
  }
  printf("\ntime taken =\t %lf\n",result);
  return 0;
}
  

In this program CLOCK_REALTIME means choose best wall clock timing of machines which is defined in time.h header file.
compile:
gcc matrix_mul_clock_gettime.c -o exec -lm
Run:
./exec