[ashu@123]

#include <stdio.h>
#include <math.h>
#include <stdlib.h>

main()
{
int step, i, k;
float I, Lxu, U, u, pi, A, B, t, dt, nUpper, nLower, dn, n, phi, Su, sum, Ak;
double r;
char filename[8];
FILE *fp;
/*================================================= ========*/
printf ("Enter name of file to write to:");
scanf("%s", filename);
fp = fopen(filename,"w");
printf ("Enter the streamwise mean velocity:");
scanf("%f", &U);
printf ("Enter the streamwise turbulent intensity (decimal):");
scanf("%f", &I);
printf ("Enter the streamwise integral length scale (m):");
scanf("%f", &Lxu);
printf ("Number of timesteps, maximum 20000:");
scanf("%d", &step);
printf ("Stepsize of time in second:");
scanf("%f", &dt);
pi = 3.141592654;
A = 4.0*I*I*Lxu*U;
B = 70.8*(Lxu/U)*(Lxu/U);
nUpper=1.0/2.0/dt;
nLower=0.001;
dn=(nUpper-nLower)/step; /* here taking N = M = step */
t = 0;
for (i=0; i<step; i++)
{
sum = 0.0;
srand48(time(NULL));
n = nLower+0.5*dn;
for(k=0; k<step; k++)
{
r = drand48();
phi=r*2.0*pi; /*phi to be randomly selected*/
Su = A/pow((1.0 + B*n*n),(5.0/6.0));
Ak = sqrt(Su*dn);
sum = sum + Ak*cos(2.0*pi*n*t + phi);
n = n+dn;
}
u = sqrt(2.0)*sum;
fprintf(fp,"%f\n",u);
t = t + dt;
}
fclose(fp);
}




Above program is to generate fluctuating velocity components.
While running above program, i am getting following error:

undefined reference to 'srand48'
undefined reference to 'drand48'

can anyone suggest me where is the problem in above program.
Thank You in advance.