[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.

[pakk]

You ask where the problem is in your program... Well, the error messages are clear...

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

Search for 'srand48' and 'drand48' and you will easily find the lines that give the problem. The first problem is in the line "srand48(time(NULL));" and the second problem in the line "r = drand48();".

A better question would be: what is the problem in your program? Well, even that is more or less clear from the error message: you use two words that are undefined. The compiler does not know what srand48 is. The compiler does not know what drand48 is. So the compiler does not know what to do.

An even better question would be: how to solve these problems? That is not shown in the error message. But some logical thinking can solve this. The problem is that you use functions that are not known to the compiler. There are two ways to solve this:
1. Tell the compiler what the functions should do;
2. Use different functions that the compiler knows.

I have no idea why you chose to use srand48 and drand48. But they are unknown to your compiler. For this work, it is sufficient to use the standard random number generator, see http://www.cplusplus.com/reference/cstdlib/rand/.