use ctypes to call a dll file in Python - openmp

Question

I write a dll file with openmp, then I use ctypes to use it in Python, but the speed becomes super slow. When I rewrite the code in C, it only takes several seconds to finish.
The main part of the dll file is:
double martini(double a[], double b[], double d_title[], double c[], double d[], double A[], int num_element)
{
double nu_start = 0;
double mu_start = 0;
double z_start = 0;
double step_nu = 2 * 3.1415926 / 100;
double step_mu = 3.1415926 / 100;
double step_z = 0;
double nu = 0;
double mu = 0;
double z = 0;
double integral_first = 0;
double d_uv = 0;
int i = 0;
int j = 0;
int k = 0;
int loop = 0;
#pragma omp parallel for default(none) shared(a, d_title, b, c, nu_start, mu_start, z_start, step_nu, step_mu) private( i,j,k,mu, nu, step_z, z, d_uv) reduction(+:integral_first)
for (loop = 0; loop < num_element; loop++)
{
for (i = 0; i < 100; i++)
{
mu = mu_start + (i + 1) * step_mu;
for (j = 0; j < 100; j++)
{
nu = nu_start + (j + 1) * step_nu;
for (k = 0; k < 1500; k++)
{
d_uv = (sin(mu) * sin(mu) * cos(nu) * cos(nu) + sin(mu) * sin(mu) * (a[loop] * sin(nu) - d_title[loop] * cos(nu)) * (a[loop] * sin(nu) - d_title[loop] * cos(nu)) + b[loop] * b[loop] * cos(mu) * cos(mu)) / (c[loop] * c[loop]);
step_z = 20 / (d_uv * 1500);
z = z_start + (k + 1) * step_z;
integral_first = integral_first + 1 / 2 * A[loop] * sin(mu) * (1 - 3 * sin(mu) * sin(mu) * cos(nu) * cos(nu)) * exp(-d_uv * z) * log(1 + z * z) * step_z * step_mu * step_nu / (c[loop] * c[loop]);
}
}
}
}
return integral_first;
}
I use CDLL() in python to load this dll file. Then this function is called. But the speed is very low.
I also write another C file where I create a, b,d... by giving them 500 random numbers. I can get the result in seconds.
Python code:
martini=CDLL('C:/Users/38914/source/repos/Dll2/x64/Release/Dll2.dll')
integral=martini.martini
integral.restype=c_double
a_func=(c_double*num_element)()
b_func=(c_double*num_element)()
d_title_func=(c_double*num_element)()
c_func=(c_double*num_element)()
d_func=(c_double*num_element)()
A_func=(c_double*num_element)()
a_func[:]=a
b_func[:]=b
d_title_func[:]=d_title
c_func[:]=c
d_func[:]=d
A_func[:]=A
num_element_func=c_int(num_element)
out=integral(a_func,b_func,d_title_func,c_func,d_func,A_func,num_element_func)
a,b,c,d,d_title and A are array I use Python to read from a txt file.