fortran_c_on_hcc.md

+++
title = "Fortran/C on HCC"
description = "How to compile and run Fortran/C program on HCC machines"
weight = "50"
+++

This quick start demonstrates how to implement a Fortran/C program on
HCC supercomputers. The sample codes and submit scripts can be
downloaded from [serial_dir.zip](/attachments/serial_dir.zip).

#### Login to a HCC Cluster

Connect to a HCC cluster]({{< relref "../../connecting/" >}}) and make a subdirectory 
called `serial_dir` under your `$WORK` directory. 

{{< highlight bash >}}
$ cd $WORK
$ mkdir serial_dir
{{< /highlight >}}

In the subdirectory `serial_dir`, save all the relevant Fortran/C codes. Here we include two demo
programs, `demo_f_serial.f90` and `demo_c_serial.c`, that compute the sum from 1 to 20. 

{{%expand "demo_f_serial.f90" %}}
{{< highlight bash >}}
Program demo_f_serial
    implicit none
    integer, parameter :: N = 20
    real*8 w
    integer i
    common/sol/ x
    real*8 x
    real*8, dimension(N) :: y
    do i = 1,N
        w = i*1d0
        call proc(w)
        y(i) = x
        write(6,*) 'i,x = ', i, y(i)
    enddo
    write(6,*) 'sum(y) =',sum(y)
Stop
End Program
Subroutine proc(w)
    real*8, intent(in) :: w
    common/sol/ x
    real*8 x
    x = w
Return
End Subroutine
{{< /highlight >}}
{{% /expand %}}


{{%expand "demo_c_serial.c" %}}
{{< highlight c >}}
//demo_c_serial
#include <stdio.h>

double proc(double w){
        double x;
        x = w;
        return x;
}

int main(int argc, char* argv[]){
    int N=20;
    double w;
    int i;
    double x;
    double y[N];
    double sum;
    for (i = 1; i <= N; i++){        
        w = i*1e0;
        x = proc(w);
        y[i-1] = x;
        printf("i,x= %d %lf\n", i, y[i-1]) ;
    }
    
    sum = 0e0;
    for (i = 1; i<= N; i++){
        sum = sum + y[i-1]; 
    }
    
    printf("sum(y)= %lf\n", sum);  
 
return 0; 
}
{{< /highlight >}}
{{% /expand %}}

---

#### Compiling the Code

The compiling of a Fortran/C++ code to executable is usually done behind
the scene in a Graphical User Interface (GUI) environment, such as
Microsoft Visual Studio. In a HCC cluster, the compiling is done
explicitly by first loading a choice compiler and then executing the
corresponding compiling command. Here we will use the GNU Complier
Collection, `gcc`, for demonstration. Other available compilers such as
`intel` or `pgi` can be looked up using the command
line `module avail`.  Before compiling the code, make sure there is no 
dependency on any numerical library in the code. If invoking a numerical 
library is necessary, contact a HCC specialist 
({{< icon name="envelope" >}}[hcc-support@unl.edu](mailto:hcc-support@unl.edu)) to
discuss implementation options.

{{< highlight bash >}}
$ module load compiler/gcc/8.2
$ gfortran demo_f_serial.f90 -o demo_f_serial.x
$ gcc demo_c_serial.c -o demo_c_serial.x
{{< /highlight >}}

The above commends load the `gcc` complier and use the compiling
commands `gfortran` or `gcc` to compile the codes to`.x` files
(executables). 

#### Creating a Submit Script

Create a submit script to request one core (default) and 1-min run time
on the supercomputer. The name of the main program enters at the last
line.

{{% panel header="`submit_f.serial`"%}}
{{< highlight bash >}}
#!/bin/sh
#SBATCH --mem-per-cpu=1024
#SBATCH --time=00:01:00
#SBATCH --job-name=Fortran
#SBATCH --error=Fortran.%J.err
#SBATCH --output=Fortran.%J.out

module load compiler/gcc/4.9  
./demo_f_serial.x
{{< /highlight >}}
{{% /panel %}}

{{% panel header="`submit_c.serial`"%}}
{{< highlight bash >}}
#!/bin/sh
#SBATCH --mem-per-cpu=1024
#SBATCH --time=00:01:00
#SBATCH --job-name=C
#SBATCH --error=C.%J.err
#SBATCH --output=C.%J.out

module load compiler/gcc/4.9
./demo_c_serial.x
{{< /highlight >}}
{{% /panel %}}

#### Submit the Job

The job can be submitted through the command `sbatch`. The job status
can be monitored by entering `squeue` with the `-u` option.

{{< highlight bash >}}
$ sbatch submit_f.serial
$ sbatch submit_c.serial
$ squeue -u <username>
{{< /highlight >}}

Replace `<username>` with your HCC username.

#### Sample Output

The sum from 1 to 20 is computed and printed to the `.out` file (see
below). 
{{%expand "Fortran.out" %}}
{{< highlight batchfile>}}
 i,x =            1   1.0000000000000000     
 i,x =            2   2.0000000000000000     
 i,x =            3   3.0000000000000000     
 i,x =            4   4.0000000000000000     
 i,x =            5   5.0000000000000000     
 i,x =            6   6.0000000000000000     
 i,x =            7   7.0000000000000000     
 i,x =            8   8.0000000000000000     
 i,x =            9   9.0000000000000000     
 i,x =           10   10.000000000000000     
 i,x =           11   11.000000000000000     
 i,x =           12   12.000000000000000     
 i,x =           13   13.000000000000000     
 i,x =           14   14.000000000000000     
 i,x =           15   15.000000000000000     
 i,x =           16   16.000000000000000     
 i,x =           17   17.000000000000000     
 i,x =           18   18.000000000000000     
 i,x =           19   19.000000000000000     
 i,x =           20   20.000000000000000     
 sum(y) =   210.00000000000000
{{< /highlight >}}
{{% /expand %}}

{{%expand "C.out" %}}
{{< highlight batchfile>}}
i,x= 1 1.000000
i,x= 2 2.000000
i,x= 3 3.000000
i,x= 4 4.000000
i,x= 5 5.000000
i,x= 6 6.000000
i,x= 7 7.000000
i,x= 8 8.000000
i,x= 9 9.000000
i,x= 10 10.000000
i,x= 11 11.000000
i,x= 12 12.000000
i,x= 13 13.000000
i,x= 14 14.000000
i,x= 15 15.000000
i,x= 16 16.000000
i,x= 17 17.000000
i,x= 18 18.000000
i,x= 19 19.000000
i,x= 20 20.000000
sum(y)= 210.000000
{{< /highlight >}}
{{% /expand %}}