Undefined reference to nlopt_... symbols - fortran

Question

I am fairly new to Fortran and this may sound like a silly question. I encounter an error when compiling the Fortran code that is posted as an example in the NLOPT Wiki.
I compile in Ubuntu 18.04 LTS using the following command:
gfortran example-nlopt.f90 -o example-nlopt -I/usr/local/include/
The following error is produced in the terminal:
/tmp/ccbAim6b.o: In function `MAIN__':
example-nlopt.f90:(.text+0x26): undefined reference to `nlo_create_'
example-nlopt.f90:(.text+0x42): undefined reference to `nlo_get_lower_bounds_'
example-nlopt.f90:(.text+0x67): undefined reference to `nlo_set_lower_bounds_'
example-nlopt.f90:(.text+0x8a): undefined reference to `nlo_set_min_objective_'
example-nlopt.f90:(.text+0xca): undefined reference to `nlo_add_inequality_constraint_'
example-nlopt.f90:(.text+0x10e): undefined reference to `nlo_add_inequality_constraint_'
example-nlopt.f90:(.text+0x12d): undefined reference to `nlo_set_xtol_rel_'
example-nlopt.f90:(.text+0x164): undefined reference to `nlo_optimize_'
example-nlopt.f90:(.text+0x305): undefined reference to `nlo_destroy_'
collect2: error: ld returned 1 exit status

Answer 1

Based on what I saw in nlopt's documentation (https://nlopt.readthedocs.io/en/latest/NLopt_Installation/#changing-the-installation-directory) it looks like you just need to specify the library to link against. Maybe try this:
gfortran -I/usr/local/include/ -L/usr/local/lib example-nlopt.f90 -o example-nlopt -lnlopt -lm
This assumes you have the libnlopt.so in /usr/local/lib, if not then point to its location with the -L option.