# 198. Compilation can fail if there is no main program or if a USEd module has not been compiled.
topic: {ref}`compiling`
```{note}
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```
gfortran m.f90
gives an error
undefined reference to `main'
if m.f90 does not contain a main program.
gfortran -c m.f90
gives an error like
Cannot open module file 'x.mod'
if the file containing module x has not previously been compiled.
---