[atrettel]

They are pretty similar, but they are definitely used differently. For one, you have to "call" a subroutine in one statement, but you can use multiple functions on the same statement (since they can return values). Functions (usually) do not change their arguments, but subroutines often do. In some sense, functions are closer to how mathematical functions work but subroutines are closer to labels for certain procedures.

[adastra22]

So they are used differently, but there isn’t a language enforced difference (other than return value)?

[adrian_b]

There is a language enforced difference.

Fortran functions correspond to "pure" functions in C/C++ and other languages, i.e. idempotent functions that do not modify arguments or global variables.

If a C/C++ function is converted to Fortran, it must be rewritten as a subroutine, unless it is a pure function.

Not all C/C++ compilers support the language extension of marking functions as pure, and even with such compilers many programmers are lazy, so they forget to mark as pure the functions where this is applicable, even if this is recommended for improving program optimization and verification.

Fortran function were pure functions because this is the mathematical sense of the term "function". The extension of the meaning of the word "function" for any kind of procedure happened decades after the creation of Fortran.

The distinction between a "void" function and other functions has negligible importance. On the other hand the distinction between "functions" in the C language sense and "pure functions" is very important and programmers should better mark as "pure" all the functions for which this is true. This is at least as important for program optimization and for checking program correctness as declaring a variable with the correct type.

[pklausler]

> Fortran functions correspond to "pure" functions in C/C++ and other languages, i.e. idempotent functions that do not modify arguments or global variables.

This is nonsense. Fortran functions aren't pure. They can have side effects.

HPF/Fortran '95 added the PURE attribute for subprograms, but it's not the default.

[pklausler]

Functions are called only within the context of expression evaluation, and Fortran allows a compiler to perform algebraic transformations on expressions. If you write X=Y*F(Z) and we can determine that Y is zero, the function call can be deleted. So side effects in functions are somewhat risky.

[atrettel]

The language enforced difference is that only functions can return a value, but other than that, they are quite similar and just called "procedures" generally. In my experience, Fortran programmers use them differently in practice, and that is more of a guideline than something enforced by the language itself.