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Fortran Compilers

There are three free/open-source compilers for Unix-like operating systems with support for modern Fortran standards:

GNU Fortran (gfortran)

Modern Fortran compiler, part of the GNU Compiler Collection.

Flang

Fortran front-end for LLVM, based on source code of PGI.

F18

A rewritten front-end for LLVM in development, contributed by NVIDIA.

LFortran

An interactive LLVM-based Fortran 2018 compiler, in pre-alpha stage.

FC

Another Fortran 2018 front-end for LLVM 10 by CompilerTree, in development.

GNU Fortran is a fork of G95, a discontinued Fortran 95 compiler that has been removed from the FreeBSD ports collection some time ago due to deprecated dependencies that are required for building. Flang in an older front-end for LLVM, missing most features of Fortran 2008 and 2018. F18 is planned to replace Flang in 2020.

Fortran compilers are available for many platforms. Most of them target Microsoft Windows and/or Linux, but won’t produce native binaries for FreeBSD and other Unix systems, like Intel Fortran, PGI Community Edition, or the abandoned Open64. Please see the Fortran Wiki and Wikipedia for an overview.

Some historic compilers for ancient Fortran are open-source as well:

• f2c, a FORTRAN 77 to C transcompiler (Linux, Unix, Windows),
• GNU Fortran 77 (g77), predecessor of gfortran (Linux, Unix),
• Open Watcom FORTRAN 77 (DOS, Linux, OS/2, Windows).

Proprietary compilers targeting CP/M, DOS (fig. 1), and Microsoft Windows can be downloaded from various web sites dedicated to historic software, like old-dos.ru or winworldpc.com.

Fig. 1: Box of Microsoft FORTRAN Compiler for DOS

On-line Compilers

Fortran code can be compiled and executed in your web browser by using online services, like:

• Rextester (GNU Fortran 5.4.0),
• OnlineGDB (GNU Fortran 5.4.0),
• Tutorials Point (GNU Fortran 7.1.1, only Fortran 95 standard).

You may want to try some of the examples in the next sections by using an online compiler.

Modern Fortran on FreeBSD

Using FreeBSD as a programming environment for modern Fortran limits the list of available compilers to GNU Fortran and Flang, as F18 is still in early stage. Both can be installed from Ports or by using the FreeBSD package manager. The Intel Fortran compiler for FreeBSD was removed in 2010.

• Installing GNU Fortran:

  # pkg install lang/gcc9
  # gfortran9 --version
  GNU Fortran (FreeBSD Ports Collection) 9.2.0
  Copyright (C) 2019 Free Software Foundation, Inc.
  This is free software; see the source for copying conditions.  There is NO
  warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

• Installing Flang:

  # pkg install devel/flang
  # flang --version
  clang version 7.0.1
  Target: x86_64-portbld-freebsd12.0
  Thread model: posix
  InstalledDir: /usr/local/bin

• Installing F18:

  # pkg install devel/f18
  # f18 -help
  f18 options:
    -Mfixed | -Mfree     force the source form
    -Mextend             132-column fixed form
    -f[no-]backslash     enable[disable] \escapes in literals
    -M[no]backslash      disable[enable] \escapes in literals
    -Mstandard           enable conformance warnings
    -r8 | -fdefault-real-8 | -i8 | -fdefault-integer-8  change default kinds of intrinsic types
    -Werror              treat warnings as errors
    -ed                  enable fixed form D lines
    -E                   prescan & preprocess only
    -fparse-only         parse only, no output except messages
    -funparse            parse & reformat only, no code generation
    -funparse-with-symbols  parse, resolve symbols, and unparse
    -fdebug-measure-parse-tree
    -fdebug-dump-provenance
    -fdebug-dump-parse-tree
    -fdebug-dump-symbols
    -fdebug-resolve-names
    -fdebug-instrumented-parse
    -fdebug-semantics    perform semantic checks
    -fdotty              print FIR as a dotty graph
    -fdebug-dump-linear-ir dump the flat linear FIR for debug
    -v -c -o -I -D -U    have their usual meanings
    -help                print this again
  Other options are passed through to the compiler.

  At the current development stage, the PGI Fortran compiler is still required for the actual compilation process.

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