LFortran User Guide

About LFortran

LFortran’s goal is an implementation of the latest Fortran standard (currently F2018) with some added extensions. It works on Linux, macOS, most BSDs and on Windows.

The LFortran compiler consists of the following components:

  • The LFortran compiler itself.

  • A runtime library supporting language elements and intrinsic functions.

  • Several module definitions which define the external environment and supply particular details about the computer running the program.

  • The capability to read additional module definitions which define the interfaces to external libraries such as OpenMPI.

  • Generation of executable code with the LLVM compiler infrastructure.

  • Generation of C, C++, WebAssembly or Julia code.

  • Interactive usage via LLVM JIT

  • Source file formatting (lfortran fmt).


The LFortran compiler is currently (mid-2023) incomplete (alpha stage) but is intended to support ISO Fortran 2018, ISO/IEC 1539:2018.


The extensions are currently in development and are planned to include:

  • Global Scope (statements outside of a program block).

  • Interactive Fortran (in alpha).

  • Jupyter integration.

  • Support for GPUs and other accelerators

Interactive Compiler

LFortran supports an interactive mode; just run the lfortran command to start it.

Invoking LFortran

The LFortran compiler supports numerous command-line flags to select compilation options, output options, link options and so on.

Compiler information

  • --print-targets, Print the registered CPU targets

  • --version, Show the current version

Source code format

  • --fixed-form, Parse the file assuming Fortran 66 format (6 spaces)

  • --fixed-form-infer, Use heuristics to infer if a file is in fixed form

Source code processing

  • --cpp, Enable C preprocessing

  • -E, Preprocess only; do not compile, assemble or link

Other inputs

  • -D <macro>=<value> Define a macro (or 1 if omitted)

  • -I <value>, Include path for include statements

  • -L <value>, Library path for shared libraries

  • -l <value>, Link library naming a linkable shared library

Compiler feature selections

  • --fast, Best performance (disable strict standard compliance)

  • --implicit-argument-casting, Allow implicit argument casting

  • --implicit-interface, Allow implicit interface

  • --implicit-typing, Allow implicit typing

  • --openmp, Enable OpenMP

  • --print-leading-space, Print leading white space if format is unspecified

  • --realloc-lhs, Reallocate left hand side automatically

  • --target <value>, Generate code for the given target

  • --backend flag is used to specify the target backend for code generation in LFortran. The supported backends are:

    • llvm: The most advanced and default backend, used for generating LLVM IR or machine code through LLVM.

    • wasm: For generating Webassembly via our custom wasm backend.

    • c: For generating C code.

    • cpp: For generating C++ code (requires the Kokkos library).

    • x86: For generating x86 machine code directly (without LLVM).

    • fortran: For generating Fortran code.

    • julia: For generating Julia code.

Compiler text outputs

  • --error-format <value>, Control how errors are produced (human, short)

  • --no-error-banner, Turn off error banner

  • --no-warnings, Turn off all warnings

  • -S, Emit assembly, do not assemble or link

  • --time-report, Show compilation time report

  • -v, Be more verbose

Compiler binary outputs

  • -c, Compile and assemble, do not link

  • --generate-object-code, Generate object code into .o files

  • -J <value>, Where to save mod files

  • -o <value>, Specify the file to place the compiler’s output into

  • --static, Create a static executable

Compiler debugging

A number of command-line options select various text outputs useful for debugging the compiler. See lfortran --help for a list.


The following commands and code demonstrate basic operation of the compiler.

lfortran helloworld.f90
Hello World!

lfortran -o hw helloworld.f90
Hello World!

cat helloworld.f90
program hello_world
    implicit none
    write (*, *) 'Hello World!'
end program hello_world

Here is a simple example with a module:

lfortran -c varray.f90
lfortran usev.f90
 sum is    7.20000000e+01

cat varray.f90
module varray
    integer :: nsize
end module varray

cat usev.f90
program usev
    use varray
    real, allocatable, dimension(:) :: A
    integer :: i
    nsize = 8
    do i = 1, nsize
        A(i) = 2.0*i
    end do
    print *, " sum is ", (A(1)+A(nsize))*nsize/2.0
end program usev

The compile command for the module requires -c to avoid automatic running of the code.

Formatting Fortran source files

The lfortran compiler will automatically format source files with the fmt option. You can select auto-indent for modules, and in-place update of the Fortran source file with the -i option (use with caution!).

lfortran fmt varray.f90
module varray
integer :: nsize
real, allocatable, dimension(:) :: A
end module varray

Or add spaces and indentation as follows:

lfortran fmt --spaces 4 --indent-unit varray.f90
module varray
    integer :: nsize
    real, allocatable, dimension(:) :: A
end module varray

Selecting the C Compiler

By default LFortran uses the clang compiler. On some systems the compiler has a version number or spelling difference. The compiler can be changed with the LFORTRAN_CC symbol:

lfortran hw.f90
Hello World!

export LFORTRAN_CC=gcc
lfortran hw.f90
Hello World!

export LFORTRAN_CC=clang-14
lfortran hw.f90
sh: clang-14: not found
...(further error messages)...

Differences from other compilers

GNU, Intel and LLVM Fortran use « standard » Fortran carriage control where the first character of each output line controls a conceptual « line printer ». A space  »  » means single-space, a zero « 0 » means double-space and one « 1 » means form-feed before printing. This is obsolete and LFortran omits this unless --print-leading-space is selected at compile-time.

There is currently no way to specify detailed compiler options to Clang such as -O3 or -flto (optimization and link-time optimization).

GNU extension declarations real*8 xvalue are accepted but deprecated. This is valid Fortran-77 but not Fortran-2018.