Skip to main content

PWR075: Avoid using GNU Fortran extensions

Issue​

Using compiler-specific extensions, such as those provided by GNU's gfortran, compromises the portability and consistency of code across different development environments.

Actions​

Refactor the code to replace existing GNU Fortran extensions with standard-compliant Fortran code. The specific steps for refactoring will depend on the particular features being replaced.

Relevance​

Fortran compilers, including GNU, often extend their support beyond the Fortran standard by introducing additional language features. While these extensions can provide enhanced functionality and convenience for programmers, they also present significant drawbacks.

Even though some extensions may be supported by multiple compilers, there will inevitably be environments where the code is not compilable, complicating future maintenance. Additionally, even when multiple vendors support the same extension, its behavior can vary, as these features are not regulated by the Fortran standard. Ultimately, Fortran code that depends on compiler-specific extensions cannot be guaranteed to be portable and consistent across different development environments.

This issue is particularly problematic when inheriting legacy code. If the code relies on compiler-specific extensions from a now unsupported or inaccessible compiler version, developers are forced to port the code to new environments, further complicating the already challenging task of maintaining legacy code.

For more information on which extensions are supported by GNU's gfortran, refer to the following resources:

Code examples​

Consider the following code, which checks if the file itself exists:

! example.f90
program example
  implicit none
  character(len = *), parameter :: file  = "example.f90"

  if(access(file, " ") == 0) then
    print *, "I exist"
  end if
end program example

This code uses access(), a GNU Fortran extension that compiles successfully with gfortran:

$ gfortran --version
GNU Fortran (Debian 14.2.0-3) 14.2.0
$ gfortran example.f90 -o example
$ ./example
 I exist

However, the same code fails to compile with LLVM's flang compiler:

$ flang-new-18 --version
Debian flang-new version 18.1.8 (9)
$ flang-new-18 example.f90 -o example
error: Semantic errors in example.f90
./example.f90:7:6: error: No explicit type declared for 'access'
    if(access(file, " ") == 0) then
       ^^^^^^

To resolve this issue, the programmer needs to provide a replacement for the functionality of the GNU extension. Fortunately, the Fortran standard provides the built-in inquire:

! solution.f90
program solution
  implicit none
  character(len = *), parameter :: file  = "solution.f90"
  logical :: exists

  inquire(file=file, exist=exists)

  if(exists) then
    print *, "I exist"
  end if
end program solution

This revised code compiles and runs successfully with flang:

$ flang-new-18 solution.f90 -o solution
$ ./solution
 I exist

References​