Fortran Build Concepts

Fortran vs C/C++

Fortran has different compilation semantics than C/C++, which affects how build systems must handle it.

Module Files

In C/C++, headers are textual includes processed by the preprocessor. In Fortran, module declarations compile to binary .mod files containing type signatures, interfaces, and metadata.

module math        ! Compiles to math.mod
    implicit none
contains
    function add(a, b) result(c)
        real, intent(in) :: a, b
        real :: c
        c = a + b
    end function add
end module math

When another file says use math, the compiler reads math.mod to verify types and interfaces. This means:

1. Modules must be compiled before their users - strict build ordering required
2. Module files are compiler-specific - gfortran .mod files are incompatible with flang
3. Module names are case-insensitive - USE MATH and use math refer to the same module

Dependency Ordering

Consider three files:

a.f90: module a (no dependencies)
b.f90: module b, uses a
c.f90: program c, uses a and b

Valid compilation order: a.f90 → b.f90 → c.f90

Invalid: b.f90 first (needs a.mod), or c.f90 first (needs both)

In C/C++, object files can be compiled in any order. In Fortran, sources must be compiled in dependency order.

How rules_fortran Handles This

Module Propagation

When one target depends on another, module files are propagated via FortranInfo:

a.f90: module a
b.f90: module b, uses a
c.f90: program c, uses a and b

fortran_library(
    name = "a",
    srcs = ["a.f90"],
)

fortran_library(
    name = "b",
    srcs = ["b.f90"],
    deps = [":a"],
)

fortran_binary(
    name = "c",
    srcs = ["c.f90"],
    deps = [":a", ":b"],
)

The deps attribute makes all modules from dependencies available when compiling.

Unlike C/C++ headers which are just textual includes, Fortran modules are compiled artifacts. The build system must ensure modules are compiled before their users - this is handled automatically through deps.

C Interoperability

rules_fortran provides CcInfo from fortran_library, enabling:

• cc_binary / cc_library / cc_test can depend on fortran_library
• fortran_binary / fortran_library / fortran_test can depend on cc_library

See the C/Fortran Interop example for details.