# Preparing your package for FPM

This document describes how you need to organize your application or library for it to successfully build with the Fortran Package Manager (fpm).

## What kind of package can fpm build?

You can use fpm to build:

• Applications (program only)
• Libraries (modules only)
• Combination of the two (programs and modules combined)

Let’s look at some examples of different kinds of package layouts that you can use with fpm.

## Example package layouts

This section describes some example package layouts that you can build with fpm. You can use them to model the layout of your own package.

### Single program

Let’s start with the simplest package imaginable—a single program without dependencies or modules. Here’s what the layout of the top-level directory looks like:

.
├── app
│   └── main.f90
└── fpm.toml


We have one source file (main.f90) in one directory (app). Its contents are:

program main
print *, 'Hello, World!'
end program main


This program prints the usual greeting to the standard output, and nothing more.

There’s another important file in the top-level directory, fpm.toml. This is fpm’s configuration file specific to your package. It includes all the data that fpm needs to build your app. In our simple case, it looks like this:

name = "hello"
version = "0.1.0"
author = "Jane Programmer"
maintainer = "jane@example.com"


The preamble includes some metadata, such as license, author, and similar, that you may have seen in other package manager configuration files. The one option that matters here right now is:

name = "hello"


This line specifies the name of your package, which determines the name of the executable file of your program. In this example, our program executable, once built, will be called hello.

Let’s now build this program using fpm:

$fpm build # gfortran (for build/debug/app/main.o) # gfortran (for build/debug/app/hello)  On the first line, we ran fpm build to compile and link the application. The latter two lines are emitted by fpm, and indicate which command was executed at each build step (gfortran), and which files have been output by it: object file main.o, and executable hello. We can now run the app with fpm run: $ fpm run
Hello, World!


If your application needs to use a module internally, but you don’t intend to build it as a library to be used in other projects, you can include the module in your program source file as well. For example:

$cat app/main.f90 module math_constants real, parameter :: pi = 4 * atan(1.) end module math_constants program main use math_constants, only: pi print *, 'Hello, World!' print *, 'pi = ', pi end program main  Now, run this using fpm run: $ fpm run
# gfortran (for build/debug/app/main.o)
# gfortran (for build/debug/app/hello)
Hello, World!
pi =    3.14159274


Notice that you can run fpm run, and if the package hasn’t been built yet, fpm build will run automatically for you. This is true if the source files have been updated since the last build. Thus, if you want to run your application, you can skip the fpm build step, and go straight to fpm run.

Although we have named our program hello, which is the same name as the package name in fpm.toml, you can name it anything you want as long as it’s permitted by the language.

In this last example, our source file defined a math_constants module inside the same source file as the main program. Let’s see how we can define an fpm package that makes this module available as a library.

### Single-module library

The package layout for this example looks like this:

.
├── fpm.toml
└── src
└── math_constants.f90


In this example we’ll build a simple math constants library that exports the number pi as a parameter:

$cat src/math_constants.f90 module math_constants real, parameter :: pi = 4 * atan(1.) end module math_constants  and our fpm.toml is the same as before. Now use fpm build to build the package: $ fpm build
# gfortran (for build/debug/library/math_constants.o build/debug/library/math_constants.mod)
# ar (for build/debug/library/math_constants.a)
ar: creating build/debug/library/math_constants.a


Based on the output of fpm build, fpm first ran gfortran to emit the binary object (math_constants.o) and module (math_constants.mod) files. Then it ran ar to create a static library archive math_constants.a. build/debug/library is thus both your include and library path, should you want to compile and link an external program with this library.

For modules in the top-level (src) directory, fpm requires that:

• The module has the same name as the source file.
• There is only one module per file.

These two requirements simplify the build process for fpm. As Fortran compilers emit module files (.mod) with the same name as the module itself (but not the source file, .f90), naming the module the same as the source file allows fpm to:

• Uniquely and exactly map a source file (.f90) to its object (.o) and module (.mod) files.
• Avoid conflicts with modules of the same name that could appear in dependency packages (more on this in a bit).

Since this is a library without executable programs, fpm run here does nothing.

In this example, our library is made of only one module. However, most real-world libraries are likely to use multiple modules. Let’s see how you can package your multi-module library.

### Multi-module library

In this example, we’ll use another module to define a 64-bit real kind parameter and make it available in math_constants to define pi with higher precision. To make this exercise worthwhile, we’ll define another math constant, Euler’s number.

Our package layout looks like this:

.
├── fpm.toml
└── src
├── math_constants.f90
└── type_kinds.f90


And our source file contents are:

$cat src/math_constants.f90 module math_constants use type_kinds, only: rk real(rk), parameter :: pi = 4 * atan(1._rk) real(rk), parameter :: e = exp(1._rk) end module math_constants$ cat src/type_kinds.f90
module type_kinds
use iso_fortran_env, only: real64
integer, parameter :: rk = real64
end module type_kinds


and there are no changes to our fpm.toml relative to previous examples.

Like before, notice that the module type_kinds is name exactly as the source file that contains it. This is important.

By now you know how to build the package:

$fpm build # gfortran (for build/debug/library/type_kinds.o build/debug/library/type_kinds.mod) # gfortran (for build/debug/library/math_constants.o build/debug/library/math_constants.mod) # ar (for build/debug/library/math_constants.a) ar: creating build/debug/library/math_constants.a  Our build path now contains: $ ls build/debug/library/
math_constants.a  math_constants.mod  math_constants.o  type_kinds.mod  type_kinds.o


And the static library includes all the object files:

$nm build/debug/library/math_constants.a math_constants.o: type_kinds.o:  The takeaways from this example are that: • fpm automatically scanned the src directory for any source files. • It also resolved the dependency order between different modules. ### Application and library Let’s now combine the two previous examples into one: We’ll build the math constants library and an executable program that uses it. We’ll use this program as a demo, and to verify that defining higher-precision constants from the previous example actually worked. Here’s the package layout for your application + library package: . ├── app │ └── main.f90 ├── fpm.toml └── src ├── math_constants.f90 └── type_kinds.f90  Our fpm.toml remains unchanged and our executable program source file is: $ cat app/main.f90
program main
use math_constants, only: e, pi
print *, 'math_constants library demo'
print *, 'pi = ', pi
print *, 'e = ', e
end program main


Let’s go straight to running the demo program:

$fpm run # gfortran (for build/debug/library/type_kinds.o build/debug/library/type_kinds.mod) # gfortran (for build/debug/library/math_constants.o build/debug/library/math_constants.mod) # ar (for build/debug/library/math_constants.a) ar: creating build/debug/library/math_constants.a # gfortran (for build/debug/app/main.o) # gfortran (for build/debug/app/math_constants) math_constants library demo pi = 3.1415926535897931 e = 2.7182818284590451  The fpm build + run process works as expected, and our program correctly outputs higher-precision constants. So far we covered how fpm builds: • A single program • A single-module library • A multi-module library • A program and a library However, all our modules so far have been organized in the top level source directory. More complex libraries may organize their modules in subdirectories. Let’s see how we can build this with fpm. ### Multi-level library In this example, we’ll define our library as a collection of modules, two of which are defined in a subdirectory: . ├── app │ └── main.f90 ├── fpm.toml └── src ├── math_constants │ ├── derived.f90 │ └── fundamental.f90 ├── math_constants.f90 └── type_kinds.f90  First, fpm.toml and src/type_kinds.f90 remain unchanged relative to the previous example. The rest of the source files are: $ cat src/math_constants.f90
module math_constants
use math_constants_fundamental, only: e, pi
use math_constants_derived, only: half_pi, two_pi
end module math_constants

$cat src/math_constants/fundamental.f90 module math_constants_fundamental use type_kinds, only: rk real(rk), parameter :: pi = 4 * atan(1._rk) real(rk), parameter :: e = exp(1._rk) end module math_constants_fundamental$ cat src/math_constants/derived.f90
module math_constants_derived
use math_constants_fundamental, only: pi
use type_kinds, only: rk
real(rk), parameter :: two_pi = 2 * pi
real(rk), parameter :: half_pi = pi / 2
end module math_constants_derived

$cat app/main.f90 program main use math_constants, only: e, pi, half_pi, two_pi print *, 'math_constants library demo' print *, 'pi = ', pi print *, '2*pi = ', two_pi print *, 'pi/2 = ', half_pi print *, 'e = ', e end program main  Our top-level math_constants module now doesn’t define the constants, but imports them from the two modules in the subdirectory. Constants e and pi we define in the math_constants_fundamental module, and two_pi and half_pi in the math_constants_derived module. From the main program, we access all the constants from the top-level module math_constants. Let’s build and run this package: $ fpm run
# gfortran (for build/debug/library/type_kinds.o build/debug/library/type_kinds.mod)
# gfortran (for build/debug/library/math_constants_fundamental.o build/debug/library/math_constants_fundamental.mod)
# gfortran (for build/debug/library/math_constants_derived.o build/debug/library/math_constants_derived.mod)
# gfortran (for build/debug/library/math_constants.o build/debug/library/math_constants.mod)
# ar (for build/debug/library/math_constants.a)
ar: creating build/debug/library/math_constants.a
# gfortran (for build/debug/app/main.o)
# gfortran (for build/debug/app/math_constants)
math_constants library demo
pi =    3.1415926535897931
2*pi =    6.2831853071795862
pi/2 =    1.5707963267948966
e =    2.7182818284590451


Again, fpm built and run the package as expected.

Recall from an earlier example that fpm required the modules in the top-level src directory to be named the same as their source file. This is why src/math_constants.f90 defines module math_constants.

For modules defined in subdirectories, there’s an additional requirement: module name must contain the path components of the directory that its source file is in. In our case, src/math_constants/fundamental.f90 defines the math_constants_fundamental module. Likewise, src/math_constants/derived.f90 defines the math_constants_derived module.

This rule applies generally to any number of nested directories and modules. For example, src/a/b/c/d.f90 must define a module called a_b_c_d.

Takeaways from this example are that:

• You can place your module source files in any levels of subdirectories inside src.
• The module name must include the path components and the source file name–for example, src/a/b/c/d.f90 must define a module called a_b_c_d.

### Be more explicit

So far we’ve let fpm use its defaults to determine the layout of our package. It determined where our library sources would live, what the name of the executable will be, and some other things. But we can be more explicit about it, and make some changes to those things.

Let’s look at what the fpm.toml file from our last example would look like if we specified everything.

name = "math_constants"
version = "0.1.0"
author = "Jane Programmer"
maintainer = "jane@example.com"

[library]
source-dir="src"

[[ executable ]]
name="math_constants"
source-dir="app"
main="main.f90"


You can see that by making these explicit in the fpm.toml we are able to change many of the settings that fpm used by default. We can change the folders where our sources are stored, we can change the name of our executable, and we can change the name of the file our program is defined in.

fpm also provides support for unit testing. By default, fpm looks for a program in test/main.f90 which it will compile and execute with the command fpm test. The tests are treated pretty much exactly like the executables. Let’s define one explicitly in our fpm.toml file. We’ll make sure that our definition of pi satisfies the property sin(pi) == 0.0. Here’s the fpm.toml file:

name = "math_constants"
version = "0.1.0"
author = "Jane Programmer"
maintainer = "jane@example.com"

[library]
source-dir="src"

[[ executable ]]
name="math_constants"
source-dir="app"
main="main.f90"

[[ test ]]
name="runTests"
source-dir="test"
main="main.f90"


where the contents of the main.f90 file are

program main
use math_constants, only: pi
print *, "sin(pi) = ", sin(pi)
end program main


With this setup, we can run our tests.

\$ fpm test
# gfortran (for build/debug/library/type_kinds.o build/debug/library/type_kinds.mod)
# gfortran (for build/debug/library/math_constants_fundamental.o build/debug/library/math_constants_fundamental.mod)
# gfortran (for build/debug/library/math_constants_derived.o build/debug/library/math_constants_derived.mod)
# gfortran (for build/debug/library/math_constants.o build/debug/library/math_constants.mod)
# ar (for build/debug/library/math_constants.a)
ar: creating build/debug/library/math_constants.a
# gfortran (for build/debug/app/main.o)
# gfortran (for build/debug/app/math_constants)
# gfortran (for build/debug/test/main.o)
# gfortran (for build/debug/test/runTests)
sin(pi) =    1.2246467991473532E-016


Inevitably, you’ll want to be able to include other libraries in your project. fpm makes this incredibly simple, by taking care of fetching and compiling your dependencies for you. You just tell it what your dependencies are, and where to find them. Let’s add a dependency to our library. Now our fpm.toml file looks like this:

name = "math_constants"
version = "0.1.0"
author = "Jane Programmer"
maintainer = "jane@example.com"

[library]
source-dir="src"

[dependencies]
helloff = { git = "https://gitlab.com/everythingfunctional/helloff.git" }

[[ executable ]]
name="math_constants"
source-dir="app"
main="main.f90"

[[ test ]]
name="runTests"
source-dir="test"
main="main.f90"


Now you can use any modules from this library anywhere in your code. Just like this:

program main
use helloff, only: create_greeting
use math_constants, only: e, pi, half_pi, two_pi
print *, 'math_constants library demo'
print *, 'pi = ', pi
print *, '2*pi = ', two_pi
print *, 'pi/2 = ', half_pi
print *, 'e = ', e
print *, create_greeting("fpm")
end program main


And now, fpm run will output the following:

 math_constants library demo
pi =    3.1415926535897931
2*pi =    6.2831853071795862
pi/2 =    1.5707963267948966
e =    2.7182818284590451
Hello, fpm!


Additionally, any users of your library will now automatically depend on your dependencies too. So if you don’t need that dependency for the library, like in the above example, then you can specify it for the specific executable like below. Then fpm will still fetch and compile it when building your executable, but users of your library won’t have to.

name = "math_constants"
version = "0.1.0"
author = "Jane Programmer"
maintainer = "jane@example.com"

[library]
source-dir="src"

[[ executable ]]
name="math_constants"
source-dir="app"
main="main.f90"
[executable.dependencies]
helloff = { git = "https://gitlab.com/everythingfunctional/helloff.git" }

[[ test ]]
name="runTests"
source-dir="test"
main="main.f90"


You can also specify dependencies for your tests in a similar way, with [test.dependencies] instead of [executable.dependencies]. There’s also another option for test dependencies. The below example makes the dependencies available for all the tests, but again your users won’t depend on these.

name = "math_constants"
version = "0.1.0"
author = "Jane Programmer"
maintainer = "jane@example.com"

[library]
source-dir="src"

[dev-dependencies]
helloff = { git = "https://gitlab.com/everythingfunctional/helloff.git" }

[[ executable ]]
name="math_constants"
source-dir="app"
main="main.f90"

[[ test ]]
name="runTests"
source-dir="test"
main="main.f90"


You can also be specific about which version of a dependency you’d like. You can specify a branch to use like helloff = { git = "https://gitlab.com/everythingfunctional/helloff.git", branch = "master" }, or a tag like helloff = { git = "https://gitlab.com/everythingfunctional/helloff.git", tag = "v1.2.3" }, or even a specific commit like helloff = { git = "https://gitlab.com/everythingfunctional/helloff.git", rev = "a1b2c3" }. You can even specify the path to another folder, if for example you’ve got another fpm package in the same repository. Like this: helloff = { path = "helloff" }. Note that you should not specify paths outside of your repository, or things won’t work for your users.

### Custom build scripts

If there is something special about your library that makes fpm unable to build it, you can provide your own build script. fpm will then simply call your build script to build the library.

To specify a build script to be used, put it in the library section of your fpm.toml file, like:

[library]
source-dir="src"
build-script="my_build_script"


fpm will set the following environment variables to specify some parameters to the build script:

• FC – The Fortran compiler to be used.
• FFLAGS – The flags that should be passed to the Fortran compiler.
• BUILD_DIR – Where the compiled files should be placed.
• INCLUDE_DIRS – The folders where any dependencies can be found, space separated. It is then the responsibility of the build script to generate the appropriate include flags.

Additionally, script will be called with the name of the archive (*.a file) that should be produced as the command line argument.

Note: If the name of the build script is Makefile or ends with .mk, then the make program will be used to run it. Not the the archive file is explicitly specified as the target to be built

Note: All file and directory names are specified with their full canonical path.