Add exit code example package

Author: perazz

example_packages/fpm_test_exit_code/README.md

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+# fpm_test_exit_code
+Test program for application exit codes
+see https://github.com/fortran-lang/fpm/issues/848
+
+This app expects to receive an integer command line argument, to check whether it is odd or even.
+It returns 0 on success (odd input), or among a few error codes otherwise.

example_packages/fpm_test_exit_code/app/main.f90

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+! Test program for application exit codes
+! see https://github.com/fortran-lang/fpm/issues/848
+
+! This app expects to receive an integer command line argument, to check whether it is odd or even.
+! It returns 0 on success (odd input), or among a few error codes otherwise.
+
+program check_odd_number
+  implicit none
+
+  integer, parameter :: SUCCESS          = 0
+  integer, parameter :: INVALID_ARGUMENT = 1
+  integer, parameter :: NOT_AN_INTEGER   = 2
+  integer, parameter :: NOT_ODD          = 3
+
+  character(len=1024) :: buffer
+  integer :: ierr,ln,the_number
+
+  ! If the argument is missing or not an integer, return an error flag
+  if (command_argument_count()/=1) stop INVALID_ARGUMENT
+
+  ! Get command argument
+  call get_command_argument(1,value=buffer,length=ln,status=ierr)
+
+  ! On invalid string
+  if (ln<1 .or. ierr/=0) stop INVALID_ARGUMENT
+
+  ! Read to int
+  read(buffer(:ln),*,iostat=ierr) the_number
+
+  ! On invalid integer
+  if (ierr/=0) stop NOT_AN_INTEGER
+
+  ! Check if it is odd or even
+  if (mod(the_number,2)==0) then
+      ! Is even
+      stop NOT_ODD
+  else
+      ! Is odd
+      stop SUCCESS
+  end if
+
+end program check_odd_number

example_packages/fpm_test_exit_code/fpm.toml

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+name = "fpm_test_exit_code"
+version = "0.1.0"
+license = "license"
+author = "Jane Doe"
+maintainer = "[email protected]"
+copyright = "Copyright 2023, Jane Doe"
+[build]
+auto-executables = true
+[install]
+library = false