Polish the code, add comments

1 files changed, 24 insertions, 3 deletions

diff --git a/fpm/src/fpm.F90 b/fpm/src/fpm.F90
index a68a0b9..c93e3aa 100644
--- a/fpm/src/fpm.F90
+++ b/fpm/src/fpm.F90
@@ -10,8 +10,26 @@ integer, parameter :: os_windows = 3
 contains
 
 integer function get_os() result(r)
+! Determines the OS type.
+!
+! Currently we use the $HOMEPATH and $HOME environment variables to determine
+! the OS type. That is not 100% accurate in all cases, but it seems to be good
+! enough for now. See the following issue for a more robust solution:
+!
+! https://github.com/fortran-lang/fpm/issues/144
+!
 character(len=100) :: val
 integer stat
+! Only Windows define $HOMEPATH by default (if a user defines $HOMEPATH on Linux
+! or macOS, then this will be wrong):
+call get_environment_variable("HOMEPATH", val, status=stat)
+if (stat == 0) then
+    r = os_windows
+    return
+end if
+
+! We assume that $HOME=/home/... is Linux, $HOME=/Users/... is macOS, otherwise
+! we assume Linux. This is only a heuristic and can easily fail.
 call get_environment_variable("HOME", val, status=stat)
 if (stat == 1) then
     print *, "$HOME does not exist"
@@ -21,12 +39,15 @@ if (stat /= 0) then
     print *, "get_environment_variable() failed"
     error stop
 end if
-if (val(1:5) == "/home") then
+if (val(1:6) == "/home/") then
     r = os_linux
-else if (val(1:6) == "/Users") then
+else if (val(1:7) == "/Users/") then
     r = os_macos
 else
-    r = os_windows
+    ! This will happen on HPC systems that typically do not use either /home nor
+    ! /Users for $HOME. Those systems are typically Linux, so for now we simply
+    ! set Linux here.
+    r = os_linux
 end if
 end function