to_c_string: move to strings, document

Author: perazz

doc/specs/stdlib_strings.md

@@ -498,3 +498,33 @@ The result is an `allocatable` length `character` scalar with up to `128` cached
 ```fortran
 {!example/strings/example_to_string.f90!}
 ```
+
+<!-- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -->
+### `to_c_string`
+
+#### Description
+
+Convert a Fortran character string to a C character array.  
+This function converts a Fortran string into a C-style string, ensuring proper null-termination for use in C functions or libraries.  
+
+#### Syntax
+
+`cstr = ` [[stdlib_strings(module):to_c_string(function)]] ` (value)`
+
+#### Status
+
+Experimental
+
+#### Class
+
+Pure function.
+
+#### Argument
+
+- `value`: Shall be a `character(len=*)` string.  
+  This is an `intent(in)` argument.  
+  The Fortran string that will be converted to a C character array.
+
+#### Result value
+
+The result is a `character(kind=c_char)` array with a dimension of `len(value) + 1` to accommodate the null terminator.

src/stdlib_strings.fypp

@@ -8,10 +8,12 @@ module stdlib_strings
     use stdlib_string_type, only: string_type, char, verify, repeat, len
     use stdlib_optval, only: optval
     use stdlib_kinds, only: sp, dp, xdp, qp, int8, int16, int32, int64, lk, c_bool
+    use iso_c_binding, only: c_char, c_null_char
     implicit none
     private
 
     public :: to_string
+    public :: to_c_string
     public :: strip, chomp
     public :: starts_with, ends_with
     public :: slice, find, replace_all, padl, padr, count, zfill
@@ -943,5 +945,17 @@ contains
 
     end function zfill_char
 
+    !> Convert a Fortran character string to a C character array
+    !>
+    !> Version: experimental    
+    pure function to_c_string(value) result(cstr)
+       character(len=*), intent(in) :: value
+       character(kind=c_char) :: cstr(len(value)+1)
+       integer :: i
+       do concurrent (i=1:len(value)) 
+           cstr(i) = value(i:i)
+       end do 
+       cstr(len(value)+1) = c_null_char
+    end function to_c_string
 
 end module stdlib_strings

src/stdlib_system_subprocess.F90

@@ -2,6 +2,8 @@ module fortran_subprocess
     use iso_c_binding  
     use iso_fortran_env, only: int64, real64
     use stdlib_system
+    use stdlib_io, only: getfile
+    use stdlib_strings, only: to_c_string
     implicit none
     public
 
@@ -199,15 +201,6 @@ subroutine launch_asynchronous(process, args, stdin)
 
     end subroutine launch_asynchronous
 
-    pure function c_string(str) result(cstr)
-       character(*), intent(in) :: str
-       character(c_char), allocatable :: cstr(:)
-       integer :: i
-       allocate(cstr(len(str)+1))
-       forall(i=1:len(str)) cstr(i) = str(i:i)
-       cstr(len(str)+1) = c_null_char
-    end function c_string
-    
     subroutine launch_synchronous(process, args, stdin)        
         class(process_type), intent(inout) :: process
         !> The command and arguments