In your code you have the two lines:

>   Max_Descripteur : Positive;
>   subtype Ch_L_Cha�ne is String (1..Max_Descripteur);

Which are "elaborated" when the program begins.  At this time and ONLY at
this time is the value of Max_escripteur used to define the size of the
array.  Since you have not defined a value for Max_Descripteur when the type
Ch_L_Cha�ne is defined the actual size of the array is unpredictable, but
likely zero.

The functionality you are looking for may be found in either of the two
packages:

  Ada.Strings.Bounded

or

  Ada.Strings.Unbounded


I hope this helps,
SteveD