16. Peculiarities in the language Fortran 90

• Using the free source form it is not permitted to have comments starting with a C or a * in column 1. This would be a violation of the free format. The new character ! (exclamation mark) has to be used.
• An advantage compared with earlier Fortran standards is that the standard now requires that the compiler can signal if the user deviates from the permitted standard.

  It is required that a Fortran 90 compiler can signal

  • use of syntax not defined in the standard.
  • violation of the syntax rules.
  • use of kinds not available.
  • use of obsolete constructs (or statements).
  • use of non-Fortran characters (for example, Swedish or Cyrillic characters) outside of character strings or comments.
  • violation of the area of validity for variable names, names of the DO-loops and the corresponding names like IF, CASE and operators.
  • the reason that a program is not accepted by the compiler.

  The above means that it is permitted to include extensions to Fortran 90. It has to be possible to ask the program to signal for any extensions outside the Fortran 90 standard.

• The compiler often performs some data flow analysis.

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