LEX

Unrecognized character ...
Invalid first character ...
Line too long ...
Non-numeric character ...
Continuation indicator ...
Label at ... invalid with continuation line indicator ...
Character constant ...
Continuation line ...
Statement at ... begins with invalid token

Although the diagnostics identify specific problems, they can be produced when general problems such as the following occur:

• The source file contains something other than Fortran code. If the code in the file does not look like many of the examples elsewhere in this document, it might not be Fortran code. (Note that Fortran code often is written in lower case letters, while the examples in this document use upper case letters, for stylistic reasons.) For example, if the file contains lots of strange-looking characters, it might be APL source code; if it contains lots of parentheses, it might be Lisp source code; if it contains lots of bugs, it might be C++ source code.
• The source file contains free-form Fortran code, but `-ffree-form' was not specified on the command line to compile it. Free form is a newer form for Fortran code. The older, classic form is called fixed form. Fixed-form code is visually fairly distinctive, because numerical labels and comments are all that appear in the first five columns of a line, the sixth column is reserved to denote continuation lines, and actual statements start at or beyond column 7. Spaces generally are not significant, so if you see statements such as `REALX,Y' and `DO10I=1,100', you are looking at fixed-form code. Comment lines are indicated by the letter `C' or the symbol `*' in column 1. (Some code uses `!' or `/*' to begin in-line comments, which many compilers support.) Free-form code is distinguished from fixed-form source primarily by the fact that statements may start anywhere. (If lots of statements start in columns 1 through 6, that's a strong indicator of free-form source.) Consecutive keywords must be separated by spaces, so `REALX,Y' is not valid, while `REAL X,Y' is. There are no comment lines per se, but `!' starts a comment anywhere in a line (other than within a character or hollerith constant). See section Source Form, for more information.
• The source file is in fixed form and has been edited without sensitivity to the column requirements. Statements in fixed-form code must be entirely contained within columns 7 through 72 on a given line. Starting them "early" is more likely to result in diagnostics than finishing them "late", though both kinds of errors are often caught at compile time. For example, if the following code fragment is edited by following the commented instructions literally, the result, shown afterward, would produce a diagnostic when compiled:

  C On XYZZY systems, remove "C" on next line:
  C     CALL XYZZY_RESET
  

  The result of editing the above line might be:

  C On XYZZY systems, remove "C" on next line:
       CALL XYZZY_RESET
  

  However, that leaves the first `C' in the `CALL' statement in column 6, making it a comment line, which is not really what the author intended, and which is likely to result in one of the above-listed diagnostics. Replacing the `C' in column 1 with a space is the proper change to make, to ensure the `CALL' keyword starts in or after column 7. Another common mistake like this is to forget that fixed-form source lines are significant through only column 72, and that, normally, any text beyond column 72 is ignored or is diagnosed at compile time. See section Source Form, for more information.
• The source file requires preprocessing, and the preprocessing is not being specified at compile time. A source file containing lines beginning with #define, #include, #if, and so on is likely one that requires preprocessing. If the file's suffix is `.f' or `.for', the file will normally be compiled without preprocessing by g77. Change the file's suffix from `.f' to `.F' (or, on systems with case-insensitive file names, to `.fpp') or from `.for' to `.fpp'. g77 compiles files with such names with preprocessing. Or, learn how to use gcc's `-x' option to specify the language `f77-cpp-input' for Fortran files that require preprocessing. See section Options Controlling the Kind of Output.
• The source file is preprocessed, and the results of preprocessing result in syntactic errors that are not necessarily obvious to someone examining the source file itself. Examples of errors resulting from preprocessor macro expansion include exceeding the line-length limit, improperly starting, terminating, or incorporating the apostrophe or double-quote in a character constant, improperly forming a hollerith constant, and so on. See section Options Controlling the Kind of Output, for suggestions about how to use, and not use, preprocessing for Fortran code.

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