(g77.info) Not My Type

Info Catalog

(g77.info) Working Programs

(g77.info) Variables Assumed To Be Zero

 
 Not My Type
 -----------
 
    A fruitful source of bugs in Fortran source code is use, or mis-use,
 of Fortran's implicit-typing feature, whereby the type of a variable,
 array, or function is determined by the first character of its name.
 
    Simple cases of this include statements like `LOGX=9.227', without a
 statement such as `REAL LOGX'.  In this case, `LOGX' is implicitly
 given `INTEGER(KIND=1)' type, with the result of the assignment being
 that it is given the value `9'.
 
    More involved cases include a function that is defined starting with
 a statement like `DOUBLE PRECISION FUNCTION IPS(...)'.  Any caller of
 this function that does not also declare `IPS' as type `DOUBLE
 PRECISION' (or, in GNU Fortran, `REAL(KIND=2)') is likely to assume it
 returns `INTEGER', or some other type, leading to invalid results or
 even program crashes.
 
    The `-Wimplicit' option might catch failures to properly specify the
 types of variables, arrays, and functions in the code.
 
    However, in code that makes heavy use of Fortran's implicit-typing
 facility, this option might produce so many warnings about cases that
 are working, it would be hard to find the one or two that represent
 bugs.  This is why so many experienced Fortran programmers strongly
 recommend widespread use of the `IMPLICIT NONE' statement, despite it
 not being standard FORTRAN 77, to completely turn off implicit typing.
 (`g77' supports `IMPLICIT NONE', as do almost all FORTRAN 77 compilers.)
 
    Note that `-Wimplicit' catches only implicit typing of *names*.  It
 does not catch implicit typing of expressions such as `X**(2/3)'.  Such
 expressions can be buggy as well--in fact, `X**(2/3)' is equivalent to
 `X**0', due to the way Fortran expressions are given types and then
 evaluated.  (In this particular case, the programmer probably wanted
 `X**(2./3.)'.)
 

Info Catalog

(g77.info) Working Programs

(g77.info) Variables Assumed To Be Zero