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Using the GNU Compiler Collection (GCC)

3.5 Options to Request or Suppress Warnings

Warnings are diagnostic messages that report constructions which are not inherently erroneous but which are risky or suggest there might have been an error.

You can request many specific warnings with options beginning `-W', for example `-Wimplicit' to request warnings on implicit declarations. Each of these specific warning options also has a negative form beginning `-Wno-' to turn off warnings; for example, `-Wno-implicit'. This manual lists only one of the two forms, whichever is not the default.

These options control the amount and kinds of warnings produced by GNU Fortran:

-fsyntax-only

Check the code for syntax errors, but don't do anything beyond that.

-pedantic

Issue warnings for uses of extensions to ANSI FORTRAN 77. `-pedantic' also applies to C-language constructs where they occur in GNU Fortran source files, such as use of `\e' in a character constant within a directive like `#include'.

Valid ANSI FORTRAN 77 programs should compile properly with or without this option. However, without this option, certain GNU extensions and traditional Fortran features are supported as well. With this option, many of them are rejected.

Some users try to use `-pedantic' to check programs for strict ANSI conformance. They soon find that it does not do quite what they want--it finds some non-ANSI practices, but not all. However, improvements to g77 in this area are welcome.

-pedantic-errors

Like `-pedantic', except that errors are produced rather than warnings.

-fpedantic

Like `-pedantic', but applies only to Fortran constructs.

-w

Inhibit all warning messages.

-Wno-globals

Inhibit warnings about use of a name as both a global name (a subroutine, function, or block data program unit, or a common block) and implicitly as the name of an intrinsic in a source file.

Also inhibit warnings about inconsistent invocations and/or definitions of global procedures (function and subroutines). Such inconsistencies include different numbers of arguments and different types of arguments.

-Wimplicit

Warn whenever a variable, array, or function is implicitly declared. Has an effect similar to using the IMPLICIT NONE statement in every program unit. (Some Fortran compilers provide this feature by an option named `-u' or `/WARNINGS=DECLARATIONS'.)

-Wunused

Warn whenever a variable is unused aside from its declaration.

-Wuninitialized

Warn whenever an automatic variable is used without first being initialized.

These warnings are possible only in optimizing compilation, because they require data-flow information that is computed only when optimizing. If you don't specify `-O', you simply won't get these warnings.

These warnings occur only for variables that are candidates for register allocation. Therefore, they do not occur for a variable whose address is taken, or whose size is other than 1, 2, 4 or 8 bytes. Also, they do not occur for arrays, even when they are in registers.

Note that there might be no warning about a variable that is used only to compute a value that itself is never used, because such computations may be deleted by data-flow analysis before the warnings are printed.

These warnings are made optional because GNU Fortran is not smart enough to see all the reasons why the code might be correct despite appearing to have an error. Here is one example of how this can happen:

SUBROUTINE DISPAT(J) IF (J.EQ.1) I=1 IF (J.EQ.2) I=4 IF (J.EQ.3) I=5 CALL FOO(I) END

If the value of J is always 1, 2 or 3, then I is always initialized, but GNU Fortran doesn't know this. Here is another common case:

SUBROUTINE MAYBE(FLAG) LOGICAL FLAG IF (FLAG) VALUE = 9.4 ... IF (FLAG) PRINT *, VALUE END

This has no bug because VALUE is used only if it is set.

-Wall

The `-Wunused' and `-Wuninitialized' options combined. These are all the options which pertain to usage that we recommend avoiding and that we believe is easy to avoid. (As more warnings are added to g77 some might be added to the list enabled by `-Wall'.)

The remaining `-W...' options are not implied by `-Wall' because they warn about constructions that we consider reasonable to use, on occasion, in clean programs.

-Wsurprising

Warn about "suspicious" constructs that are interpreted by the compiler in a way that might well be surprising to someone reading the code. These differences can result in subtle, compiler-dependent (even machine-dependent) behavioral differences. The constructs warned about include:

• Expressions having two arithmetic operators in a row, such as `X*-Y'. Such a construct is nonstandard, and can produce unexpected results in more complicated situations such as `X**-Y*Z'. g77 along with many other compilers, interprets this example differently than many programmers, and a few other compilers. Specifically, g77 interprets `X**-Y*Z' as `(X**(-Y))*Z', while others might think it should be interpreted as `X**(-(Y*Z))'.

  A revealing example is the constant expression `2**-2*1.', which g77 evaluates to .25, while others might evaluate it to 0., the difference resulting from the way precedence affects type promotion.

  (The `-fpedantic' option also warns about expressions having two arithmetic operators in a row.)

• Expressions with a unary minus followed by an operand and then a binary operator other than plus or minus. For example, `-2**2' produces a warning, because the precedence is `-(2**2)', yielding -4, not `(-2)**2', which yields 4, and which might represent what a programmer expects.

  An example of an expression producing different results in a surprising way is `-I*S', where I holds the value `-2147483648' and S holds `0.5'. On many systems, negating I results in the same value, not a positive number, because it is already the lower bound of what an INTEGER(KIND=1) variable can hold. So, the expression evaluates to a positive number, while the "expected" interpretation, `(-I)*S', would evaluate to a negative number.

  Even cases such as `-I*J' produce warnings, even though, in most configurations and situations, there is no computational difference between the results of the two interpretations--the purpose of this warning is to warn about differing interpretations and encourage a better style of coding, not to identify only those places where bugs might exist in the user's code.

• DO loops with DO variables that are not of integral type--that is, using REAL variables as loop control variables. Although such loops can be written to work in the "obvious" way, the way g77 is required by the Fortran standard to interpret such code is likely to be quite different from the way many programmers expect. (This is true of all DO loops, but the differences are pronounced for non-integral loop control variables.)

  @xref{Loops}, for more information.

-Werror

Make all warnings into errors.

-W

Turns on "extra warnings" and, if optimization is specified via `-O', the `-Wuninitialized' option. (This might change in future versions of g77

"Extra warnings" are issued for:

• Unused parameters to a procedure (when `-Wunused' also is specified).

• Overflows involving floating-point constants (not available for certain configurations).

See section `Options to Request or Suppress Warnings' in Using the GNU Compiler Collection (GCC), for information on more options offered by the GBE shared by g77 gcc and other GNU compilers.

Some of these have no effect when compiling programs written in Fortran:

-Wcomment

-Wformat

-Wparentheses

-Wswitch

-Wtraditional

-Wshadow

-Wid-clash-len

-Wlarger-than-len

-Wconversion

-Waggregate-return

-Wredundant-decls

These options all could have some relevant meaning for GNU Fortran programs, but are not yet supported.