Programming with GNU Software

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Warnings

Let's look at a few flags to control the level of warning we can control. Listed here are some of the more useful and commonly used warning options; if you want to view the full set of warnings available, the GCC man pages contain much more information.

All of the warning options are prefixed by -W. So (for example) -Wall literally stands for all warnings, and is not some euphemism for a wall!

-w Inhibit all warning messages.
-pedantic Print all warnings that are required by ANSI standard C.
-pedantic-errors Like -pedantic, except that the warnings are instead treated as errors.
-Wall This a very general warning option: it encompasses a whole range of features, each of which are listed below in following -W... options¹:
-Wimplicit-int Any function declarations without a return-type will be reported; their return type will default to int.
-Wimplicit-function-declaration Warns whenever a function is encountered in a block of code, but has not yet been declared.
-Wimplicit Same as -Wimplicit-int -Wimplicit-function-declaration.
-Wmain Warns if main:
- takes any other arguments other than (a) an int followed by char **, or (b) no arguments, or
- returns any value other than int.
-Wreturn-type This flag warns that any function not declaring a return-type defaults to int, and also warns of any non-void functions that do not return a value.
-Wunused Warns if any:
- local variable is declared but not used;
- a static function is defined, but is not used; or
- a statement computes a result, but the result is never used.
-Wswitch Warns if
- an enumeration is used in a switch statement, but at least one of the elements of the enumeration has not been used in the switch; or
- some value not catered for by the enumeration is used in the same switch statement.
-Wcomment Warns if any comment started with /* contains an inner /*; for example, /* some comment... /* */.
-Wformat If any print-related functions are called (for example printf), type-mismatching of arguments is reported where it occurs.
-Wchar-subscripts -
-Wuninitialized An automatic variable is used without first being initialized.
-Wparentheses Warns if parentheses are omitted in certain contexts.

Notes de bas de page

-Wtrigraphs has been omitted; you shouldn't really need to use this anyway.

`-w`	Inhibit all warning messages.
`-pedantic`	Print all warnings that are required by ANSI standard C.
`-pedantic-errors`	Like `-pedantic`, except that the warnings are instead treated as errors.
`-Wall`	This a very general warning option: it encompasses a whole range of features, each of which are listed below in following `-W...` options¹:
`-Wimplicit-int`	Any function declarations without a return-type will be reported; their return type will default to `int`.
`-Wimplicit-function-declaration`	Warns whenever a function is encountered in a block of code, but has not yet been declared.
`-Wimplicit`	Same as `-Wimplicit-int` `-Wimplicit-function-declaration`.
`-Wmain`	Warns if `main`: - takes any other arguments other than (a) an `int` followed by `char **`, or (b) no arguments, or - returns any value other than `int`.
`-Wreturn-type`	This flag warns that any function not declaring a return-type defaults to `int`, and also warns of any non-void functions that do not return a value.
`-Wunused`	Warns if any: - local variable is declared but not used; - a static function is defined, but is not used; or - a statement computes a result, but the result is never used.
`-Wswitch`	Warns if - an enumeration is used in a switch statement, but at least one of the elements of the enumeration has not been used in the switch; or - some value not catered for by the enumeration is used in the same switch statement.
`-Wcomment`	Warns if any comment started with `/` contains an inner `/`; for example, `/* some comment... /* */`.
`-Wformat`	If any print-related functions are called (for example `printf`), type-mismatching of arguments is reported where it occurs.
`-Wchar-subscripts`	-
`-Wuninitialized`	An automatic variable is used without first being initialized.
`-Wparentheses`	Warns if parentheses are omitted in certain contexts.