5.7.5.6 Wextra Option

A nonvolatile automatic variable might be changed by a call to longjmp. These warnings are possible only in optimizing compilation. The compiler sees only the calls to setjmp. It cannot know where longjmp will be called. In fact, a signal handler could call it at any point in the code. As a result, a warning may be generated even when there is in fact no problem, because longjmp cannot in fact be called at the place that would cause a problem.

A function could exit both via return value; and return;. Completing the function body without passing any return statement is treated as return;.

An expression-statement or the left-hand side of a comma expression contains no side effects. To suppress the warning, cast the unused expression to void. For example, an expression such as x[i,j] causes a warning, but x[(void)i,j] does not.

An unsigned value is compared against zero with < or <=.

A comparison like x<=y<=z appears. This is equivalent to (x<=y ? 1 : 0) <= z, which is a different interpretation from that of an ordinary mathematical notation.

Storage-class specifiers like static are not the first things in a declaration. According to the C Standard, this usage is obsolescent.

If -Wall or -Wunused is also specified, warn about unused arguments.

A comparison between signed and unsigned values could produce an incorrect result when the signed value is converted to unsigned (but won’t warn if -Wno-sign-compare is also specified).

An aggregate has a partly bracketed initializer. For example, the following code would evoke such a warning, because braces are missing around the initializer for x.h:

struct s { int f, g; };
struct t { struct s h; int i; };
struct t x = { 1, 2, 3 };

An aggregate has an initializer that does not initialize all members. For example, the following code would cause such a warning, because x.h would be implicitly initialized to zero:

struct s { int f, g, h; };
struct s x = { 3, 4 };