7.6.4.3 The -W Option

Use the -W command line option to print extra warning messages for these events:

• 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] will cause a warning, but x[(void)i,j] will 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 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 don’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 };