gcc消除编译时的warning

6.61.11 Diagnostic Pragmas

GCC allows the user to selectively enable or disable certain types of diagnostics, and change the kind of the diagnostic. For example, a project’s policy might require that all sources compile with -Werror but certain files might have exceptions allowing specific types of warnings. Or, a project might selectively enable diagnostics and treat them as errors depending on which preprocessor macros are defined.

Gcc允许使用者选择性地使能、禁止,或改变诊断的类型。打个比方就是，一个项目中可能要把诊断结果当做error对待，或者禁止，允许所有warnings。

pragma GCC diagnostic kind option

Modifies the disposition of a diagnostic. Note that not all diagnostics are modifiable; at the moment only warnings (normally controlled by ‘-W…’) can be controlled, and not all of them. Use -fdiagnostics-show-option to determine which diagnostics are controllable and which option controls them.
kind is ‘error’ to treat this diagnostic as an error, ‘warning’ to treat it like a warning (even if -Werror is in effect), or ‘ignored’ if the diagnostic is to be ignored. option is a double quoted string that matches the command-line option.

          #pragma GCC diagnostic warning "-Wformat"
          #pragma GCC diagnostic error "-Wformat"
          #pragma GCC diagnostic ignored "-Wformat"

Note that these pragmas override any command-line options. GCC keeps track of the location of each pragma, and issues diagnostics according to the state as of that point in the source file. Thus, pragmas occurring after a line do not affect diagnostics caused by that line.

#pragma GCC diagnostic push
#pragma GCC diagnostic pop

Causes GCC to remember the state of the diagnostics as of each push, and restore to that point at each pop. If a pop has no matching push, the command-line options are restored.

使GCC记住每次push时的诊断状态，并在每次pop时恢复到该点。如果pop没有匹配的push，则恢复命令行选项。

          #pragma GCC diagnostic error "-Wuninitialized"
            foo(a);                       /* error is given for this one */
          #pragma GCC diagnostic push
          #pragma GCC diagnostic ignored "-Wuninitialized"
            foo(b);                       /* no diagnostic for this one */
          #pragma GCC diagnostic pop
            foo(c);                       /* error is given for this one */
          #pragma GCC diagnostic pop
            foo(d);                       /* depends on command-line options */

GCC also offers a simple mechanism for printing messages during compilation.

GCC还提供了一种简单机制来输出编译信息。

#pragma message string

Prints string as a compiler message on compilation. The message is informational only, and is neither a compilation warning nor an error.

          #pragma message "Compiling " __FILE__ "..."

string may be parenthesized, and is printed with location information. For example,

带位置信息的输出string

          #define DO_PRAGMA(x) _Pragma (#x)
          #define TODO(x) DO_PRAGMA(message ("TODO - " #x))

          #TODO(Remember to fix this)

prints ‘/tmp/file.c:4: note: #pragma message: TODO - Remember to fix this’.