各个层次的gcc警告
从上到下覆盖


变量(代码)级:指定某个变量警告


int a __attribute__ ((unused));
指定该变量为"未使用的".即使这个变量没有被使用,编译时也会忽略则个警告输出.


文件级:在源代码文件中诊断(忽略/警告)


语法:


#pragma GCC diagnostic [error|warning|ignored] "-W<警告选项>"
诊断-忽略:(关闭警告)


#pragma  GCC diagnostic ignored  "-Wunused"
#pragma  GCC diagnostic ignored  "-Wunused-parameter"
诊断-警告:(开启警告)


#pragma  GCC diagnostic warning  "-Wunused"
#pragma  GCC diagnostic warning  "-Wunused-parameter"
诊断-错误:(开启警告-升级为错误)


#pragma  GCC diagnostic error  "-Wunused"
#pragma  GCC diagnostic error  "-Wunused-parameter"
用法:


在文件开头处关闭警告,在文件结尾出再开启警告,这样可以忽略该文件中的指定警告.


项目级:命令行/编译参数指定


警告:
gcc main.c -Wall 忽略:
gcc mian.c -Wall -Wno-unused-parameter //开去all警告,但是忽略 -unused-parameter警告


选项格式: -W[no-]<警告选项>
如 : -Wno-unused-parameter # no- 表示诊断时忽略这个警告

来源：https://github.com/zodiac1111/note/blob/master/note/gcc/gcc-ignored-warning.markdown#%E6%96%87%E4%BB%B6%E7%BA%A7%E5%9C%A8%E6%BA%90%E4%BB%A3%E7%A0%81%E6%96%87%E4%BB%B6%E4%B8%AD%E8%AF%8A%E6%96%AD%E5%BF%BD%E7%95%A5%E8%AD%A6%E5%91%8A

 

 

6.59.10 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.

#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.

          #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.

#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,

          #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’.

来源：http://gcc.gnu.org/onlinedocs/gcc/Diagnostic-Pragmas.html