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’.