Ubuntu下Apache重启错误:Could not reliably determine解决
2016-01-15 11:22
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Preprocessor:
The
To prevent bad effects from a double include, it is common to surround the body in the include file with the following (where
#ifdefand
#ifndef
The
#ifdef(if defined) and
#ifndef(if not defined) preprocessor commands are used to test if a preprocessor variable has been "defined". There are two common uses for this, with slightly different patterns.
Prevent multiple definitions in header files
When there definitions in a header file that can not be made twice, the code below should be used. A header file may be included twice other include files include it, or an included file includes it and the source file includes it again.To prevent bad effects from a double include, it is common to surround the body in the include file with the following (where
MYHEADER_His replaced by a name that is appropriate for your program).
#ifndef MYHEADER_H #define MYHEADER_H . . . // This will be seen by the compiler only once #endif /* MYHEADER_H */
Turning debugging code off and on
Debugging code is necessary in programs, however, it is not usually appropriate to leave it in the delivered code. The preprocessor #ifdef commmand can surround the debugging code. If DEBUG is defined as below (probably in an include file) all debugging statement surrounded by the #ifdef DEBUG statement will be active. However, if it isn't defined, none of the statements will make it thru the preprocessor.#define DEBUG . . . #ifdef DEBUG . . . // debugging output #endif
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