For example: #include, this command to the beginning of the called number # preprocessor.
=== C language macro definition (#define usage) ====
#define command called a macro definition, it is also a commanding C language preprocessor. The so-called macro definition, an identifier is used to represent a string, if there is the identifier in the code behind, then replace all of the specified string.
#define N 100
Some explanations for the use of #define
1. The macro definition is used to represent a string macro name, time and expansion of a macro string to replace the macro name, this is just a simple and crude replacement.
2. The macro definition is not a description or statement, end of the line does not have to add a semicolon.
3. The macro definition must uninstall function outside the scope of which is defined macro commands to the source end, such as to the scope which can stop command # undef.
==========================
=========== ========= C language conditional compilation
#if #elif #else #endif
Example code:
#include<stdio.h>
int main () {
#if _WIN32
printf("win32");
#elif _linux_
printf ( "linux system");
#else
printf ( "qita");
#endif
return 0;
}