C

C Preprocessor is a text substitution tool, it instructs the compiler to do required pre-processing before the code is compiled

Replace Macro, include header files

means copying the contents of another file in the current one

#include <filename>

/usr/local/include
libdir/gcc/target/version/include
/usr/target/include
/usr/include

#include "filename"

Search the current path
If not find, search the standard search path

#include macroName

macroName is a macro name pointing to a header file

Define search path

-I dir, define search path in makefile
INCLUDE_PATH, define an environment variable

#define STDIO <stdio.h>
#include STDIO

int main()
{
	printf("Hello World!\n");

	return 0;
}

Replace identifier with replacement content before compiling program

#include <stdio.h>

#define MESSAGE "Hello World!"
#define PI 3.14

int main()
{
	printf("%s\n", MESSAGE);
	printf("%f\n", PI);

	return 0;
}

Define function

#include <stdio.h>

#define doubleNum(a) a*2

int main()
{
	printf("%d\n", doubleNum(10));

	return 0;
}

#, convert a parameter to a string

#include <stdio.h>

#define OUTPUT(a) printf(#a)

int main()
{
	OUTPUT(Hello\n);

	return 0;
}

##, joined a token with a string

#include <stdio.h>

#define FUNCTION(name, a) int fun_##name() { return a;}

FUNCTION(v1, 12);
FUNCTION(v2, 2);

int main()
{
	printf("%d\n", fun_v1());
	printf("%d\n", fun_v2());

	return 0;
}

#include <stdio.h>

int main()
{

#ifndef SYSTEM
#define SYSTEM 1

	if(SYSTEM == 1)
		printf("Linux system ...\n");
#endif

#ifndef SYSTEM
	printf("Windows system ...\n");
#elif SYSTEM == 1
	printf("Linux system ...\n");
#else
	printf("Mac system ...\n");
#endif
}

causes preprocessing to stop at the location where the directive is encountered

//#define PI 3.14

int main()
{
#ifndef PI
#error NOT DEFINE PI
#endif
	return 0;
}

Issues special commands to the compiler

// main.c
#include <stdio.h>

int main(void)
{
    #pragma omp parallel
    {
    printf("Hello, world.\n");
    }

  return 0;
}

gcc -fopenmp main.c -o hello

#include <stdio.h>

int main() {

   printf("File :%s\n", __FILE__ ); // file name
   printf("Date :%s\n", __DATE__ ); // current date
   printf("Time :%s\n", __TIME__ ); // current time
   printf("Line :%d\n", __LINE__ ); // current line number
   printf("ANSI :%d\n", __STDC__ ); // 1, if the compiler complies with the ANSI standard

}

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