The C language preprocessor

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The C compiler also uses a preprocessor that processes the source program before the compiler gets access to the transformed program. The idea started in ALGOL 68 (a complex language) but became popular with C.

Some preprocessor directives, introduced by the hash character "#" include the following in C.

#include header files (as modules)
#define to define preprocessor
#error for custom error messages (related to the compile)
#pragma for non-standard or compiler-specific directives.
#if, etc.

C also has a macro expansion capability that makes providing meaningful error messages more difficult.

A common directive at the top of most C programs is the following.

#include

The stdio stands for "standard input output" and contains routines, such as printf and scanf to do fundamental input and output. If the directive is not present, the C compiler will not know the meaning of printf or scanf and will issue detected error messages.

Compiler preprocessor directives can be used to conditionally include or exclude source code in the compilation.

#define test #define debug #undef test #undef debug

Here is the C code.

#include <stdio.h>
#define test
int main(void)
{
	int x, y, z;
	printf("Enter two integers: ");
	#ifdef test
	x = 2;
	y = 3;
	printf("(simulate x=%d y=%d)", x, y);
	#else
	scanf("%i %i", &x, &y);
	#endif
	z = x + y;
	printf("\nThe sum of %d and %d is %d.", x, y, z);
	}

Here is the output of the C code.

Enter two integers: (simulate x=2 y=3)
The sum of 2 and 3 is 5.

Note the following;

Defined symbols can be used to toggle test mode and non-test mode on and off.
Such symbols can be defined in a file and included using the include directive so that an entire project can be made and the test mode changed in just one place.

Many languages have similar directives for pre-processing, file inclusion, etc.

Many languages include pre-defined symbols and values. C defines the following (and many others).

__unix__ _WIN32 __FILE__ __LINE__

A cross-platform program for, say Windows or Linux, can be achieved by using conditional compilation to include code suitable for each platform.

The "__FILE__" and "__LINE__" values can be used to provide source file debugging information.
The "__DATE__" and "__TIME__" values can be used to provide information about the version as to static date and time as to when the compile took place.

Here is the C code.

#include <stdio.h>
int main(void)
{
	printf("\napha");
	printf("\nbeta");
	printf("\nline=%d file=%s", __LINE__, __FILE__);
	printf("\ndate=%s time=%s", __DATE__, __TIME__);
	printf("\ngamma");
	printf("\ndelta");
	}

Here is the output of the C code.

apha
beta
line=7 file=D:\W\PROGRAM1.C\lang52-02.c
date=Apr  4 2020 time=10:24:46
gamma
delta

A macro expansion replaces text with other text (i.e., a sample string rewriting system).

C does not have constants (named literals) but the same effect can be achieved using macro expansion.

Here is the C code.

#include <stdio.h>
#define PI 3.14159
int main(void)
{
	printf("\nPI=%d", PI);
	printf("\nPI=%f", PI);
	}

Here is the output of the C code.

PI=-266631570
PI=3.141590

Note how C does not raise an error if an incorrect type specifier is used in a printf. Rather, it does to output anyway.

Here is the C code.

#include <stdio.h>
#define PI 3.14159
#define TORADS(D) (D*PI/180)
int main(void)
{
	double degrees1 = 90.0;
	printf("\ndegrees=%.3lf", degrees1);
	printf("\nradians=%.3lf", TORADS(90.0));
	printf("\nradians=%.3lf", TORADS(degrees1));
	}

Here is the output of the C code.

degrees=90.000
radians=1.571
radians=1.571

The second printf does the conversion at compile time.
The third printf does the conversion at run time (unless the compiler is smart).

Some other popular programming languages provide preprocessor directives.

C#, VB.NET, .NET languages in general
VBA, Visual Basic
Delphi, Turbo Pascal
Common Lisp
Ada, Perl, JavaScript, Ruby, PL/SQL

Some popular programming languages do not provide preprocessor directives.

Java, Lua, Python

#include <> #include "" #define #define =

#undef #ifdef #ifndef #if

#elif

#else #endif

Note: The C/C++ macro preprocessor is much more powerful than the subset of capabilities listed above.