C Expert Programming Chapter 1 C: Through the Fog of Time and Space 1.3 The Standard I/O Library and the C Preprocessor

Standard I/O Libraries and C Preprocessors
Some functions not implemented by C compilers must be implemented in other ways.In C, they are processed at runtime, either in application code or in a runtime library.In the C language, the vast majority of library functions or auxiliary programs need to be called explicitly.For example, in C (where necessary), the programmer must manage the use of dynamic memory, create arrays of various sizes, test array bounds, and perform range checking.

C originally did not define I/O, but provided by library functions.Later, this actually became the standard mechanism.
Portable I/O, written by Mike Lesk, first appeared in 1972, optimized and tailored to become the standard I/O library.

C Preprocessor
Initiator: Alan Snyder
Time: 1972
Three main functions implemented:
String replacement.Usually used to give a constant a symbolic name.

The header file contains.(The general declarations are separated into header files and can be used by many source files, although the convention uses a .h extension for header files)

An extension to the generic code template.Unlike functions, macros (marco) can receive arguments of different types in successive invocations (the actual arguments of the macro are just output as-is). In the expansion of a macro, whitespace can make a big difference to the expansion.

// 1.
#Define a (y) a_expanded (y)
a (x);a (y) a_expanded(y)
a(x);
is expanded to
(y) a_expanded(y)(x);

You might think that using curly braces in macros, like in other parts of C, would combine redundant statements into a single compound statement, but that's not the case.Preprocessors like macros should only be used in moderation.

//2.
/*People: Steve Bourne
*Location: Bell Labs
*Shell (command interpreter) for UNIX v7
*Use the preprocessor to make CThe language looks more like Algol-68
*/
#define STRING char *
#define IF if (
#define THEN ) {
#define ELSE } else (
#define FI ;}
#define WHILE while (
#define DO ) {
#define OD ;}
#define INT int
#define BEGIN {
    #define END }
    
    INT compare(s1, s2) STRING s1; STRING s2;
    BEGIN
        WHILE *s1++ == *s2
DO IF *s2++ == 0
      THEN return(0)
      FI
  OD
  return (*--s1 - *s2);
END
 
  /*Translation, just replace it as it is*/
int compare(s1, s2) char *s1; char *s2;
{
while (*s1++ == *s2 ) {
if (*s2++ == 0) {
return (0);
}
;}
return (*--s1 - *s2);
} }

Corresponding C code:
int compare(s1, s2)
char *s1, *s2;
{
while (*s1++ == *s2) {
if (*s2 ++ == 0) Return (0);
}
Return (*-s1-*s2);
}
/*Shell does not use malloc, but uses sbrk to manage the heap storage itself, Steve explained,
* *The reason why he uses this special memory allocator is to improve the efficiency of string processing
* */
 
  /*Bource's creation of this variant of C actually led to the whimsical International Obfuscated C Code Competition
 * (The International Obfuscated C Code Competition)
  */

Macros are best used only to name constants and provide a shorthand notation for some appropriate construct.Macro names should be capitalized so that
can be easily distinguished from function calls.Never use the C preprocessor to modify the underlying structure of the language, because then C is not C.