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A few directives

All preprocessor directives, or commands, are preceded by a hash mark (#). One example you have already seen in previous chapters is the #include directive:

#include <stdio.h>

This directive tells the preprocessor to include the file stdio.h; in other words, to treat it as though it were part of the program text.

A file to be included may itself contain #include directives, thus encompassing other files. When this happens, the included files are said to be nested.

Here are a few other directives:

#if ... #endif

The #if directive is followed by an expression on the same line. The lines of code between #if and #endif will be compiled only if the expression is true. This is called conditional compilation.

#else

This is part of an #if preprocessor statement and works in the same way with #if that the regular C else does with the regular if.

#line constant filename

This causes the compiler to act as though the next line is line number constant and is part of the file filename. Mainly used for debugging.

#error

This forces the compiler to abort. Also intended for debugging.

Below is an example of conditional compilation. The following code displays 23 to the screen.

#include <stdio.h>

#define CHOICE 500

int my_int = 0;

#if (CHOICE == 500)
void set_my_int()
{
  my_int = 23;
}
#else
void set_my_int()
{
  my_int = 17;
}
#endif

int main ()
{
  set_my_int();
  printf("%d\n", my_int);

  return 0;
}