4.2 Macros with values

In addition to being defined, a macro can also be given a value. This value is inserted into the source code at each point where the macro occurs. The following program uses a macro NUM, to represent a number which will be printed:

#include <stdio.h>

int
main (void)
{
  printf ("Value of NUM is %d\n", NUM);
  return 0;
}

Note that macros are not expanded inside strings--only the occurrence of NUM outside the string is substituted by the preprocessor.

To define a macro with a value, the -D command-line option can be used in the form -DNAME=VALUE. For example, the following command line defines NUM to be 100 when compiling the program above:

$ gcc -Wall -DNUM=100 dtestval.c
$ ./a.out 
Value of NUM is 100

This example uses a number, but a macro can take values of any form. Whatever the value of the macro is, it is inserted directly into the source code at the point where the macro name occurs. For example, the following definition expands the occurrences of NUM to 2+2 during preprocessing:

$ gcc -Wall -DNUM="2+2" dtestval.c
$ ./a.out 
Value of NUM is 4

After the preprocessor has made the substitution NUM ==> 2+2 this is equivalent to compiling the following program:

#include <stdio.h>

int
main (void)
{
  printf ("Value of NUM is %d\n", 2+2);
  return 0;
}

Note that it is a good idea to surround macros by parentheses whenever they are part of an expression. For example, the following program uses parentheses to ensure the correct precedence for the multiplication 10*NUM:

#include <stdio.h>

int
main (void)
{
  printf ("Ten times NUM is %d\n", 10 * (NUM));
  return 0;
}

With these parentheses, it produces the expected result when compiled with the same command line as above:

$ gcc -Wall -DNUM="2+2" dtestmul10.c
$ ./a.out 
Ten times NUM is 40

Without parentheses, the program would produce the value 22 from the literal form of the expression 10*2+2 = 22, instead of the desired value 10*(2+2) = 40.

When a macro is defined with -D alone, gcc uses a default value of 1. For example, compiling the original test program with the option -DNUM generates an executable which produces the following output:

$ gcc -Wall -DNUM dtestval.c
$ ./a.out 
Value of NUM is 1

A macro can be defined with an empty value using quotes on the command line, -DNAME="". Such a macro is still treated as defined by conditionals such as #ifdef, but expands to nothing.

A macro containing quotes can be defined using shell-escaped quote characters. For example, the command-line option -DMESSAGE='"Hello, World!"' defines a macro MESSAGE which expands to the sequence of characters "Hello, World!". The outer shell-quotes '...' protect the C-quotes of the string "Hello, World!". For an explanation of the different types of quoting and escaping used in the shell see the "GNU Bash Reference Manual", section Further reading.