26.5.1 Object-like Macros

An object-like macro is a simple identifier that will be replaced by a code fragment. It is called object-like because in most cases the use of the macro looks like reference to a data object in code that uses it. These macros are most commonly used to give symbolic names to numeric constants.

The way to define macros with the #define directive. #define is followed by the name of the macro and then the token sequence it should be an abbreviation for, which is variously referred to as the macro’s body, expansion or replacement list. For example,

#define BUFFER_SIZE 1024

defines a macro named BUFFER_SIZE as an abbreviation for the token 1024. If somewhere after this #define directive there comes a C statement of the form

foo = (char *) malloc (BUFFER_SIZE);

then preprocessing will recognize and expand the macro BUFFER_SIZE, so that compilation will see the tokens:

foo = (char *) malloc (1024);

By convention, macro names are written in upper case. Programs are easier to read when it is possible to tell at a glance which names are macros. Macro names that start with ‘__’ are reserved for internal uses, and many of them are defined automatically, so don’t define such macro names unless you really know what you’re doing. Likewise for macro names that start with ‘_’ and an upper-case letter.

The macro’s body ends at the end of the #define line. You may continue the definition onto multiple lines, if necessary, using backslash-newline. When the macro is expanded, however, it will all come out on one line. For example,

#define NUMBERS 1, \
                2, \
                3
int x[] = { NUMBERS };
     → int x[] = { 1, 2, 3 };

The most common visible consequence of this is surprising line numbers in error messages.

There is no restriction on what can go in a macro body provided it decomposes into valid preprocessing tokens. Parentheses need not balance, and the body need not resemble valid C code. (If it does not, you may get error messages from the C compiler when you use the macro.)

Preprocessing scans the program sequentially. A macro definition takes effect right after its appearance. Therefore, the following input

foo = X;
#define X 4
bar = X;

produces

foo = X;
bar = 4;

When preprocessing expands a macro name, the macro’s expansion replaces the macro invocation, then the expansion is examined for more macros to expand. For example,

#define TABLESIZE BUFSIZE
#define BUFSIZE 1024
TABLESIZE
     → BUFSIZE
     → 1024

TABLESIZE is expanded first to produce BUFSIZE, then that macro is expanded to produce the final result, 1024.

Notice that BUFSIZE was not defined when TABLESIZE was defined. The #define for TABLESIZE uses exactly the expansion you specify—in this case, BUFSIZE—and does not check to see whether it too contains macro names. Only when you use TABLESIZE is the result of its expansion scanned for more macro names.

This makes a difference if you change the definition of BUFSIZE at some point in the source file. TABLESIZE, defined as shown, will always expand using the definition of BUFSIZE that is currently in effect:

#define BUFSIZE 1020
#define TABLESIZE BUFSIZE
#undef BUFSIZE
#define BUFSIZE 37

Now TABLESIZE expands (in two stages) to 37.

If the expansion of a macro contains its own name, either directly or via intermediate macros, it is not expanded again when the expansion is examined for more macros. This prevents infinite recursion. See Self-Referential Macros, for the precise details.