26.5.3 Macro Arguments

Function-like macros can take arguments, just like true functions. To define a macro that uses arguments, you insert parameters between the pair of parentheses in the macro definition that make the macro function-like. The parameters must be valid C identifiers, separated by commas and optionally whitespace.

To invoke a macro that takes arguments, you write the name of the macro followed by a list of actual arguments in parentheses, separated by commas. The invocation of the macro need not be restricted to a single logical line—it can cross as many lines in the source file as you wish. The number of arguments you give must match the number of parameters in the macro definition. When the macro is expanded, each use of a parameter in its body is replaced by the tokens of the corresponding argument. (The macro body is not required to use all of the parameters.)

As an example, here is a macro that computes the minimum of two numeric values, as it is defined in many C programs, and some uses.

#define min(X, Y)  ((X) < (Y) ? (X) : (Y))
  x = min(a, b);      → x = ((a) < (b) ? (a) : (b));
  y = min(1, 2);      → y = ((1) < (2) ? (1) : (2));
  z = min(a+28, *p);  → z = ((a+28) < (*p) ? (a+28) : (*p));

In this small example you can already see several of the dangers of macro arguments. See Macro Pitfalls, for detailed explanations.

Leading and trailing whitespace in each argument is dropped, and all whitespace between the tokens of an argument is reduced to a single space. Parentheses within each argument must balance; a comma within such parentheses does not end the argument. However, there is no requirement for square brackets or braces to balance, and they do not prevent a comma from separating arguments. Thus,

macro (array[x = y, x + 1])

passes two arguments to macro: array[x = y and x + 1]. If you want to supply array[x = y, x + 1] as an argument, you can write it as array[(x = y, x + 1)], which is equivalent C code. However, putting an assignment inside an array subscript is to be avoided anyway.

All arguments to a macro are completely macro-expanded before they are substituted into the macro body. After substitution, the complete text is scanned again for macros to expand, including the arguments. This rule may seem strange, but it is carefully designed so you need not worry about whether any function call is actually a macro invocation. You can run into trouble if you try to be too clever, though. See Argument Prescan, for detailed discussion.

For example, min (min (a, b), c) is first expanded to

  min (((a) < (b) ? (a) : (b)), (c))

and then to

((((a) < (b) ? (a) : (b))) < (c)
 ? (((a) < (b) ? (a) : (b)))
 : (c))

(The line breaks shown here for clarity are not actually generated.)

You can leave macro arguments empty without error, but many macros will then expand to invalid code. You cannot leave out arguments entirely; if a macro takes two arguments, there must be exactly one comma at the top level of its argument list. Here are some silly examples using min:

min(, b)        → ((   ) < (b) ? (   ) : (b))
min(a, )        → ((a  ) < ( ) ? (a  ) : ( ))
min(,)          → ((   ) < ( ) ? (   ) : ( ))
min((,),)       → (((,)) < ( ) ? ((,)) : ( ))

min()      error→ macro "min" requires 2 arguments, but only 1 given
min(,,)    error→ macro "min" passed 3 arguments, but takes just 2

Whitespace is not a preprocessing token, so if a macro foo takes one argument, foo () and foo ( ) both supply it an empty argument.

Macro parameters appearing inside string literals are not replaced by their corresponding actual arguments.

#define foo(x) x, "x"
foo(bar)        → bar, "x"

See the next subsection for how to insert macro arguments into a string literal.

The token following the macro call and the last token of the macro expansion do not become one token even if it looks like they could:

#define foo()  abc
foo()def        → abc def