3.10. Macro Pitfalls

3.10.1. Misnesting

When a macro is called with arguments, the arguments are substituted into the macro body and the result is checked, together with the rest of the input file, for more macro calls. It is possible to piece together a macro call coming partially from the macro body and partially from the arguments. For example,

#define twice(x) (2*(x)) #define call_with_1(x) x(1) call_with_1 (twice) ==> twice(1) ==> (2*(1))

Macro definitions do not have to have balanced parentheses. By writing an unbalanced open parenthesis in a macro body, it is possible to create a macro call that begins inside the macro body but ends outside of it. For example,

#define strange(file) fprintf (file, "%s %d", … strange(stderr) p, 35) ==> fprintf (stderr, "%s %d", p, 35)

The ability to piece together a macro call can be useful, but the use of unbalanced open parentheses in a macro body is just confusing, and should be avoided.

3.10.2. Operator Precedence Problems

You may have noticed that in most of the macro definition examples shown above, each occurrence of a macro argument name had parentheses around it. In addition, another pair of parentheses usually surround the entire macro definition. Here is why it is best to write macros that way.

Suppose you define a macro as follows,

#define ceil_div(x, y) (x + y - 1) / y

whose purpose is to divide, rounding up. (One use for this operation is to compute how many int objects are needed to hold a certain number of char objects.) Then suppose it is used as follows:

a = ceil_div (b & c, sizeof (int)); ==> a = (b & c + sizeof (int) - 1) / sizeof (int);

This does not do what is intended. The operator-precedence rules of C make it equivalent to this:

a = (b & (c + sizeof (int) - 1)) / sizeof (int);

What we want is this:

a = ((b & c) + sizeof (int) - 1)) / sizeof (int);

Defining the macro as

#define ceil_div(x, y) ((x) + (y) - 1) / (y)

provides the desired result.

Unintended grouping can result in another way. Consider sizeof ceil_div(1, 2). That has the appearance of a C expression that would compute the size of the type of ceil_div (1, 2), but in fact it means something very different. Here is what it expands to:

sizeof ((1) + (2) - 1) / (2)

This would take the size of an integer and divide it by two. The precedence rules have put the division outside the sizeof when it was intended to be inside.

Parentheses around the entire macro definition prevent such problems. Here, then, is the recommended way to define ceil_div:

#define ceil_div(x, y) (((x) + (y) - 1) / (y))

3.10.3. Swallowing the Semicolon

Often it is desirable to define a macro that expands into a compound statement. Consider, for example, the following macro, that advances a pointer (the argument p says where to find it) across whitespace characters:

#define SKIP_SPACES(p, limit) \ { char *lim = (limit); \ while (p < lim) { \ if (*p++ != ' ') { \ p--; break; }}}

Here backslash-newline is used to split the macro definition, which must be a single logical line, so that it resembles the way such code would be laid out if not part of a macro definition.

A call to this macro might be SKIP_SPACES (p, lim). Strictly speaking, the call expands to a compound statement, which is a complete statement with no need for a semicolon to end it. However, since it looks like a function call, it minimizes confusion if you can use it like a function call, writing a semicolon afterward, as in SKIP_SPACES (p, lim);

This can cause trouble before else statements, because the semicolon is actually a null statement. Suppose you write

if (*p != 0) SKIP_SPACES (p, lim); else …

The presence of two statements--the compound statement and a null statement--in between the if condition and the else makes invalid C code.

The definition of the macro SKIP_SPACES can be altered to solve this problem, using a do … while statement. Here is how:

#define SKIP_SPACES(p, limit) \ do { char *lim = (limit); \ while (p < lim) { \ if (*p++ != ' ') { \ p--; break; }}} \ while (0)

Now SKIP_SPACES (p, lim); expands into

do {…} while (0);

which is one statement. The loop executes exactly once; most compilers generate no extra code for it.

3.10.4. Duplication of Side Effects

Many C programs define a macro min, for "minimum", like this:

#define min(X, Y) ((X) < (Y) ? (X) : (Y))

When you use this macro with an argument containing a side effect, as shown here,

next = min (x + y, foo (z));

it expands as follows:

next = ((x + y) < (foo (z)) ? (x + y) : (foo (z)));

where x + y has been substituted for X and foo (z) for Y.

The function foo is used only once in the statement as it appears in the program, but the expression foo (z) has been substituted twice into the macro expansion. As a result, foo might be called two times when the statement is executed. If it has side effects or if it takes a long time to compute, the results might not be what you intended. We say that min is an unsafe macro.

The best solution to this problem is to define min in a way that computes the value of foo (z) only once. The C language offers no standard way to do this, but it can be done with GNU extensions as follows:

#define min(X, Y) \ ({ typeof (X) x_ = (X); \ typeof (Y) y_ = (Y); \ (x_ < y_) ? x_ : y_; })

The ({ … }) notation produces a compound statement that acts as an expression. Its value is the value of its last statement. This permits us to define local variables and assign each argument to one. The local variables have underscores after their names to reduce the risk of conflict with an identifier of wider scope (it is impossible to avoid this entirely). Now each argument is evaluated exactly once.

If you do not wish to use GNU C extensions, the only solution is to be careful when using the macro min. For example, you can calculate the value of foo (z), save it in a variable, and use that variable in min:

#define min(X, Y) ((X) < (Y) ? (X) : (Y)) … { int tem = foo (z); next = min (x + y, tem); }

(where we assume that foo returns type int).

3.10.5. Self-Referential Macros

A self-referential macro is one whose name appears in its definition. Recall that all macro definitions are rescanned for more macros to replace. If the self-reference were considered a use of the macro, it would produce an infinitely large expansion. To prevent this, the self-reference is not considered a macro call. It is passed into the preprocessor output unchanged. Consider an example:

#define foo (4 + foo)

where foo is also a variable in your program.

Following the ordinary rules, each reference to foo will expand into (4 + foo); then this will be rescanned and will expand into (4 + (4 + foo)); and so on until the computer runs out of memory.

The self-reference rule cuts this process short after one step, at (4 + foo). Therefore, this macro definition has the possibly useful effect of causing the program to add 4 to the value of foo wherever foo is referred to.

In most cases, it is a bad idea to take advantage of this feature. A person reading the program who sees that foo is a variable will not expect that it is a macro as well. The reader will come across the identifier foo in the program and think its value should be that of the variable foo, whereas in fact the value is four greater.

One common, useful use of self-reference is to create a macro which expands to itself. If you write

#define EPERM EPERM

then the macro EPERM expands to EPERM. Effectively, it is left alone by the preprocessor whenever it's used in running text. You can tell that it's a macro with #ifdef. You might do this if you want to define numeric constants with an enum, but have #ifdef be true for each constant.

If a macro x expands to use a macro y, and the expansion of y refers to the macro x, that is an indirect self-reference of x. x is not expanded in this case either. Thus, if we have

#define x (4 + y) #define y (2 * x)

then x and y expand as follows:

x ==> (4 + y) ==> (4 + (2 * x)) y ==> (2 * x) ==> (2 * (4 + y))

Each macro is expanded when it appears in the definition of the other macro, but not when it indirectly appears in its own definition.

3.10.6. Argument Prescan

Macro arguments are completely macro-expanded before they are substituted into a macro body, unless they are stringified or pasted with other tokens. After substitution, the entire macro body, including the substituted arguments, is scanned again for macros to be expanded. The result is that the arguments are scanned twice to expand macro calls in them.

Most of the time, this has no effect. If the argument contained any macro calls, they are expanded during the first scan. The result therefore contains no macro calls, so the second scan does not change it. If the argument were substituted as given, with no prescan, the single remaining scan would find the same macro calls and produce the same results.

You might expect the double scan to change the results when a self-referential macro is used in an argument of another macro (Section 3.10.5 Self-Referential Macros): the self-referential macro would be expanded once in the first scan, and a second time in the second scan. However, this is not what happens. The self-references that do not expand in the first scan are marked so that they will not expand in the second scan either.

You might wonder, "Why mention the prescan, if it makes no difference? And why not skip it and make the preprocessor faster?" The answer is that the prescan does make a difference in three special cases:

• Nested calls to a macro.

  We say that nested calls to a macro occur when a macro's argument contains a call to that very macro. For example, if f is a macro that expects one argument, f (f (1)) is a nested pair of calls to f. The desired expansion is made by expanding f (1) and substituting that into the definition of f. The prescan causes the expected result to happen. Without the prescan, f (1) itself would be substituted as an argument, and the inner use of f would appear during the main scan as an indirect self-reference and would not be expanded.

• Macros that call other macros that stringify or concatenate.

  If an argument is stringified or concatenated, the prescan does not occur. If you want to expand a macro, then stringify or concatenate its expansion, you can do that by causing one macro to call another macro that does the stringification or concatenation. For instance, if you have

  #define AFTERX(x) X_ ## x #define XAFTERX(x) AFTERX(x) #define TABLESIZE 1024 #define BUFSIZE TABLESIZE

  then AFTERX(BUFSIZE) expands to X_BUFSIZE, and XAFTERX(BUFSIZE) expands to X_1024. (Not to X_TABLESIZE. Prescan always does a complete expansion.)

• Macros used in arguments, whose expansions contain unshielded commas.

  This can cause a macro expanded on the second scan to be called with the wrong number of arguments. Here is an example:

  #define foo a,b #define bar(x) lose(x) #define lose(x) (1 + (x))

  We would like bar(foo) to turn into (1 + (foo)), which would then turn into (1 + (a,b)). Instead, bar(foo) expands into lose(a,b), and you get an error because lose requires a single argument. In this case, the problem is easily solved by the same parentheses that ought to be used to prevent misnesting of arithmetic operations:

  #define foo (a,b) or #define bar(x) lose((x))

  The extra pair of parentheses prevents the comma in foo's definition from being interpreted as an argument separator.

3.10.7. Newlines in Arguments

The invocation of a function-like macro can extend over many logical lines. However, in the present implementation, the entire expansion comes out on one line. Thus line numbers emitted by the compiler or debugger refer to the line the invocation started on, which might be different to the line containing the argument causing the problem.

Here is an example illustrating this:

#define ignore_second_arg(a,b,c) a; c ignore_second_arg (foo (), ignored (), syntax error);

The syntax error triggered by the tokens syntax error results in an error message citing line three--the line of ignore_second_arg-- even though the problematic code comes from line five.

We consider this a bug, and intend to fix it in the near future.