ctraps.pdf

C Traps and Pitfalls*

Andrew Koenig

AT&T Bell Laboratories

Murray Hill, New Jersey 07974

ABSTRACT

The C language is like a carving knife: simple, sharp, and extremely useful in

skilled hands. Like any sharp tool, C can injure people who don’t know how to handle it.

This paper shows some of the ways C can injure the unwary, and how to avoid injury.

0. Introduction

The C language and its typical implementations are designed to be used easily by experts. The lan-

guage is terse and expressive. There are few restrictions to keep the user from blundering. A user who has

blundered is often rewarded by an effect that is not obviously related to the cause.

In this paper, we will look at some of these unexpected rewards. Because they are unexpected, it

may well be impossible to classify them completely. Nevertheless, we have made a rough effort to do so

by looking at what has to happen in order to run a C program. We assume the reader has at least a passing

acquaintance with the C language.

Section 1 looks at problems that occur while the program is being broken into tokens. Section 2 fol-

lows the program as the compiler groups its tokens into declarations, expressions, and statements. Section

3 recognizes that a C program is often made out of several parts that are compiled separately and bound

together. Section 4 deals with misconceptions of meaning: things that happen while the program is actually

running. Section 5 examines the relationship between our programs and the library routines they use. In

section 6 we note that the program we write is not really the program we run; the preprocessor has gotten at

it first. Finally, section 7 discusses portability problems: reasons a program might run on one implementa-

tion and not another.

1. Lexical Pitfalls

The first part of a compiler is usually called a lexical analyzer . This looks at the sequence of charac-

ters that make up the program and breaks them up into tokens . A token is a sequence of one or more char-

acters that have a (relatively) uniform meaning in the language being compiled. In C, for instance, the

token -> has a meaning that is quite distinct from that of either of the characters that make it up, and that is

independent of the context in which the -> appears.

For another example, consider the statement:

if (x > big) big = x;

Each non-blank character in this statement is a separate token, except for the keyword if and the two

instances of the identifier big .

In fact, C programs are broken into tokens twice. First the preprocessor reads the program. It must

tokenize the program so that it can find the identifiers, some of which may represent macros. It must then

replace each macro invocation by the result of evaluating that macro. Finally, the result of the macro

replacement is reassembled into a character stream which is given to the compiler proper. The compiler

then breaks the stream into tokens a second time.

__________________

* This paper, greatly expanded, is the basis for the book C Traps and Pitfalls (Addison-Wesley, 1989, ISBN

0–201–17928–8); interested readers may wish to refer there as well.

In this section, we will explore some common misunderstandings about the meanings of tokens and

the relationship between tokens and the characters that make them up. We will talk about the preprocessor

later.

1.1. = is not ==

Programming languages derived from Algol, such as Pascal and Ada, use := for assignment and =

for comparison. C, on the other hand, uses = for assignment and == for comparison. This is because

assignment is more frequent than comparison, so the more common meaning is given to the shorter symbol.

Moreover, C treats assignment as an operator, so that multiple assignments (such as a=b=c ) can be

written easily and assignments can be embedded in larger expressions.

This convenience causes a potential problem: one can inadvertently write an assignment where one

intended a comparison. Thus, this statement, which looks like it is checking whether x is equal to y :

if (x = y)

foo();

actually sets x to the value of y and then checks whether that value is nonzero. Or consider the following

loop that is intended to skip blanks, tabs, and newlines in a file:

while (c == ’ ’ || c = ’\t’ || c == ’\n’)

c = getc (f);

The programmer mistakenly used = instead of == in the comparison with ’\t’ . This ‘‘comparison’’ actu-

ally assigns ’\t’ to c and compares the (new) value of c to zero. Since ’\t’ is not zero, the ‘‘compari-

son’’ will always be true, so the loop will eat the entire file. What it does after that depends on whether the

particular implementation allows a program to keep reading after it has reached end of file. If it does, the

loop will run forever.

Some C compilers try to help the user by giving a warning message for conditions of the form e1 =

e2 . To avoid warning messages from such compilers, when you want to assign a value to a variable and

then check whether the variable is zero, consider making the comparison explicit. In other words, instead

of:

if (x = y)

foo();

write:

if ((x = y) != 0)

foo();

This will also help make your intentions plain.

1.2. & and | are not && or ||

It is easy to miss an inadvertent substitution of = for == because so many other languages use = for

comparison. It is also easy to interchange & and && , or | and || , especially because the & and | operators

in C are different from their counterparts in some other languages. We will look at these operators more

closely in section 4.

1.3. Multi-character Tokens

Some C tokens, such as / , * , and = , are only one character long. Other C tokens, such as /* and == ,

and identifiers, are several characters long. When the C compiler encounters a / followed by an * , it must

be able to decide whether to treat these two characters as two separate tokens or as one single token. The C

reference manual tells how to decide: ‘‘If the input stream has been parsed into tokens up to a given charac-

ter, the next token is taken to include the longest string of characters which could possibly constitute a

token.’’ Thus, if a / is the first character of a token, and the / is immediately followed by a * , the two

characters begin a comment, regardless of any other context.

The following statement looks like it sets y to the value of x divided by the value pointed to by p :

y = x/*p /* p points at the divisor */;

In fact, /* begins a comment, so the compiler will simply gobble up the program text until the */ appears.

In other words, the statement just sets y to the value of x and doesn’t even look at p . Rewriting this state-

ment as

y = x / *p /* p points at the divisor */;

or even

y = x/(*p) /* p points at the divisor */;

would cause it to do the division the comment suggests.

This sort of near-ambiguity can cause trouble in other contexts. For example, older versions of C use

=+ to mean what present versions mean by += . Such a compiler will treat

a=-1;

as meaning the same thing as

a =- 1;

a = a - 1;

This will surprise a programmer who intended

a = -1;

On the other hand, compilers for these older versions of C would interpret

a=/*b;

a =/ * b ;

even though the /* looks like a comment.

1.4. Exceptions

Compound assignment operators such as += are really multiple tokens. Thus,

a + /* strange */ = 1

means the same as

a += 1

These operators are the only cases in which things that look like single tokens are really multiple tokens. In

particular,

p - > a

is illegal. It is not a synonym for

p -> a

As another example, the >> operator is a single token, so >>= is made up of two tokens, not three.

On the other hand, those older compilers that still accept =+ as a synonym for += treat =+ as a single

token.

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1.5. Strings and Characters

Single and double quotes mean very different things in C, and there are some contexts in which con-

fusing them will result in surprises rather than error messages.

A character enclosed in single quotes is just another way of writing an integer. The integer is the one

that corresponds to the given character in the implementation’s collating sequence. Thus, in an ASCII

implementation, ’a’ means exactly the same thing as 0141 or 97. A string enclosed in double quotes, on

the other hand, is a short-hand way of writing a pointer to a nameless array that has been initialized with the

characters between the quotes and an extra character whose binary value is zero.

The following two program fragments are equivalent:

printf ("Hello world\n");

char hello[] = {’H’, ’e’, ’l’, ’l’, ’o’, ’ ’,

’w’, ’o’, ’r’, ’l’, ’d’, ’\n’, 0};

printf (hello);

Using a pointer instead of an integer (or vice versa) will often cause a warning message, so using

double quotes instead of single quotes (or vice versa) is usually caught. The major exception is in function

calls, where most compilers do not check argument types. Thus, saying

printf(’\n’);

instead of

printf ("\n");

will usually result in a surprise at run time.

Because an integer is usually large enough to hold several characters, some C compilers permit mul-

tiple characters in a character constant. This means that writing ’yes’ instead of "yes" may well go

undetected. The latter means ‘‘the address of the first of four consecutive memory locations containing y ,

e , s , and a null character, respectively.’’ The former means ‘‘an integer that is composed of the values of

the characters y , e , and s in some implementation-defined manner.’’ Any similarity between these two

quantities is purely coincidental.

2. Syntactic Pitfalls

To understand a C program, it is not enough to understand the tokens that make it up. One must also

understand how the tokens combine to form declarations, expressions, statements, and programs. While

these combinations are usually well-defined, the definitions are sometimes counter-intuitive or confusing.

In this section, we look at some syntactic constructions that are less than obvious.

2.1. Understanding Declarations

I once talked to someone who was writing a C program that was going to run stand-alone in a small

microprocessor. When this machine was switched on, the hardware would call the subroutine whose

address was stored in location 0.

In order to simulate turning power on, we had to devise a C statement that would call this subroutine

explicitly. After some thought, we came up with the following:

(*(void(*)())0)();

Expressions like these strike terror into the hearts of C programmers. They needn’t, though, because

they can usually be constructed quite easily with the help of a single, simple rule: declare it the way you use

it.

Every C variable declaration has two parts: a type and a list of stylized expressions that are expected

to evaluate to that type. The simplest such expression is a variable:

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float f, g;

indicates that the expressions f and g , when evaluated, will be of type float . Because the thing declared

is an expression, parentheses may be used freely:

float ((f));

means that ((f)) evaluates to a float and therefore, by inference, that f is also a float .

Similar logic applies to function and pointer types. For example,

float ff();

means that the expression ff() is a float , and therefore that ff is a function that returns a float .

Analogously,

float *pf;

means that *pf is a float and therefore that pf is a pointer to a float .

These forms combine in declarations the same way they do in expressions. Thus

float *g(), (*h)();

says that *g() and (*h)() are float expressions. Since () binds more tightly than * , *g() means

the same thing as *(g()) : g is a function that returns a pointer to a float , and h is a pointer to a func-

tion that returns a float .

Once we know how to declare a variable of a given type, it is easy to write a cast for that type: just

remove the variable name and the semicolon from the declaration and enclose the whole thing in parenthe-

ses. Thus, since

float *g();

declares g to be a function returning a pointer to a float , (float *()) is a cast to that type.

Armed with this knowledge, we are now prepared to tackle (*(void(*)())0)() . We can ana-

lyze this statement in two parts. First, suppose that we have a variable fp that contains a function pointer

and we want to call the function to which fp points. That is done this way:

(*fp)();

If fp is a pointer to a function, *fp is the function itself, so (*fp)() is the way to invoke it. The paren-

theses in (*fp) are essential because the expression would otherwise be interpreted as *(fp()) . We

have now reduced the problem to that of finding an appropriate expression to replace fp .

This problem is the second part of our analysis. If C could read our mind about types, we could

write:

(*0)();

This doesn’t work because the * operator insists on having a pointer as its operand. Furthermore, the

operand must be a pointer to a function so that the result of * can be called. Thus, we need to cast 0 into a

type loosely described as ‘‘pointer to function returning void.’’

If fp is a pointer to a function returning void, then (*fp)() is a void value, and its declaration

would look like this:

void (*fp)();

Thus, we could write:

void (*fp)();

(*fp)();

at the cost of declaring a dummy variable. But once we know how to declare the variable, we know how to

cast a constant to that type: just drop the name from the variable declaration. Thus, we cast 0 to a ‘‘pointer

to function returning void’’ by saying:

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