C Traps and Pitfalls
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 language
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 follows
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 implementation
and not another.
1. Lexical Pitfalls
The first part of a compiler is usually called a lexical analyzer. This looks at the sequence of characters
that make up the program and breaks them up into tokens. A token is a sequence of one or more characters
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’’ actually
assigns ’\t’ to c and compares the (new) value of c to zero. Since ’\t’ is not zero, the ‘‘comparison’’
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 character,
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 statement
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;
or
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;
as
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 confusing
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 multiple
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 function
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 parentheses.
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 analyze
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 parentheses
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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(void(*)())0
and we can now replace fp by (void(*)())0:
(*(void(*)())0)();
The semicolon on the end turns the expression into a statement.
At the time we tackled this problem, there was no such thing as a typedef declaration. Using it,
we could have solved the problem more clearly:
typedef void (*funcptr)();
(* (funcptr) 0)();
2.2. Operators Don’t Always Have the Precedence You Want
Suppose that the manifest constant FLAG is an integer with exactly one bit turned on in its binary
representation (in other words, a power of two), and you want to test whether the integer variable flags
has that bit turned on. The usual way to write this is:
if (flags & FLAG) ...
The meaning of this is plain to most C programmers: an if statement tests whether the expression in the
parentheses evaluates to 0 or not. It might be nice to make this test more explicit for documentation purposes:
if (flags & FLAG != 0) ...
The statement is now easier to understand. It is also wrong, because != binds more tightly than &, so the
interpretation is now:
if (flags & (FLAG != 0)) ...
This will work (by coincidence) if FLAG is 1 or 0 (!), but not for any other power of two.*
Suppose you have two integer variables, h and l, whose values are between 0 and 15 inclusive, and
you want to set r to an 8-bit value whose low-order bits are those of l and whose high-order bits are those
of h. The natural way to do this is to write:
r = h<<4 r =" h" r =" (h" r =" h" b ="="" z =" a" a =" b" b =" c;" a =" b;" c="getc(in)" c="getc(in))" t="BTYPE(pt1-">aty)==STRTY) || t==UNIONTY ){
This was intended to assign a value to t and then see if t is equal to STRTY or UNIONTY. The actual
- 6 -
effect is quite different.*
The precedence of the C logical operators comes about for historical reasons. B, the predecessor of
C, had logical operators that corresponded rougly to C’s & and | operators. Although they were defined to
act on bits, the compiler would treat them as && and || if they were in a conditional context. When the
two usages were split apart in C, it was deemed too dangerous to change the precedence much.**
2.3. Watch Those Semicolons!
An extra semicolon in a C program usually makes little difference: either it is a null statement, which
has no effect, or it elicits a diagnostic message from the compiler, which makes it easy to remove. One
important exception is after an if or while clause, which must be followed by exactly one statement.
Consider this example:
if (x[i] > big);
big = x[i];
The semicolon on the first line will not upset the compiler, but this program fragment means something
quite different from:
if (x[i] > big)
big = x[i];
The first one is equivalent to:
if (x[i] > big) { }
big = x[i];
which is, of course, equivalent to:
big = x[i];
(unless x, i, or big is a macro with side effects).
Another place that a semicolon can make a big difference is at the end of a declaration just before a
function definition. Consider the following fragment:
struct foo {
int x;
}
f()
{
. . .
}
There is a semicolon missing between the first } and the f that immediately follows it. The effect of this is
to declare that the function f returns a struct foo, which is defined as part of this declaration. If the
semicolon were present, f would be defined by default as returning an integer.†
2.4. The Switch Statement
C is unusual in that the cases in its switch statement can flow into each other. Consider, for example,
the following program fragments in C and Pascal:
__________________
* Thanks to Guy Harris for pointing this out to me.
** Dennis Ritchie and Steve Johnson both pointed this out to me.
† Thanks to an anonymous benefactor for this one.
- 7 -
switch (color) {
case 1: printf ("red");
break;
case 2: printf ("yellow");
break;
case 3: printf ("blue");
break;
}
case color of
1: write (’red’);
2: write (’yellow’);
3: write (’blue’)
end
Both these program fragments do the same thing: print red, yellow, or blue (without starting a
new line), depending on whether the variable color is 1, 2, or 3. The program fragments are exactly analogous,
with one exception: the Pascal program does not have any part that corresponds to the C break
statement. The reason for that is that case labels in C behave as true labels: control can flow unimpeded
right through a case label.
Looking at it another way, suppose the C fragment looked more like the Pascal fragment:
switch (color) {
case 1: printf ("red");
case 2: printf ("yellow");
case 3: printf ("blue");
}
and suppose further that color were equal to 2. Then, the program would print yellowblue, because
control would pass naturally from the second printf call to the statement after it.
This is both a strength and a weakness of C switch statements. It is a weakness because leaving
out a break statement is easy to do, and often gives rise to obscure program misbehavior. It is a strength
because by leaving out a break statement deliberately, one can readily express a control structure that is
inconvenient to implement otherwise. Specifically, in large switch statements, one often finds that the
processing for one of the cases reduces to some other case after a relatively small amount of special handling.
For example, consider a program that is an interpreter for some kind of imaginary machine. Such a
program might contain a switch statement to handle each of the various operation codes. On such a
machine, it is often true that a subtract operation is identical to an add operation after the sign of the second
operand has been inverted. Thus, it is nice to be able to write something like this:
case SUBTRACT:
opnd2 = -opnd2;
/* no break */
case ADD:
. . .
As another example, consider the part of a compiler that skips white space while looking for a token.
Here, one would want to treat spaces, tabs, and newlines identically except that a newline should cause a
line counter to be incremented:
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case ’\n’:
linecount++;
/* no break */
case ’\t’:
case ’ ’:
. . .
2.5. Calling Functions
Unlike some other programming languages, C requires a function call to have an argument list, even
if there are no arguments. Thus, if f is a function,
f();
is a statement that calls the function, but
f;
does nothing at all. More precisely, it evaluates the address of the function, but does not call it.*
2.6. The Dangling else Problem
We would be remiss in leaving any discussion of syntactic pitfalls without mentioning this one.
Although it is not unique to C, it has bitten C programmers with many years of experience.
Consider the following program fragment:
if (x == 0)
if (y == 0) error();
else {
z = x + y;
f (&z);
}
The programmer’s intention for this fragment is that there should be two main cases: x = 0 and x¹0.
In the first case, the fragment should do nothing at all unless y = 0, in which case it should call error. In
the second case, the program should set z = x + y and then call f with the address of z as its argument.
However, the program fragment actually does something quite different. The reason is the rule that
an else is always associated with the closest unmatched if. If we were to indent this fragment the way it
is actually executed, it would look like this:
if (x == 0) {
if (y == 0)
error();
else {
z = x + y;
f (&z);
}
}
In other words, nothing at all will happen if x¹0. To get the effect implied by the indentation of the original
example, write:
__________________
* Thanks to Richard Stevens for pointing this out.
- 9 -
if (x == 0) {
if (y == 0)
error();
} else {
z = x + y;
f (&z);
}
3. Linkage
A C program may consist of several parts that are compiled separately and then bound together by a
program usually called a linker, linkage editor, or loader. Because the compiler normally sees only one file
at a time, it cannot detect errors whose recognition would require knowledge of several source program
files at once.
In this section, we look at some errors of that type. Some C implementations, but not all, have a program
called lint that catches many of these errors. It is impossible to overemphasize the importance of
using such a program if it is available.
3.1. You Must Check External Types Yourself
Suppose you have a C program divided into two files. One file contains the declaration:
int n;
and the other contains the declaration:
long n;
This is not a valid C program, because the same external name is declared with two different types in the
two files. However, many implementations will not detect this error, because the compiler does not know
about the contents of either of the two files while it is compiling the other. Thus, the job of checking type
consistency can only be done by the linker (or some utility program like lint); if the operating system has a
linker that doesn’t know about data types, there is little the C compiler can do to force it.
What actually happens when this program is run? There are many possibilities:
1. The implementation is clever enough to detect the type clash. One would then expect to see a diagnostic
message explaining that the type of n was given differently in two different files.
2. You are using an implementation in which int and long are really the same type. This is typically
true of machines in which 32-bit arithmetic comes most naturally. In this case, your program will
probably work as if you had said long (or int) in both declarations. This would be a good example
of a program that works only by coincidence.
3. The two instances of n require different amounts of storage, but they happen to share storage in such
a way that the values assigned to one are valid for the other. This might happen, for example, if the
linker arranged for the int to share storage with the low-order part of the long. Whether or not this
happens is obviously machine- and system-dependent. This is an even better example of a program
that works only by coincidence.
4. The two instances of n share storage in such a way that assigning a value to one has the effect of
apparently assigning a different value to the other. In this case, the program will probably fail.
Another example of this sort of thing happens surprisingly often. One file of a program will contain
a declaration like:
char filename[] = "/etc/passwd";
and another will contain this declaration:
char *filename;
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Although arrays and pointers behave very similarly in some contexts, they are not the same. In the
first declaration, filename is the name of an array of characters. Although using the name will generate
a pointer to the first element of that array, that pointer is generated as needed and not actually kept around.
In the second declaration, filename is the name of a pointer. That pointer points wherever the
programmer makes it point. If the programmer doesn’t give it a value, it will have a zero (null) value by
default.
The two declarations of filename use storage in different ways; they cannot coexist.
One way to avoid type clashes of this sort is to use a tool like lint if it is available. In order to be able
to check for type clashes between separately compiled parts of a program, some program must be able to
see all the parts at once. The typical compiler does not do this, but lint does.
Another way to avoid these problems is to put external declarations into include files. That way,
the type of an external object only appears once.*
4. Semantic Pitfalls
A sentence can be perfectly spelled and written with impeccable grammar and still be meaningless.
In this section, we will look at ways of writing programs that look like they mean one thing but actually
mean something quite different.
We will also discuss contexts in which things that look reasonable on the surface actually give undefined
results. We will limit ourselves here to things that are not guaranteed to work on any C implementation.
We will leave those that might work on some implementations but not others until section 7, which
looks at portability problems.
4.1. Expression Evaluation Sequence
Some C operators always evaluate their operands in a known, specified order. Others don’t. Consider,
for instance, the following expression:
a < will="" be="" evaluated="" if="" a="" is="" indeed="" less="" than="" c="">
evaluated to determine the value of the whole expression. On the other hand, if a is greater than or equal to
b, then c
To evaluate a
machines, it may even evaluate them in parallel.
Only the four C operators &&, ||, ?:, and , specify an order of evaluation. && and || evaluate the
left operand first, and the right operand only if necessary. The ?: operator takes three operands: a?b:c
evaluates a first, and then evaluates either b or c, depending on the value of a. The , operator evaluates its
left operand and discards its value, then evaluates its right operand.†
All other C operators evaluate their operands in undefined order. In particular, the assignment operators
do not make any guarantees about evaluation order.
For this reason, the following way of copying the first n elements of array x to array y doesn’t work:
i = 0;
while (i < i =" 0;" i =" 0;" i =" 0;" i =" 0;" i="1;" s =" sqrt" s =" sqrt" i="0;" r =" malloc" r =" malloc" p = "xyz" q =" p;" p ="="">>1 cannot possibly be zero.
5. Library Functions
Every useful C program must use library functions, because there is no way of doing input or output
built into the language. In this section, we look at some cases where some widely-available library functions
behave in ways that the programmer might not expect.
5.1. Getc Returns an Integer
Consider the following program:
#include
main()
{
char c;
while ((c = getchar()) != EOF)
putchar (c);
}
This program looks like it should copy its standard input to its standard output. In fact, it doesn’t
quite do this.
The reason is that c is declared as a character rather than as an integer. This means that it is impossible
for c to hold every possible character as well as EOF.
Thus there are two possibilities. Either some legitimate input character will cause c to take on the
same value as EOF, or it will be impossible for c to have the value EOF at all. In the former case, the program
will stop copying in the middle of certain files. In the latter case, the program will go into an infinite
loop.
Actually, there is a third case: the program may work by coincidence. The C Reference Manual
defines the result of the expression
((c = getchar()) != EOF)
quite rigorously. Section 6.1 states:
When a longer integer is converted to a shorter or to a char, it is truncated on the left; excess bits
are simply discarded.
Section 7.14 states:
There are a number of assignment operators, all of which group right-to-left. All require an lvalue as
their left operand, and the type of an assignment expression is that of its left operand. The value is
the value stored in the left operand after the assignment has taken place.
The combined effect of these two sections is to require that the result of getchar be truncated to a character
value by discarding the high-order bits, and that this truncated value then be compared with EOF. As part of
this comparison, the value of c must be extended to an integer, either by padding on the left with zero bits
or by sign extension, as appropriate.
However, some compilers do not implement this expression correctly. They properly assign the
low-order bits of the value of getchar to c. However, instead of then comparing c to EOF, they compare the
entire value of getchar! A compiler that does this will make the sample program shown above appear to
work ‘‘correctly.’’
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5.2. Buffered Output and Memory Allocation
When a program produces output, how important is it that a human be able to see that output immediately?
It depends on the program.
For example, if the output is going to a terminal and is asking the person sitting at that terminal to
answer a question, it is crucial that the person see the output in order to be able to know what to type. On
the other hand, if the output is going to a file, from where it will eventually be sent to a line printer, it is
only important that all the output get there eventually.
It is often more expensive to arrange for output to appear immediately than it is to save it up for a
while and write it later on in a large chunk. For this reason, C implementations typically afford the programmer
some control over how much output is to be produced before it is actually written.
That control is often vested in a library function called setbuf. If buf is a character array of appropriate
size, then
setbuf (stdout, buf);
tells the I/O library that all output written to stdout should henceforth use buf as an output buffer, and that
output directed to stdout should not actually be written until buf becomes full or until the programmer
directs it to be written by calling fflush. The appropriate size for such a buffer is defined as BUFSIZ in
Thus, the following program illustrates the obvious way to use setbuf in a program that copies its
standard input to its standard output:
#include
main()
{
int c;
char buf[BUFSIZ];
setbuf (stdout, buf);
while ((c = getchar()) != EOF)
putchar (c);
}
Unfortunately, this program is wrong, for a subtle reason.
To see where the trouble lies, ask when the buffer is flushed for the last time. Answer: after the main
program has finished, as part of the cleaning up that the library does before handing control back to the
operating system. But by that time, the buffer has already been freed!
There are two ways to prevent this sort of trouble.
First, make the buffer static, either by declaring it explicitly as static:
static char buf[BUFSIZ];
or by moving the declaration outside the main program entirely.
Another possibility is to allocate the buffer dynamically and never free it:
char *malloc();
setbuf (stdout, malloc (BUFSIZ));
Note that in this latter case, it is unnecessary to check if malloc was successful, because if it fails it will
return a null pointer. A null pointer is an acceptable second argument to setbuf; it requests that stdout be
unbuffered. This will work slowly, but it will work.
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6. The Preprocessor
The programs we run are not the programs we write: they are first transformed by the C preprocessor.
The preprocessor gives us a way of abbreviating things that is important for two major reasons (and several
minor ones).
First, we may want to be able to change all instances of a particular quantity, such as the size of a
table, by changing one number and recompiling the program.*
Second, we may want to define things that appear to be functions but do not have the execution overhead
normally associated with a function call. For example, getchar and putchar are usually implemented
as macros to avoid having to call a function for each character of input or output.
6.1. Macros are not Functions
Because macros can be made to appear almost as if they were functions, programmers are sometimes
tempted to regard them as truly equivalent. Thus, one sees things like this:
#define max(a,b) ((a)>(b)?(a):(b))
Notice all the parentheses in the macro body. They defend against the possibility that a or b might be
expressions that contain operators of lower precedence than >.
The main problem, though, with defining things like max as macros is that an operand that is used
twice may be evaluated twice. Thus, in this example, if a is greater than b, a will be evaluated twice: once
during the comparison, and again to calculate the value yielded by max.
Not only can this be inefficient, it can also be wrong:
biggest = x[0];
i = 1;
while (i < biggest =" max" biggest =" ((biggest)">(x[i++])?(biggest):(x[i++]));
First, biggest is compared to x[i++]. Since i is 1 and x[1] is 3, the relation is false. As a side effect,
i is incremented to 2.
Because the relation is false, the value of x[i++] is now assigned to biggest. However, i is now
2, so the value assigned to biggest is the value of x[2], which is 1.
One way around these worries is to ensure that the arguments to the max macro do not have any side
effects:
biggest = x[0];
for (i = 1; i < biggest =" max"> in the Eighth Edition of the Unix system:
#define putc(x,p) (--(p)->_cnt>=0?(*(p)->_ptr++=(x)):_flsbuf(x,p))
The first argument to putc is a character to be written to a file; the second argument is a pointer to an
internal data structure that describes the file. Notice that the first argument, which could easily be something
like *z++, is carefully evaluated only once, even though it appears in two separate places in the
macro body, while the second argument is evaluated twice (in the macro body, x appears twice, but since
__________________
* The preprocessor also makes it easy to group such manifest constants together to make them easier to find.
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the two occurrences are on opposite sides of a : operator, exactly one of them will be evaluated in any single
instance of putc). Since it is unusual for the file argument to putc to have side effects, this rarely
causes trouble. Nevertheless, it is documented in the user’s manual: ‘‘Because it is implemented as a
macro, putc treats a stream argument with side effects improperly. In particular, putc(c,*f++) doesn’t
work sensibly.’’ Notice that putc(*c++,f) works fine in this implementation.
Some C implementations are less careful. For instance, not everyone handles putc(*c++,f) correctly.
As another example, consider the toupper function that appears in many C libraries. It translates a
lower-case letter to the corresponding upper-case letter while leaving other characters unchanged. If we
assume that all the lower-case letters and all the upper-case letters are contiguous (with a possible gap
between the cases), we get the following function:
toupper(c)
{
if (c >= ’a’ && c <= ’z’) c += ’A’ - ’a’; return c; } In most C implementations, the subroutine call overhead is much longer than the actual calculations, so the implementor is tempted to make it a macro: #define toupper(c) ((c)>=’a’ && (c)<=’z’? (c)+(’A’-’a’): (c)) This is indeed faster than the function in many cases. However, it will cause a surprise for anyone who tries to use toupper(*p++). Another thing to watch out for when using macros is that they may generate very large expressions indeed. For example, look again at the definition of max: #define max(a,b) ((a)>(b)?(a):(b))
Suppose we want to use this definition to find the largest of a, b, c, and d. If we write the obvious:
max(a,max(b,max(c,d)))
this expands to:
((a)>(((b)>(((c)>(d)?(c):(d)))?(b):(((c)>(d)?(c):(d)))))?
(a):(((b)>(((c)>(d)?(c):(d)))?(b):(((c)>(d)?(c):(d))))))
which is surprisingly large. We can make it a little less large by balancing the operands:
max(max(a,b),max(c,d))
which gives:
((((a)>(b)?(a):(b)))>(((c)>(d)?(c):(d)))?
(((a)>(b)?(a):(b))):(((c)>(d)?(c):(d))))
Somehow, though, it seems easier to write:
biggest = a;
if (biggest < biggest =" b;" biggest =" c;" biggest =" d;" p =" malloc" p ="="" q =" a" r =" a">0.
What relationships might we want to hold between a, b, p, and q?
1. Most important, we want q*b + r == a, because this is the relation that defines the remainder.
2. If we change the sign of a, we want that to change the sign of q, but not the absolute value.
3. We want to ensure that r>=0 and r
hash table, it is important to be able to know that it will always be a valid index.
These three properties are clearly desirable for integer division and remainder operations. Unfortunately,
they cannot all be true at once.
Consider 3/2, giving a quotient of 1 and a remainder of 1. This satisfies property 1. What should be
the value of - 3/2? Property 2 suggests that it should be - 1, but if that is so, the remainder must also be
- 1, which violates property 3. Alternatively, we can satisfy property 3 by making the remainder 1, in
which case property 1 demands that the quotient be - 2. This violates property 2.
Thus C, and any language that implements truncating integer division, must give up at least one of
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these three principles.
Most programming languages give up number 3, saying instead that the remainder has the same sign
as the dividend. This makes it possible to preserve properties 1 and 2. Most C implementations do this in
practice, also.
However, the C language definition only guarantees property 1, along with the property that ïrï<ïbï and that r³0 whenever a³0 and b > 0. This property is less restrictive than either property 2 or property 3,
and actually permits some rather strange implementations that would be unlikely to occur in practice (such
as an implementation that always truncates the quotient away from zero).
Despite its sometimes unwanted flexibility, the C definition is enough that we can usually make integer
division do what we want, provided that we know what we want. Suppose, for example, that we have a
number n that represents some function of the characters in an identifier, and we want to use division to
obtain a hash table entry h such that 0£h < h =" n"> - HASHSIZE, so we can write:
h = n % HASHSIZE;
if (h < c =" getchar())" c =" getchar())">= ’a’ && (c) <= ’z’? (c) + ’A’ - ’a’: (c)) #define tolower(c) ((c) >= ’A’ && (c) <= ’Z’? (c) + ’a’ - ’A’: (c)) but realized that this would cause c to be evaluated anywhere between one and three times for each call, which would play havoc with expressions like toupper(*p++). Instead, he decided to rewrite toupper and tolower as functions. Toupper now looked something like this: int toupper (c) int c; { if (c >= ’a’ && c <= ’z’) return c + ’A’ - ’a’; return c; } and tolower looked similar. This change had the advantage of robustness, at the cost of introducing function call overhead into each use of these functions. Our hero realized that some people might not be willing to pay the cost of this overhead, so he re-introduced the macros with new names: #define _toupper(c) ((c)+’A’-’a’) #define _tolower(c) ((c)+’a’-’A’) This gave users a choice of convenience or speed. There was just one problem in all this: the people at Berkeley never followed suit, nor did some other C implementors. This means that a program written on an AT&T system that uses toupper or tolower, and assumes that it will be able to pass an argument that is not a letter of the appropriate case, may stop working on some other C implementation. This sort of failure is very hard to trace for someone who does not know this bit of history. 7.8. Free First, then Reallocate Most C implementations provide users with three memory allocation functions called malloc, realloc, and free. Calling malloc(n) returns a pointer to n characters of newly-allocated memory that the programmer can use. Giving free a pointer to memory previously returned by malloc makes that memory available for re-use. Calling realloc with a pointer to an allocated area and a new size stretches or shrinks the memory to the new size, possibly copying it in the process. Or so one might think. The truth is actually somewhat more subtle. Here is an excerpt from the description of realloc that appears in the System V Interface Definition: - 25 - Realloc changes the size of the block pointed to by ptr to size bytes and returns a pointer to the (possibly moved) block. The contents will be unchanged up to the lesser of the new and old sizes. The Seventh Edition of the reference manual for the UNIX system contains a copy of the same paragraph. In addition, it contains a second paragraph describing realloc: Realloc also works if ptr points to a block freed since the last call of malloc, realloc, or calloc; thus sequences of free, malloc and realloc can exploit the search strategy of malloc to do storage compaction. Thus, the following is legal under the Seventh Edition: free (p); p = realloc (p, newsize); This idiosyncrasy remains in systems derived from the Seventh Edition: it is possible to free a storage area and then reallocate it. By implication, freeing memory on these systems is guaranteed not to change its contents until the next time memory is allocated. Thus, on these systems, one can free all the elements of a list by the following curious means: for (p = head; p != NULL; p = p->next)
free ((char *) p);
without worrying that the call to free might invalidate p->next.
Needless to say, this technique is not recommended, if only because not all C implementations preserve
memory long enough after it has been freed. However, the Seventh Edition manual leaves one thing
unstated: the original implementation of realloc actually required that the area given to it for reallocation be
free first. For this reason, there are many C programs floating around that free memory first and then reallocate
it, and this is something to watch out for when moving a C program to another implementation.
7.9. An Example of Portability Problems
Let’s take a look at a problem that has been solved many times by many people. The following program
takes two arguments: a long integer and a (pointer to a) function. It converts the integer to decimal
and calls the given function with each character of the decimal representation.
void
printnum (n, p)
long n;
void (*p)();
{
if (n < n =" -n;">= 10)
printnum (n/10, p);
(*p) (n % 10 + ’0’);
}
This program is fairly straightforward. First we check if n is negative; if so, we print a sign and
make n positive. Next, we test if n³10. If so, its decimal representation has two or more digits, so we call
printnum recursively to print all but the last digit. Finally, we print the last digit.
This program, for all its simplicity, has several portability problems. The first is the method it uses to
convert the low-order decimal digit of n to character form. Using n%10 to get the value of the low-order
digit is fine, but adding ’0’ to it to get the corresponding character representation is not. This addition
assumes that the machine collating sequence has all the digits in sequence with no gaps, so that ’0’+5 has
the same value as ’5’, and so on. This assumption, while true of the ASCII and EBCDIC character sets,
might not be true for some machines. The way to avoid that problem is to use a table:
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void
printnum (n, p)
long n;
void (*p)();
{
if (n < n =" -n;">= 10)
printnum (n/10, p);
(*p) ("0123456789"[n % 10]);
}
The next problem involves what happens if n < q =" n" r =" n"> 0) {
r -= 10;
q++;
}
if (n <= -10) printneg (q, p); (*p) ("0123456789"[-r]); } 8. This Space Available There are many ways for C programmers to go astray that have not been mentioned in this paper. If you find one, please contact the author. It may well be included, with an acknowledging footnote, in a future revision. References The C Programming Language (Kernighan and Ritchie, Prentice-Hall 1978) is the definitive work on C. It contains both an excellent tutorial, aimed at people who are already familiar with other high-level languages, and a reference manual that describes the entire language succinctly. While the language has expanded slightly since 1978, this book is still the last word on most subjects. This book also contains the ‘‘C Reference Manual’’ we have mentioned several times in this paper. The C Puzzle Book (Feuer, Prentice-Hall, 1982) is an unusual way to hone one’s syntactic skills. The book is a collection of puzzles (and answers) whose solutions test the reader’s knowledge of C’s fine points. C: A Reference Manual (Harbison and Steele, Prentice Hall 1984) is mostly intended as a reference source for implementors. Other users may also find it useful, particularly because of its meticulous cross references.
