The Preprocessor in C

Advanced Programming in C Unit 5

Unit 5 The Preprocessor
Structure
5.0 Introduction
5.1 File Inclusion
Self Assessment Questions
5.2 Macro Definition and Substitution
5.2.1 Macros with Arguments
5.2.2 Nesting of Macros
Self Assessment Questions
5.3 Conditional Compilation
Self Assessment Questions
5.4 Summary
5.5 Terminal Questions
5.6 Answers to Self Assessment Questions
5.7 Answers to Terminal Questions
5.8 Exercises
5.0 Introduction
Preprocessor is a unique feature of C language. The C preprocessor
provides several tools that are unavailable in other highlevel
languages.
Conceptually, the ``preprocessor'' is a translation phase that is applied to
your source code before the compiler proper gets its hands on it. (Once
upon a time, the preprocessor was a separate program, much as the
compiler and linker may still be separate programs today.) Generally, the
preprocessor performs textual substitutions on your source code, in three
sorts of ways:
· File inclusion: inserting the contents of another file into your source file,
as if you had typed it all in there.
· Macro substitution: replacing instances of one piece of text with another.
Advanced Programming in C Unit 5
Sikkim Manipal University Page No. 116
· Conditional compilation: Arranging that, depending on various
circumstances, certain parts of your source code are seen or not seen
by the compiler at all.
The next three sections will introduce these three preprocessing functions.
The syntax of the preprocessor is different from the syntax of the rest of C in
several respects. First of all, the preprocessor is ``line based.'' Each of the
preprocessor directives we're going to learn about (all of which begin with
the # character) must begin at the beginning of a line, and each ends at the
end of the line. (The rest of C treats line ends as just another whitespace
character, and doesn't care how your program text is arranged into lines.)
Secondly, the preprocessor does not know about the structure of Cabout
functions, statements, or expressions. It is possible to play strange tricks
with the preprocessor to turn something which does not look like C into C (or
vice versa). It's also possible to run into problems when a preprocessor
substitution does not do what you expected it to, because the preprocessor
does not respect the structure of C statements and expressions (but you
expected it to). For the simple uses of the preprocessor we'll be discussing,
you shouldn't have any of these problems, but you'll want to be careful
before doing anything tricky or outrageous with the preprocessor. (As it
happens, playing tricky and outrageous games with the preprocessor is
considered sporting in some circles, but it rapidly gets out of hand, and can
lead to bewilderingly impenetrable programs.)
Objectives
At the end of this unit you will be able to understand:
· What is a preprocessor
· How to include Files in the source program
· Macro definition and substitution
· Use of parentheses in macro definitions
Advanced Programming in C Unit 5
Sikkim Manipal University Page No. 117
5.1 File Inclusion
A line of the form
#include <filename.h>
or
#include "filename.h"
causes the contents of the file filename.h to be read, parsed, and compiled
at that point. (After filename.h is processed, compilation continues on the
line following the #include line.) For example, suppose you got tired of
retyping external function prototypes such as
extern int getline(char [], int);
at the top of each source file. You could instead place the prototype in a
header file, perhaps getline.h, and then simply place
#include "getline.h"
at the top of each source file where you called getline. (You might not find it
worthwhile to create an entire header file for a single function, but if you had
a package of several related function, it might be very useful to place all of
their declarations in one header file.) As we may have mentioned, that's
exactly what the Standard header files such as stdio.h are collections of
declarations (including external function prototype declarations) having to do
with various sets of Standard library functions. When you use #include to
read in a header file, you automatically get the prototypes and other
declarations it contains, and you should use header files, precisely so that
you will get the prototypes and other declarations they contain.
The difference between the <> and "" forms is where the preprocessor
searches for filename.h. As a general rule, it searches for files enclosed in
<> in central, standard directories, and it searches for files enclosed in "" in
the ``current directory,'' or the directory containing the source file that's
Advanced Programming in C Unit 5
Sikkim Manipal University Page No. 118
doing the including. Therefore, "" is usually used for header files you've
written, and <> is usually used for headers which are provided for you
(which someone else has written).
The extension ``.h'', by the way, simply stands for ``header,'' and reflects the
fact that #include directives usually sit at the top (head) of your source files,
and contain global declarations and definitions which you would otherwise
put there. (That extension is not mandatory, you can theoretically name your
own header files anything you wish, but .h is traditional, and recommended.)
As we've already begun to see, the reason for putting something in a header
file, and then using #include to pull that header file into several different
source files, is when the something (whatever it is) must be declared or
defined consistently in all of the source files. If, instead of using a header file,
you typed the something in to each of the source files directly, and the
something ever changed, you'd have to edit all those source files, and if you
missed one, your program could fail in subtle (or serious) ways due to the
mismatched declarations (i.e. due to the incompatibility between the new
declaration in one source file and the old one in a source file you forgot to
change). Placing common declarations and definitions into header files
means that if they ever change, they only have to be changed in one place,
which is a much more workable system.
What should you put in header files?
· External declarations of global variables and functions. We said that a
global variable must have exactly one defining instance, but that it can
have external declarations in many places. We said that it was a grave
error to issue an external declaration in one place saying that a variable
or function has one type, when the defining instance in some other place
actually defines it with another type. (If the two places are two source
files, separately compiled, the compiler will probably not even catch the
Advanced Programming in C Unit 5
Sikkim Manipal University Page No. 119
discrepancy.) If you put the external declarations in a header file,
however, and include the header wherever it's needed, the declarations
are virtually guaranteed to be consistent. It's a good idea to include the
header in the source file where the defining instance appears, too, so
that the compiler can check that the declaration and definition match.
(That is, if you ever change the type, you do still have to change it in two
places: in the source file where the defining instance occurs, and in the
header file where the external declaration appears. But at least you don't
have to change it in an arbitrary number of places, and, if you've set
things up correctly, the compiler can catch any remaining mistakes.)
· Preprocessor macro definitions (which we'll meet in the next section).
· Structure definitions
· Typedef declarations
However, there are a few things not to put in header files:
· Defining instances of global variables. If you put these in a header file,
and include the header file in more than one source file, the variable will
end up multi defined.
· Function bodies (which are also defining instances). You don't want to
put these in headers for the same reasonit's
likely that you'll end up
with multiple copies of the function and hence ``multiply defined'' errors.
People sometimes put commonlyused
functions in header files and then
use #include to bring them (once) into each program where they use that
function, or use #include to bring together the several source files
making up a program, but both of these are poor ideas. It's much better
to learn how to use your compiler or linker to combine together
separatelycompiled
object files.
Since header files typically contain only external declarations, and should
not contain function bodies, you have to understand just what does and
doesn't happen when you #include a header file. The header file may
Advanced Programming in C Unit 5
Sikkim Manipal University Page No. 120
provide the declarations for some functions, so that the compiler can
generate correct code when you call them (and so that it can make sure that
you're calling them correctly), but the header file does not give the compiler
the functions themselves. The actual functions will be combined into your
program at the end of compilation, by the part of the compiler called the
linker. The linker may have to get the functions out of libraries, or you may
have to tell the compiler/linker where to find them. In particular, if you are
trying to use a thirdparty
library containing some useful functions, the library
will often come with a header file describing those functions. Using the
library is therefore a twostep
process: you must #include the header in the
files where you call the library functions, and you must tell the linker to read
in the functions from the library itself.
Self Assessment Questions
i. # include is called ________
ii. State true or false:
Nesting of included files is not allowed.
iii. State true or false:
Defining instances of global variables is not recommended in the header
files.
5.2 Macro Definition and Substitution
A preprocessor line of the form
#define name text
defines a macro with the given name, having as its value the given
replacement text. After that (for the rest of the current source file), wherever
the preprocessor sees that name, it will replace it with the replacement text.
The name follows the same rules as ordinary identifiers (it can contain only
letters, digits, and underscores, and may not begin with a digit). Since
macros behave quite differently from normal variables (or functions), it is
Advanced Programming in C Unit 5
Sikkim Manipal University Page No. 121
customary to give them names which are all capital letters (or at least which
begin with a capital letter). The replacement text can be absolutely anythingit's
not restricted to numbers, or simple strings, or anything.
The most common use for macros is to propagate various constants around
and to make them more selfdocumenting.
We've been saying things like
char line[100];
...
getline(line, 100);
but this is neither readable nor reliable; it's not necessarily obvious what all
those 100's scattered around the program are, and if we ever decide that
100 is too small for the size of the array to hold lines, we'll have to
remember to change the number in two (or more) places. A much better
solution is to use a macro:
#define MAXLINE 100
char line[MAXLINE];
...
getline(line, MAXLINE);
Now, if we ever want to change the size, we only have to do it in one place,
and it's more obvious what the words MAXLINE sprinkled through the
program mean than the magic numbers 100 did.
Since the replacement text of a preprocessor macro can be anything, it can
also be an expression, although you have to realize that, as always, the text
is substituted (and perhaps evaluated) later. No evaluation is performed
when the macro is defined. For example, suppose that you write something
like
#define A 2
#define B 3
#define C A + B
Advanced Programming in C Unit 5
Sikkim Manipal University Page No. 122
(this is a pretty meaningless example, but the situation does come up in
practice). Then, later, suppose that you write
int x = C * 2;
If A, B, and C were ordinary variables, you'd expect x to end up with the
value 10. But let's see what happens.
The preprocessor always substitutes text for macros exactly as you have
written it. So it first substitutes the replacement text for the macro C,
resulting in
int x = A + B * 2;
Then it substitutes the macros A and B, resulting in
int x = 2 + 3 * 2;
Only when the preprocessor is done doing all this substituting does the
compiler get into the act. But when it evaluates that expression (using the
normal precedence of multiplication over addition), it ends up initializing x
with the value 8!
To guard against this sort of problem, it is always a good idea to include
explicit parentheses in the definitions of macros which contain expressions.
If we were to define the macro C as
#define C (A + B)
then the declaration of x would ultimately expand to
int x = (2 + 3) * 2;
and x would be initialized to 10, as we probably expected.
Notice that there does not have to be (and in fact there usually is not) a
semicolon at the end of a #define line. (This is just one of the ways that the
Advanced Programming in C Unit 5
Sikkim Manipal University Page No. 123
syntax of the preprocessor is different from the rest of C.) If you accidentally
type
#define MAXLINE 100; /* WRONG */
then when you later declare
char line[MAXLINE];
the preprocessor will expand it to
char line[100;]; /* WRONG */
which is a syntax error. This is what we mean when we say that the
preprocessor doesn't know much of anything about the syntax of Cin
this
last example, the value or replacement text for the macro MAXLINE was the
4 characters 1 0 0 ; , and that's exactly what the preprocessor substituted
(even though it didn't make any sense).
Simple macros like MAXLINE act sort of like little variables, whose values
are constant (or constant expressions). It's also possible to have macros
which look like little functions (that is, you invoke them with what looks like
function call syntax, and they expand to replacement text which is a function
of the actual arguments they are invoked with) but we won't be looking at
these yet.
5.2.1 Macros with arguments
The preprocessor permits us to define more complex and more useful form
of replacements. It takes the form:
#define identifier(f1, f2, …, fn) string
Notice that there is no space between the macro identifier and the left
parenthesis. The identifiers f1, f2, …, fn are the formal macro arguments
that are analogous to the formal arguments in a function definition.
There is a basic difference between the simple replacement discussed
above and the replacement of macros with arguments. Subsequent
occurrence of a macro with arguments is known as a macro call. When a
Advanced Programming in C Unit 5
Sikkim Manipal University Page No. 124
macro is called, the preprocessor substitutes the string, replacing the formal
parameters with actual parameters.
A simple example of a macro with arguments is
#define CUBE(x) (x*x*x)
If the following statement appears later in the program
volume=CUBE(side);
then the preprocessor would expand this statement to:
volume=(side*side*side);
Consider the following statement:
volume=CUBE(a+b);
This would expand to:
volume=(a+b*a+b*a+b);
which would obviously not produce the correct results. This is because the
preprocessor performs a blind text substitution of the argument a+b in place
of x. This shortcoming can be corrected by using parentheses for each
occurrence of a formal argument in the string.
Example:
#define CUBE(x) ((x)*(x)*(x))
This would result in correct expansion of CUBE(a+b) as shown below:
volume=((a+b)*(a+b)*(a+b));
Some commonly used definitions are:
#define MAX(a,b) (((a) > (b))?(a):(b))
#define MIN(a,b) (((a) <(b))?(a):(b))
#define ABS(x) (((x) > 0)?(x):((
x)))
5.2.2 Nesting of Macros
We can also use one macro in the definition of another macro. That is,
macro definitions may be nested. For instance, consider the following macro
definitions:
Advanced Programming in C Unit 5
Sikkim Manipal University Page No. 125
#define M 5
#define N M+1
#define SQUARE(x) ((x)*(x))
#define CUBE(x) (SQUARE(x)*(x))
#define SIXTH(x) (CUBE(x)*CUBE(x))
Self Assessment Questions
i. What is the role of preprocessor in macro substitution?
ii. State true or false:
Macros and functions are synonyms.
iii. State true or false:
Nesting of macros is not allowed.
5.3 Conditional Compilation
The last preprocessor directive we're going to look at is #ifdef. If you have
the sequence
#ifdef name
program text
#else
more program text
#endif
in your program, the code that gets compiled depends on whether a
preprocessor macro by that name is defined or not. If it is (that is, if there
has been a #define line for a macro called name), then ``program text'' is
compiled and ``more program text'' is ignored. If the macro is not defined,
``more program text'' is compiled and ``program text'' is ignored. This looks
a lot like an if statement, but it behaves completely differently: an if
statement controls which statements of your program are executed at run
time, but #ifdef controls which parts of your program actually get compiled.
Advanced Programming in C Unit 5
Sikkim Manipal University Page No. 126
Just as for the if statement, the #else in an #ifdef is optional. There is a
companion directive #ifndef, which compiles code if the macro is not defined
(although the ``#else clause'' of an #ifndef directive will then be compiled if
the macro is defined). There is also an #if directive which compiles code
depending on whether a compiletime
expression is true or false. (The
expressions which are allowed in an #if directive are somewhat restricted,
however, so we won't talk much about #if here.)
Conditional compilation is useful in two general classes of situations:
· You are trying to write a portable program, but the way you do
something is different depending on what compiler, operating system, or
computer you're using. You place different versions of your code, one for
each situation, between suitable #ifdef directives, and when you compile
the program in a particular environment, you arrange to have the macro
names defined which select the variants you need in that environment.
(For this reason, compilers usually have ways of letting you define
macros from the invocation command line or in a configuration file, and
many also predefine certain macro names related to the operating
system, processor, or compiler in use. That way, you don't have to
change the code to change the #define lines each time you compile it in
a different environment.)
For example, in ANSI C, the function to delete a file is remove. On older
Unix systems, however, the function was called unlink. So if filename is
a variable containing the name of a file you want to delete, and if you
want to be able to compile the program under these older Unix systems,
you might write
#ifdef unix
unlink(filename);
#else
remove(filename);
Advanced Programming in C Unit 5
Sikkim Manipal University Page No. 127
#endif
Then, you could place the line
#define unix
at the top of the file when compiling under an old Unix system. (Since all
you're using the macro unix for is to control the #ifdef, you don't need to
give it any replacement text at all. Any definition for a macro, even if the
replacement text is empty, causes an #ifdef to succeed.)
(In fact, in this example, you wouldn't even need to define the macro
unix at all, because C compilers on old Unix systems tend to predefine it
for you, precisely so you can make tests like these.)
· You want to compile several different versions of your program, with
different features present in the different versions. You bracket the code
for each feature with #ifdef directives, and (as for the previous case)
arrange to have the right macros defined or not to build the version you
want to build at any given time. This way, you can build the several
different versions from the same source code. (One common example is
whether you turn debugging statements on or off. You can bracket each
debugging printout with #ifdef DEBUG and #endif, and then turn on
debugging only when you need it.)
For example, you might use lines like this:
#ifdef DEBUG
printf("x is %d\n", x);
#endif
to print out the value of the variable x at some point in your program to
see if it's what you expect. To enable debugging printouts, you insert the
line
#define DEBUG
at the top of the file, and to turn them off, you delete that line, but the
debugging printouts quietly remain in your code, temporarily deactivated,
Advanced Programming in C Unit 5
Sikkim Manipal University Page No. 128
but ready to reactivate if you find yourself needing them again later.
(Also, instead of inserting and deleting the #define line, you might use a
compiler flag such as DDEBUG
to define the macro DEBUG from the
compiler invocating line.)
Conditional compilation can be very handy, but it can also get out of
hand. When large chunks of the program are completely different
depending on, say, what operating system the program is being
compiled for, it's often better to place the different versions in separate
source files, and then only use one of the files (corresponding to one of
the versions) to build the program on any given system. Also, if you are
using an ANSI Standard compiler and you are writing ANSIcompatible
code, you usually won't need so much conditional compilation, because
the Standard specifies exactly how the compiler must do certain things,
and exactly which library functions it much provide, so you don't have to
work so hard to accommodate the old variations among compilers and
libraries.
5.4 Summary
Preprocessor is a unique feature of C language. The C preprocessor
provides several tools that are unavailable in other highlevel
languages.
The preprocessor is a program that processes the source code before it
passes through the compiler. Macro substitution is a process where an
identifier in a program is replaced by a predefined text. The preprocessor
permits us to define more complex and more useful form of replacements
with macros with arguments. Nesting of macros is also allowed to simplify
the complex macro definitions. An external file containing functions or macro
definitions can be included as a part of a program.
Advanced Programming in C Unit 5
Sikkim Manipal University Page No. 129
5.5 Terminal Questions
1. What is the difference between a macro call and a function call?
2. List some of the Compiler Control preprocessor directives.
3. Distinguish between the following macro definitions:
#include “filename” and #include <filename>
4. What is the preprocessor directive used to undefine a macro?
5. Write a macro definition to find the square of a number.
5.6 Answers to Self Assessment Questions
5.1 i. Preprocessor directive
ii. False
iii. True
5.2 i. Text substitution according to the macro definition.
ii. False
iii. False
5.7 Answers to Terminal Questions
1. When a macro is called, the macro definition with string substitution will
be inserted in the place of call in the source program whereas in a
function call the execution control will transfer to the function.
2. #ifdef, #ifndef, #if, #else
3. Preprocessor searches for files enclosed in <> in central, standard
directories, and it searches for files enclosed in "" in the current directory.
4. #undef
5. #define SQUARE(x) ((x)*(x))
Advanced Programming in C Unit 5
Sikkim Manipal University Page No. 130
5.8 Exercises
1. Write a macro definition to find the largest of three numbers.
2. Write a macro definition to compare two numbers.
3. What are the advantages of using macro definitions in a program?
4. Define a macro, PRINT_VALUE which can be used to print two values of
arbitrary type.
5. Identify errors, if any, in the following macro definition.
#define ABS(x) (x>0)?(x):(x)