csc 352 an introduction to the c preprocessor saumya debray dept. of computer science the university...
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CSc 352An Introduction to the C Preprocessor
Saumya DebrayDept. of Computer Science
The University of Arizona, [email protected]
The C preprocessor and its role
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cpp(C preprocessor)
cc1(C compiler)
sourceprogram
compiledcode
C compiler (e.g., gcc)
expandedcode
• expand some kinds of characters• discard whitespace and comments
– each comment is replaced with a single space
• process directives:– file inclusion (#include)– macro expansion (#define)– conditional compilation (#if, #ifdef, …)
text: Ch. 14
#include
• Specifies that the preprocessor should read in the contents of the specified file– usually used to read in type definitions, prototypes, etc.– proceeds recursively
• #includes in the included file are read in as well
• Two forms:– #include <filename>
• searches for filename from a predefined list of directories• the list can be extended via “gcc –I dir”
– #include “filename”• looks for filename specified as a relative or absolute path
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#include : Example
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a predefined include file that:• comes with the system• gives type declarations,
prototypes for library routines (printf)
where does it come from?– man 3 printf :
#include: cont’d
• We can also define our own header files:– a header file has file-extension ‘.h’– these header files typically contain “public” information
• type declarations• macros and other definitions • function prototypes
– often, the public information associated with a code file foo.c will be placed in a header file foo.h
– these header files are included by files that need that public information
#include “myheaderfile.h”
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Macros
• A macro is a symbol that is recognized by the preprocessor and replaced by the macro body
– Structure of simple macros:#define identifier replacement_list
– Examples:
#define BUFFERSZ 1024#define WORDLEN 64
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Using simple macros
• We just use the macro name in place of the value, e.g.:
#define BUFLEN 1024#define Pi 3.1416…char buffer[BUFLEN];…area = Pi * r * r;
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NOT: #define BUFLEN = 1024 #define Pi 3.1416;
Example 1
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Example 2
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we can “macroize” symbols selectively
Parameterized macros
• Macros can have parameters– these resemble functions in some ways:
• macro definition ~ formal parameters• macro use ~ actual arguments
– Form:#define macroName(arg1, …, argn) replacement_list
– Example:#define deref(ptr) *ptr#define MAX(x,y) x > y ? x : y
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no space here!(else preprocessor will assume we’re defining
a simple macro
Example
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Macros vs. functions
• Macros may be (slightly) faster– don’t incur the overhead of function call/return– however, the resulting code size is usually larger
• this can lead to loss of speed
• Macros are “generic”– parameters don’t have any associated type– arguments are not type-checked
• Macros may evaluate their arguments more than once– a function argument is only evaluated once per call
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Macros vs. Functions: Argument Evaluation
• Macros and functions may behave differently if an argument is referenced multiple times:– a function argument is evaluated once, before the call– a macro argument is evaluated each time it is encountered
in the macro body.
• Example:
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int dbl(x) { return x + x;}…u = 10; v = dbl(u++);printf(“u = %d, v = %d”, u, v);
prints: u = 11, v = 20
#define Dbl(x) x + x…u = 10; v = Dbl(u++);printf(“u = %d, v = %d”, u, v);
prints: u = 12, v = 21
Dbl(u++) expands to:u++ + u++
Properties of macros
• Macros may be nested– in definitions, e.g.:
#define Pi 3.1416#define Twice_Pi 2*Pi
– in uses, e.g.:#define double(x) x+x#define Pi 3.1416…if ( x > double(Pi) ) …
• Nested macros are expanded recursively
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Pitfalls of nested macros
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Oops!
What happened?
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textual replacement!
Avoiding the problem
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What happened
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Header Files
• Have a file extension “.h”• Contain shared definitions
– typedefs– macros– function prototypes
• referenced via “#include” directives
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Header files: example
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typedefs
• Allow us to define aliases for types• Syntax:
typedef old_type_name new_type_name;• new_type_name becomes an alias for old_type_name
• Example:– typedef int BasePay;– typedef struct node { int value; struct node *next; } node;
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Example
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defines “wcnode” as an alias for “struct wc”
we can use “wcnode” in place of“struct wc”
but not here, since “wcnode” has not yet
been defined
What if a file is #included multiple times?
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foo.h
bar1.h bar2.h
bar.c
Example of multiple inclusions
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Problems with multiple inclusions
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Solution to multiple inclusion problem
• Use conditional compilation to ensure that a header file is “really included” at most once– header file’s responsibility to protect itself from multiple-
inclusion problems– uses a conditional-compilation directive #ifndef– in effect sets a flag when a file is included so we don’t
include it again– relies on convention
• we need to understand it so we don’t break it
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Conditional Compilation: #ifdef
#ifdef identifierline1
…
linen
#endif• macros can be defined by the compiler:
– gcc –D macroName– gcc –D macroName=definition
• macros can be defined without giving them a specific value, e.g.:– #define macroName
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line1 … linen will be included if identifier has been defined as a macro; otherwise nothing will
happen.
Conditional Compilation: #ifndef
#ifndef identifierline1…linen
#endif
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line1 … linen will be included if identifier is NOT defined as a macro; otherwise
nothing will happen.
Solution to multiple inclusion problem
The header file is written as follows:
#ifndef file_specific_flag#define file_specific_flag…contents of file…#endif
• file_specific_flag usually constructed from the name of the header file:
E.g.: file = foo.h flag = _FOO_H_– try to avoid macro names starting with ‘_’
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indicates whether or not this file has been
included already
Another use of #ifdefs
• They can be useful for controlling debugging output– Example 1: guard debugging code with #ifdefs:
#ifdef DEBUG…debug message…#endif
– Example 2: use the debug macro to control what debugging code appears in the program:
#ifdef DEBUG#define DMSG(msg) printf(msg) // debugging output#else#define DMSG(msg) {} // empty statement#endif
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straightforward, but needs discipline to use consistently
Example 1(a)
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Example 1(b)
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to address the “too many arguments to macro” problem
“too many arguments” issue resolved, but the macro
expansion isn’t working quite the way we want
Example 1(c)
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Generalizing #ifdef
#if constant-expressionline1
…linen
#endif
line1 … linen included if constant-expression evaluates to a non-zero value
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Common uses:• #if 1or• #if 0