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Advanced UNIXAdvanced UNIX

Objectives of these slides:Objectives of these slides:– look at the features of the C preprocessorlook at the features of the C preprocessor

240-491 Special Topics in Comp. Eng. 1Semester 2, 2000-2001

11. The Preprocessor

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OverviewOverview

1.1. Preprocessor Features Preprocessor Features2.2. Including Files Including Files3.3. Symbolic Constants Symbolic Constants4.4. Macros Macros5.5. Conditional Compilation Conditional Compilation

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1. Preprocessor Features1. Preprocessor Features

The preprocessor is part of the compilerThe preprocessor is part of the compiler– it modifies the C program it modifies the C program beforebefore the program is the program is

compiledcompiled

The modifications involve text The modifications involve text substitutionsubstitution– text in the program is replaced by other texttext in the program is replaced by other text

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Substitution is actually based on replacing C Substitution is actually based on replacing C tokenstokens by other text. by other text.

Examples of tokens in a line of C codeExamples of tokens in a line of C code– the code:the code:

x = inc(foo);x = inc(foo);– the tokens:the tokens:x = inc ( foo ) ;x = inc ( foo ) ;

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1.1. Main Kinds of Substitutions1.1. Main Kinds of Substitutions

File InclusionFile Inclusion Symbolic ConstantsSymbolic Constants MacrosMacros Conditional CompilationConditional Compilation

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1.2. Seeing the Substitution1.2. Seeing the Substitution It is possible to execute only the preprocessor part of It is possible to execute only the preprocessor part of gccgcc: :

$ gcc $ gcc -E-E examp.c examp.c

This call to This call to gccgcc will output the modified version of will output the modified version of examp.cexamp.c

– can be used for checking that the right substitutions are can be used for checking that the right substitutions are occurringoccurring

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2. Including Files 2. Including Files (#include)(#include)

Some examples:Some examples:#include <stdio.h> /* from /usr/include */#include <stdio.h> /* from /usr/include */#include “mydefs.h” /* from current dir */#include “mydefs.h” /* from current dir */

#include <sys/file.h> /* from /usr/include/sys */#include <sys/file.h> /* from /usr/include/sys */

Each #include line in the program will bereplaced by the contents of the named header file.

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2.1. What’s in a header file?2.1. What’s in a header file? Headers are C code (so Headers are C code (so have a lookhave a look).). Typical code:Typical code:

– symbolic constant definitions (symbolic constant definitions (#define#define lines) lines)– typedeftypedef declarations declarations– externextern declarations (see later)declarations (see later)– function prototypesfunction prototypes

You will write your own header files when coding large, You will write your own header files when coding large, multiple file programs multiple file programs – see latersee later

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3. Symbolic Constants3. Symbolic Constants (#define) (#define) Some examples:Some examples:

#define TRUE 1#define TRUE 1#define FALSE 0#define FALSE 0#define SIZE 10#define SIZE 10#define MAX (SIZE*2 + 1)#define MAX (SIZE*2 + 1)

Common mistake – adding a ‘Common mistake – adding a ‘;;’’# define SIZE 10;# define SIZE 10;while (x < SIZE) ...while (x < SIZE) ...

becomesbecomeswhile (x < 10while (x < 10;;) ...) ...

The preprocessor replaces SIZEby 10;

a constantdefined usinganother constant

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4. Macros4. Macros Some examples:Some examples:

#define sum(x, y)#define sum(x, y) ((x) + (y)) ((x) + (y))#define cube(x)#define cube(x) ((x) * (x) * (x)) ((x) * (x) * (x))#define print(str)#define print(str) { printf(“%s\n”, { printf(“%s\n”, str); }str); }

Use:Use: Translation:Translation:c = sum(a, b);c = sum(a, b); c = ((a) + (b));c = ((a) + (b));y = cube(a);y = cube(a); y = ((a) * (a) * (a));y = ((a) * (a) * (a));print(argv[1]);print(argv[1]); { printf(“%s\n”, argv[1]); }{ printf(“%s\n”, argv[1]); }

the macro body

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4.1. Rules you 4.1. Rules you MustMust Follow Follow

1. Don’t place ‘;’s at the end of a macro body.1. Don’t place ‘;’s at the end of a macro body. 2. Bracket every parameter in the macro body.2. Bracket every parameter in the macro body. 3. Bracket the entire macro body.3. Bracket the entire macro body. 4. Avoid side-effects in macro calls.4. Avoid side-effects in macro calls. 5. Add braces around multiple statements in a 5. Add braces around multiple statements in a

macro body; have a ‘;’ after every statement.macro body; have a ‘;’ after every statement.

When in doubt: add some brackets

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4.2. Macros are 4.2. Macros are notnot Functions Functions

A macro ‘call’ is textually A macro ‘call’ is textually replacedreplaced by the by the macro body inside the preprocessor.macro body inside the preprocessor.

There is There is no overheadno overhead of a function call at run- of a function call at run-time.time.

Macro replacement causes the Macro replacement causes the sizesize of the of the program to increase.program to increase.

LotsLots of tricky errors are possible. of tricky errors are possible.

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4.3. Multi-line Macros4.3. Multi-line Macros

Big macros use ‘\’ to run over multiple lines:Big macros use ‘\’ to run over multiple lines:

#define swap_int(s1, s2) { int temp; \#define swap_int(s1, s2) { int temp; \ temp = s1; \ temp = s1; \ s1 = s2; \ s1 = s2; \ s2 = temp; } s2 = temp; }

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4.4. Some Common Errors4.4. Some Common Errors

1. Including space after the macro name.1. Including space after the macro name.

2. Missing parameter brackets.2. Missing parameter brackets.

3. Missing macro body brackets.3. Missing macro body brackets.

4. Including side effects in macros.4. Including side effects in macros.

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4.4.1. Space after macro name4.4.1. Space after macro name

Example:Example:#define abs (x) ((x)> 0) ? (x) : (-(x)))#define abs (x) ((x)> 0) ? (x) : (-(x)))/* WRONG *//* WRONG */

Use:Use:x = x = absabs(2)(2);;

Expansion:Expansion:x = x = (x) ((x)> 0) ? (x) : (-(x)))(x) ((x)> 0) ? (x) : (-(x)))(2);(2);

replaced by

space after abs

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4.4.2. Missing parameter brackets4.4.2. Missing parameter brackets Example:Example:

#define cube(x) #define cube(x) x * x * xx * x * x /* WRONG *//* WRONG */

Use:Use:y = cube(z + 1);y = cube(z + 1);

Expansion:Expansion: y = z + 1 * z + 1 * z + 1;y = z + 1 * z + 1 * z + 1;

same as:same as:y = z + (1 * z) + (1 * z) + 1;y = z + (1 * z) + (1 * z) + 1;

no bracketsaround the x's

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CorrectCorrect Version: Version:#define cube(x) #define cube(x) ((((xx)) * * ((xx)) * * ((xx))))

Use:Use:y = cube(z + 1);y = cube(z + 1);

Expansion:Expansion:y = (y = (((z + 1z + 1)) * * ((z + 1z + 1)) * * ((z + 1z + 1))););

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4.4.3. Missing macro body brackets4.4.3. Missing macro body brackets Example:Example:

#define inc(x)#define inc(x) (x) + 1(x) + 1 /* WRONG *//* WRONG */

Use:Use:p = 3 * inc(q);p = 3 * inc(q);

Expansion:Expansion: p = 3 * (q) + 1;p = 3 * (q) + 1;

same as:same as:p = (3 *(q)) + 1;p = (3 *(q)) + 1;

no brackets aroundthe entire macro body

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CorrectCorrect version: version:#define inc(x)#define inc(x) (((x) + 1(x) + 1))

Use:Use:p = 3 * inc(q);p = 3 * inc(q);

Expansion:Expansion:p = 3 * p = 3 * (((q) + 1(q) + 1));;

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4.4.4. Side effects4.4.4. Side effects Example:Example:

#define cube(x)#define cube(x) ((x) * (x) * (x))((x) * (x) * (x))

Use:Use:int a = 2;int a = 2;c = cube(a++);c = cube(a++);

Expansion:Expansion:int a = 2;int a = 2;c = ((a++) * (a++) * (a++))c = ((a++) * (a++) * (a++))

a's value isside-effectedthree times

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4.5. More Examples4.5. More Examples /* macros instead of functions *//* macros instead of functions */

#define max(x, y)#define max(x, y) ((x) > (y) ? (x) : (y))((x) > (y) ? (x) : (y))#define sign(x)#define sign(x) ((x) < 0 ? -1 : \((x) < 0 ? -1 : \

((x) > 0 ? 1 : 0))((x) > 0 ? 1 : 0))

/* macros for control flow *//* macros for control flow */#define forever#define forever for(;;)for(;;)#define repeat#define repeat do {do {#define until(x)#define until(x) } while (!(x))} while (!(x))

/* nicer names *//* nicer names */#define TRUE#define TRUE11#define FALSE#define FALSE 00

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to beused asa pair

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/* graphics macro *//* graphics macro */#define line(x1,y1,x2,y2) { moveto(x1, y1) ; \#define line(x1,y1,x2,y2) { moveto(x1, y1) ; \

lineto(x2, y2); } lineto(x2, y2); }

/* macro instead of function, with type info *//* macro instead of function, with type info */#define swap(x, y, TYPE)#define swap(x, y, TYPE){ TYPE temp; \{ TYPE temp; \

temp = x; \ temp = x; \ x = y; \ x = y; \ y = temp; } y = temp; }

/* hide pointers without using functions *//* hide pointers without using functions */#define LEFT(ptr)#define LEFT(ptr) (ptr->left)(ptr->left)#define RIGHT(ptr)#define RIGHT(ptr)(ptr->right)(ptr->right)#define DATA(ptr)#define DATA(ptr) (ptr->data)(ptr->data)

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4.6. Undefining4.6. Undefining

Cancel a definition:Cancel a definition:#undef name#undef name

– applies from this line in the text downwardsapplies from this line in the text downwards

Can redefine later:Can redefine later:#define name 10#define name 10

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5. Conditional Compilation5. Conditional Compilation

Specify which blocks of code to Specify which blocks of code to compilecompile– some parts of the program can be some parts of the program can be ignoredignored

by the compilerby the compiler

Used for debugging and portability.Used for debugging and portability.

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5.1. If Tests5.1. If Tests

General format:General format:#if#if expression expression

...... /* code to be included if /* code to be included if expression is true */ expression is true */

#endif#endif

Expressions are C-like, but can only refer to Expressions are C-like, but can only refer to symbolic constants and/or macros.symbolic constants and/or macros.

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ExamplesExamples#define MAX 9#define MAX 9 : :#if#if MAX < 10 MAX < 10

........ /* code to include *//* code to include */#endif#endif

#define X #define X 55#define Y#define Y 77#define DEBUG 2#define DEBUG 2 : :#if#if X < Y && DEBUG == 1 X < Y && DEBUG == 1

... ... /* code to include */ /* code to include */ #endif#endif

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Leaving Out CodeLeaving Out Code

Example:Example:##if if 00

...... /* lines of code to /* lines of code to be left out */ be left out */

#endif#endif

Useful for commenting out lines that Useful for commenting out lines that contain lots of commentscontain lots of comments– cannot nest comments in Ccannot nest comments in C

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5.2. Multi-way Branches5.2. Multi-way Branches

Format:Format:#if expr1#if expr1

......#elif#elif expr2 expr2

......#elif expr3#elif expr3

...... : :#else#else

......#endif#endif

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5.3. Definition Test5.3. Definition Test

#ifdef#ifdef name name Test if Test if namename defined defined#ifndef#ifndef name name Test if Test if namename notnot defined defined

Used for debugging:Used for debugging:#define DEBUG#define DEBUG : :#ifdef DEBUG#ifdef DEBUG printf(“Some debug message\n”): printf(“Some debug message\n”):#endif#endif

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Or:Or:#define DEBUG 1#define DEBUG 1 : :#if DEBUG == 1#if DEBUG == 1 printf(“Some debug message\n”); printf(“Some debug message\n”);#endif#endif

In this version, we must supply a value for In this version, we must supply a value for DEBUG.DEBUG.

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Prevent multiple includes:Prevent multiple includes:#ifndef#ifndef INCLUDED_TYPES INCLUDED_TYPES #define INCLUDE_TYPES #define INCLUDE_TYPES #include “types.h” #include “types.h”#endif#endif

Implement version differences:Implement version differences:#ifdef#ifdef UNIX UNIX char *version = “UNIX version”; char *version = “UNIX version”;#else#else char *version = “DOS version”; char *version = “DOS version”;#endif#endif

only include types.hif INCLUDE_TYPESisn't already defined

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5.4. Creating Symbolic Constants5.4. Creating Symbolic Constants

Can use Can use #define#define

oror Create (and initialise) on the command line:Create (and initialise) on the command line:

$ gcc $ gcc -DDEBUG-DDEBUG -o examp examp.c -o examp examp.c/* define DEBUG *//* define DEBUG */

$ gcc $ gcc -DMAX=2-DMAX=2 -o foo foo.c -o foo foo.c/* define MAX with value 2 *//* define MAX with value 2 */

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5.5. Error Reporting5.5. Error Reporting (#error) (#error)

Example:Example:#ifndef UNIX#ifndef UNIX #error#error UNIX version not chosen. UNIX version not chosen.#endif#endif

Or:Or:#if #if !defined!defined(UNIX)(UNIX) #error#error UNIX version not chosen UNIX version not chosen#endif#endif