Preprocessing, Compiling, Assembling, and Linking Introduction In this lesson will examine Architecture of C program Introduce C preprocessor and preprocessor directives How to use preprocessor s directives to manage program Have examined process Creating program Have developed program Written in C Source code Next step Translate into something computer can use Called object code Things to think about along the way How to accommodate different Versions Called localization Features Targets - Machines Operating Systems The First Step Model the process Examine at three levels Each with increasing detail Start with top level Begin with source file End with object or machine code Also called object file or machine code file Machine code will be unique to specific computer or microprocessor Transformation from source to object Called compilation or compiling Top Level - 1 of 22 -
Level 1 Level 2: The Pieces Let s begin with preprocessor Preprocessor Simple and handy tool Its job is to process C source code Before compiler Reads source program - 2 of 22 -
Translates it into machine code Question might be Why do we have to do this Overview Many of useful features and capabilities of C Not implemented by compiler Rather Selected by user Brought in on demand by preprocessor When program written User includes various directives to preprocessor Preprocessor Reads source file Interprets directives Effects operations specified by directives directives tell the preprocessor Which library files to include Which user written files to include Which portions of the program to include or exclude We may want slightly different versions of program For different applications May want to conditionally include debug code Specify certain constant identifiers Called symbolic constants Make reading and managing program easier Structure From above discussion we see Preprocessor has Input C source code Containing embedded preprocessor directives Output Preprocessed C source file Input to compiler Implementation Separate Program - 3 of 22 -
Reads original C source file Looks for lines beginning with # symbol Evaluates each such line Writes out C source to compiler Based upon directives included in line Single Program Performs Preprocessing Compilation In single pass Preprocessor Language Preprocessor language specified as set of directives Directives typically begin in column 1 (caution) of source file Depends upon version of preprocessor As it goes through source file line by line Preprocessor looks for lines beginning with the special character # Syntax Completely independent of the C language Number of directives Approximately 12 15 Shown in following table Preprocessor in Action Examine each line in program source Those that do not begin with # Viewed as source text These are ignored and sent directly to output Those that begin with # Expand Transform As directed by the command - 4 of 22 -
Assuming process runs correctly Must result in a C program Preprocessor does not correct User design errors Syntactic errors Grammatical errors Directive Definition #define #undef #include #if #ifdef #ifndef #elif #else #endif #line defined name defined(name) Define a preprocessor macro or symbolic constant Undefine or remove a preprocessor macro Insert contents of another source file Conditionally include contents of another source file Conditionally include contents of another source file if macro name is defined Conditionally include contents of another source file if macro name is not defined Conditionally include contents of another source file if macro name is defined and previous #if, #ifdef, #ifndef, or #elif failed Alternative action if preceding #if, #ifdef, #ifndef, or #elif directive fails Closes #if, #ifdef, #ifndef, or #elif construct Return line number for compiler message Directive that returns 1 if name is defined as preprocessor macro and 0 otherwise # operator Directive to replace macro parameter with string constant containing parameter s value ## operator Create single token from two adjacent tokens #pragma #error Specify proprietary information to the compiler Return a compile time error with associated message - 5 of 22 -
Lexical Conventions Line beginning with # Preprocessor command Name of command must follow # ISO C - International Standards Organization White space can precede or follow # on the same source line Older versions do not permit Line with only # ISO C Null directive Treated as blank line Older versions May be different Remainder of the line following the command May contain command args Args subject to macro replacement If no args required Remainder of line should be empty White space and comments allowed Often old compilers will ignore Preprocessor lines are recognized Before macro expansion Will talk about macros shortly If macro expands into something that looks like preprocessor directive Directive not recognized #define STRLIB #include<string.h> STRLIB 1. #define processed STRLIB is interpreted to mean #include<string.h> 2. STRLIB substitution executed Based upon the definition in previous line 3. Token sequence #include<string.h> passed to compiler as code - 6 of 22 -
The preprocessor recognizes the line continuation character Commands can extend to multiple lines with the \ character #define DOLLAR $ #define BACKSLASH \ #define MODULUS Results in 2 lines not 3 as might be expected Line 2 continued on to line 3 these interpreted as single line #define SWAP(a, b) { } a ^= b; \ b ^= a; \ a ^= b; \ Preprocessor Directives File Inclusion Directive #include Simplest preprocessor directive Has two forms syntax #include <filename> #include filename Either form Replaces the current line with Entire contents of the named file If complete path not given Search determined by form used < > Search in certain standard places System type places - 7 of 22 -
Determined by implementation Defined by search rules Specific location set at time of compiler installation First search some local places Current directory Second Certain standard places General intent < > Standard implementation files Programmer written files Included file May contain #include commands Number Implementation dependent ANSI C requires support for 8 minimum Error if included file cannot be found Third form of #include recognized syntax #include preprocessor tokens The tokens undergo normal macro expansion Result must match one of the first two forms #define COMMS G:/mySystem/include/comms.h #include COMMS Causes the preprocessor to look in directory and for the file specified G:/mySystem/include/comms.h Note: the forward slash / or back slash \ used to separate directories along a directory path depends upon operating system. Typically UNIX or LINUX derivatives use the forward slash and Windows derivatives use the back slash - 8 of 22 -
Macro Substitution Directives #define #undef syntax i. #define name text ii. #define name (arg 1, arg 2,... arg n,) text iii. #undef name #define i. The first form of #define directive Causes name To be defined as a macro to the preprocessor Instructs the preprocessor To replace all (unquoted) occurrences of name with text name Must be an identifier as defined by the C language U/L case letters... text Called the body of the macro Process called macro substitution Simple macros Common use Symbolic constants #define MAXSIZE 2048 #define PI 3.14 #define TWOPI 6.28 #define TWOPI (3.14*2.0) #define TWOPI (PI + PI) int myarray [MAXSIZE]; circumference = TWOPI * radius area = PI * pow(r,2); - 9 of 22 -
Macros with Parameters Second form of #define directive Declares a formal parameter list Parameter list Immediately follows macro name No intervening whitespace If whitespace Definition assumed to be macro with no args Enclosed in () Separated by commas Args in the parameter list Must be identifiers No two the same Need not be used in macro body Parameter list may be empty Using a Parameterized Macro Macro invoked Writing name Left parenthesis 1 actual arg for each formal parameter Separated by commas Right parenthesis If no formal parameters Must include empty arg list Whitespace may appear Between Name Left parenthesis Formal arg May contain Properly balanced parenthesis Commas If within set of parenthesis Braces and subscripting brackets Cannot contain commas Do not have to balance - 10 of 22 -
#define sum(x,y) ((x) + (y)) x = sum(2*a, b) / sum (c,d); x = sum(2 * g(a,b), h(a,b)) / sum (c,d); #define getmodem() getc(modemin) while ((c = getmodem())!= EOF) Can define a macro that takes arbitrary statement as its argument #define assign(anystatement) anystatement assign( {a = 1 ; b = 2;}) assign (c = 0; d = 1; e = 2;) #define max(a,b) ((A) > (B)? (A) : (B)) max (3, 4); max (6, 5); Potential problems Consider max(i++, j++); Appears to be simple use of max () Observe ((A) > (B)) replaced by ((i++) > (j++)) (A) : (B) replaced by ((i++) : (j++)) Potentially each variable is incremented twice #undef The #undef macro Companion to #define - 11 of 22 -
syntax #undef name Used to make name No longer defined Causes preprocessor to forget Macro definition of name Once name is undefined Can be given new definition Using #define Not an error To undefine a name that is not defined Macro expansion Not performed within #undef directive Conditional Compilation Directives #if #else, #elif #endif syntax #if constant-expression #else, #elif constant-expression #endif Conditional Compilation directives Based upon computed condition Allow lines of source code to be Passed through Eliminated Used to control the way the source code Assembled Compiled Semantics - 12 of 22 -
As expected #if constant-expression constant-expression Must evaluate to constant arithmetic value May include macro substitution if constant-expression non- zero Subsequent C code lines Intended to be included in program All C source lines Sent to preprocessor output Until #else, #elif, or #endif Expression encountered #else, #elif constant-expression #else Like familiar if - else If if previous conditions fail Lines follow #else are included #elif constant-expression Equivalent to else if Like if constant-expression evaluated Consequences are the same as #if #endif Closes the #if sequence Let the variable SYSTEM identify the host system LINUX OSX UNIX WIN7-13 of 22 -
Want different header file included depending upon system Each defines system specific information #if LINUX #define HDR linuxheader.h #elif OSX #define HDR osxheader.h #elif UNIX #define HDR unixheader.h #else #define HDR win7header.h Conditional Directives Directives #ifdef #ifndef syntax #ifdef name #ifndef name Conditional directives Test if an identifier Defined Not defined #ifdef Equivalent to if 1 if the identifier is defined if 0 if the identifier is not defined #ifndef Equivalent to if 0 if the identifier is defined if 1 if the identifier is not defined or undefined - 14 of 22 -
Want different debug code included depending upon system Conditionally include debug code Don t want to Include in the final version Take out For future upgrades Each defines system specific information #define LINUX 0 #define WIN7 0 #define UNIX 0 #define OSX 1 #if def LINUX #endif Linux debug code #if def WIN7 #endif Win 7 debug code #if def UNIX #endif Unix debug code #if def OSX #endif osx debug code Program Multiple files Several files share Common.h file May want to Debug separately Use for multiple targets Use for different programs In final build Will have multiple definitions for variable if.h file included multiple times - 15 of 22 -
May have added debug code to source For use during development Want to remove for release Bad style to individually comment out Each line of debug code Preprocessor can help preproc0.c #include <string.h> #include <stdio.h> #define DEBUG // commenting out this line will // prevent debug code from inclusion in final build int main() { char* mystring = "Hello"; #ifdef DEBUG #endif printf ("The string length is %d\n", strlen(mystring)); } return 0; Miscellaneous Directives Directives #line #error #pragma #line - 16 of 22 -
syntax #line line-number filename #line line-number If program built from Multiple other files Sometimes useful to annotate With line numbers from original file Instead of normal sequential numbering Info provided by #line directive Used to instantiate the LINE FILE LINE Line number of current source program Decimal integer constant FILE Name of current source file String constant preproc1.c #include <stdio.h> #include <string.h> int main() { char* mystring = "Hello"; #line 123 "myfile" printf ("This line is %d from %s\n", LINE, FILE ); printf ("The string length is %d\n", strlen(mystring)); return 0; } - 17 of 22 -
#error Used to write Compile time error message syntax #error error-message error-message is subject to macro expansion Typically used in conditionals Warn of inconsistencies Constraint violations Incomplete information preproc2.c #include <stdio.h> #include <string.h> #define SYSTEM Linux #ifndef SYSTEM #endif #error "You must specify the system type" int main() { } char* mystring = "Hello"; printf ("The string length is %d\n", strlen(mystring)); return 0; #pragma Used to Add new preprocessor or compiler functionality Provide implementation defined information to the compiler syntax #pragma tokens No restrictions on tokens Compilers should ignore what they do not understand - 18 of 22 -
args to directive Subject to macro expansion No agreement on standard pragmas #pragma pagesize (number of lines) MS Visual C++ Sets the number of lines desired per page of source listing #pragma pages (<pages>) Generate <pages> (formfeeds) in source listing Default value is 1 #pragma inline Compile with fast calling convention Typedef Names C provides facility for creating new data type names called typedef Typedef creates alias or synonym for existing type syntax typedef typename identifier After declaration identifier becomes synonym for typename Caution Typedef does not create a new type It is merely a synonym or alias for an existing type Cannot redefine the built-in meaning of a type typedef int double is illegal typedef int* INTPTR; INTPTR is not a pointer to an int It may be used where ever and int* can be used INTPTR myptr; myptr is now a pointer to an integer - 19 of 22 -
typedef Very useful in simplifying complicated declarations Thus Helps to simplify program Makes intent more obvious Use carefully Rather than clarify Overuse can serve to confuse The Compiler Compiler is a tool for translating programs Into variety of forms One such form Assembly language the instruction set for the machine As we saw in level 2 diagram above Top level program Can be made up of number of modules Module can be C source file Standard library file Defined as part of the language such as Math library String library Library that manages all input and output Custom library Under the Hood As program compiled Compiler has a lot of record keeping to do Translation Unit As compilation process proceeds Each.c or source file compiled individually Called translation unit Symbol Table As each source file compiled Table of identifiers of symbols within program created Called symbol table - 20 of 22 -
How compiler keeps track of All identifiers used Where in memory variables placed Allocate Memory Yes or No Each symbol name entered into symbol table Declaration brings name into name space No memory allocated Definition brings name into name space Sufficient memory allocated to hold variable If definition appears in different translation unit Identify as extern Want only single definition memory allocation For each variable or function body in system Prior to this stage Program did not depend upon machine Now program in form that will execute only on particular machine The Assembler Assembler is tool we use for converting Assembly language into machine language Program expressed as collection of 0 s and 1 s machine understands The Linker Although program now in machine language Not ready to be executed Problem All variables and data structures we use Must reside in computer memory Each needs an address in memory Question Which address should we use Unfortunately Cannot always use the same address What if someone else wants to use same address - 21 of 22 -
To solve problem assembler generates Relocatable code Code that can be placed anywhere in memory Second question arises at this time We d like to be able to use existing code Our own Other peoples How do we get this into our program without typing in each time Tool called linker loader can help with both problems Does two jobs 1. Links collection of program modules together 2. Resolves address problems Summary In this lesson examined Architecture of C program Introduced C preprocessor and preprocessor directives How to use preprocessors directives to manage program Should now be comfortable working with basic C preprocessor directives Know when and how to use Aware if tools compiler, assembler, and linker and role they play in building C program - 22 of 22 -