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The compilation process
When using a high level language compiler with an IBM PC or UNIX system, it is all too easy to forget all the stages that are encountered when source code is compiled into an execut-able file. Not only is a suitable compiler needed, but the appropriate run-time libraries and linking loader to combine all the modules are also required. The problem is that these may be well integrated for the native system, PC or work-station, but this may not be the case for a VMEbus system, where the hardware configuration may well be unique. Such cross-compilation methods, where software for another proc-essor or target is generated on a different machine, are attrac-tive if a suitable PC or workstation is available, but can require work to create the correct software environment. However, the popularity of this method, as opposed to the more traditional use of a dedicated development system, has increased dra-matically. It is now common for operating systems to support cross-compilation directly, rather than leaving the user to piece it all together.
Like many compilers, such as PASCAL or C, the high level language only generates a subset of its facilities and commands from built-in routines and relies on libraries to provide the full range of functions. These libraries use the simple commands to create well-known functions, such as printf and scanf from the C language, which print and interpret data. As a result, even a simple high level language program involves several stages and requires access to many special files.
The first stage involves pre-processing the source, where include files are added to it. These files define constants, standard functions and so on. The output of the pre-processor is fed into the compiler, where it produces an assembler file using the native instruction codes for the processor. This file may have references to other software files, called libraries. The assembler file is next assembled and converted into an object file.
This contains the hexadecimal coding for the instruc-tions, except that memory addresses and file references are not completed; these are resolved by the loader (sometimes known as a linker) that finally creates an executable file. The loader calculates all the memory addresses and takes software rou-tines from library files to supply the standard functions called by the program.
The pre-processor, as its name suggests, processes the source code before it goes through the compiler. It allows the programmer to define constants, variable types and other information. It also includes other files (include files) and combines them into the program source. These tasks can be conditionally performed, depending on the value of constants, and so on. The pre-processor is programmed using one of five basic commands which are inserted into the C source.
#define identifier string
This statement replaces all occurrences of identifier with string. The normal convention is to put the identifier in capital letters so it can easily be recognised as a pre-processor state-ment. In this example it has been used to define the values of TRUE and FALSE. The main advantage of this is usually the ability to make C code more readable by defining names to be certain values. Statements like if i == 1 can be replaced in the code by i == TRUE which makes their meaning far easier to understand. This technique is also used to define constants, which also make the code easier to understand.
One important point to remember is that the substitu-tion is literal, i.e. the identifier is replaced by the string, irre-spective of whether the substitution makes sense. While this is not usually a problem with constants, some programs use #define to replace part or complete program lines. If the wrong substitution or definition is made, the resulting program line may cause errors which are not immediately apparent from looking at the program lines. This can also cause problems with different compiler syntax where the definition is valid and accepted by one compiler but rejected by another. This prob-lem can be solved by using the #define to define different versions. This is usually done with using the #if def variation of the #define statement.
It is possible to supply definitions from the C compiler command line direct to the pre-processor, without having to edit the file to change the definitions, and so on. This often allows features for debugging to be switched on or off, as required. Another use for this command is with macros.
#define MACRO() statement
#define MACRO() statement
It is possible to define a macro which is used to condense code either for space reasons or to improve its legibility. The format is #define, followed by the macro name and the argu-ments, within brackets, that it will use in the statement. There should be no space between the name and the brackets. The statement follows the bracket. It is good practice to put each argument within the statement in brackets, to ensure that no problems are encountered with strange arguments.
#define SQ(a) ((a)*(a))
#define MAX(i,j) ((i) > ( j) ? (i) : (j))
x = SQ(56);
z = MAX(x,y);
#include “filename” #include <filename>
This statement takes the contents of a file name and includes it as part of the program code. This is frequently used to define standard constants, variable types, and so on, which may be used either directly in the program source or are expected by any library routines that are used. The difference between the two forms is in the file location. If the file name is in quotation marks, the current directory is searched, followed by the standard directory — usually /usr/include. If angle brackets are used instead, only the standard directory is searched.
Included files are usually called header files and can themselves have further #include statements. The examples show what happens if a header file is not included.
#ifdef identifier code
This statement conditionally includes code, depending on whether the identifier has been previously defined using a #define statement. This is extremely useful for conditionally altering the program, depending on definitions. It is often used to insert machine dependent software into programs. In the example, the source was edited to comment out the CPU_68000 definition so that cache control information was included and a congratulations message printed. If the CPU_68040 defini-tion had been commented out and the CPU_68000 enabled, the reverse would have happened — no cache control software is generated and an update message is printed. Note that #ifndef is true when the identifier does not exist and is the opposite of #ifdef. The #else and its associated code routine can be removed if not needed.
#define CPU_68040 /*define CPU_68000 */ #ifdef CPU_68040
/* insert code to switch on caches */ else
/* Do nothing ! */ #endif
printf(“Considered upgrading to an MC68040\n”); #else
printf(“Congratulations !\n”); #endif
#if expression code
This statement is similar to the previous #ifdef, except that an expression is evaluated to determine whether code is included. The expression can be any valid C expression but should be restricted to constants only. Variables cannot be used because the pre-processor does not know what values they have. This is used to assign values for memory locations and for other uses which require constants to be changed. The total memory for a program can be defined as a constant and, through a series of #if statements, other constants can be defined, e.g. the size of data arrays, buffers and so on. This allows the pre-processor to define resources based on a single constant and using different algorithms — without the need to edit all the constants.
This is where the processed source code is turned into assembler modules ready for the linker to combine them with the run-time libraries. There are several ways this can be done. The first may be to generate object files directly without going through a separate assembler stage. The usual approach is to create an assembler source listing which is then run through an assembler to create an object file. During this process, it is sometimes possible to switch on automatic code optimisers which examine the code and modify it to produce higher performance.
The standard C compiler for UNIX systems is called cc and from its command line, C programs can be pre-processed, compiled, assembled and linked to create an executable file. Its basic options shown below have been used by most compiler writers and therefore are common to most compilers, irrespec-tive of the platform. This procedure can be stopped at any point and options given to each stage, as needed. The options for the compiler are:
-c Compiles as far as the linking stage and leaves the object file (suffix .o). This is used to compile programs to form part of a library.
-p Instructs the compiler to produce code which counts the number of times each routine is called. This is the profiling option which is used with the prof utility to give statistics on how many subroutines are called. This information is extremely useful for finding out which parts of a program are consuming most of the processing time.
-f Links the object program with the floating point software rather than using a hardware processor. This option is largely historic as many processors now have floating point co-processors. If the system does not, this option performs the calculations in software — but more slowly.
-g Generates symbolic debug information for debuggers like sdb. Without this information, the debugger can only work at assembler level and not print variable values and so on. The symbolic information is passed through the compilation process and is stored in the executable file it produces.
-O Switch on the code optimiser to optimise the program and improve its performance. An environment variable OPTIM controls which of two levels is used. If OPTIM=HL (high level), only the higher level code is optimised. If OPTIM=BOTH, the high level and object code optimisers are both invoked. If OPTIM is not set, only the object code optimiser is used. This option cannot be used with the -g flag.
-Wc,args Passes the arguments args to the compiler process indicated by c, where c is one of p012al and stands for pre-processor, compiler first pass, compiler second pass, optimiser, assembler and linker, respectively.
-S Compiles the named C programs and generates an assembler language output file only. This file is suffixed .s. This is used to generate source listings and allows the programmer to relate the assembler code generated by the compiler back to the original C source. The standard compiler does not insert the C source into assembler output, it only adds line references.
-E Only runs the pre-processor on the named C programs and sends the result to the standard output.
-P Only runs the pre-processor on the named C programs and puts the result in the corresponding files suffixed .i.
-Dsymbol Defines a symbol to the pre-processor. This mechanism is useful in defining a constant which is then evaluated by the pre-processor, without having to edit the original source.
-Usymbol Undefine symbol to the pre-processor. This is useful in disabling pre-processor statements.
-ldir Provides an alternative directory for the pre-processor to find #include files. If the file name is in quotes, the pre-processor searches the current directory first, followed by dir and finally the standard directories.
Here is an example C program and the assembler listing it produced on an MC68010-based UNIX system. The assem-bler code uses M68000 UNIX mnemonics.
$cat math.c main()
int a,b,c; a=2;
def main; val main; scl 2; type 044; endef
def ~bf; val ~; scl 101; line 2; endef
def a; val -4+S%1; scl 1; type 04;
def b; val -8+S%1; scl 1; type 04;
def c; val -12+S%1; scl 1; type 04;
def ~ef; val ~; scl 101; line 9; endef
def main; val ~; scl -1; endef
After the compiler and pre-processor have finished their passes and have generated an assembler source file, the assem-bler is used to convert this to hexadecimal. The UNIX assem-bler differs from many other assemblers in that it is not as powerful and does not have a large range of built-in macros and other facilities. It also frequently uses a different op code syntax from that normally used or specified by a processor manufacturer. For example, the Motorola MC68000 MOVE instruction becomes mov for the UNIX assembler. In some cases, even source and destination operand positions are swapped and some instructions are not supported. The assem-bler has several options:
-o objfile Puts the assembler output into file objfile instead of replacing the input file’s .s suffix with .o.
-n Turns off long/short address optimisation. The default is to optimise and this causes the assembler to use short addressing modes whenever possible. The use of this option is very machine dependent.
-m Runs the m4 macro pre-processor on the source file.
-V Writes the assembler’s version number on standard error output.
Linking and loading
On their own, object files cannot be executed as the object file generated by the assembler contains the basic pro-gram code but is not complete. The linker, or loader as it is also called, takes the object file and searches library files to find the routines it calls. It then calculates all the address references and incorporates any symbolic information. Its final task is to create a file which can be executed. This stage is often referred to as linking or loading. The linker gives the final control to the programmer concerning where sections are located in memory, which routines are used (and from which libraries) and how unresolved references are reconciled.
Symbols, references and relocation
When the compiler encounters a printf() or similar statement in a program, it creates an external reference which the linker interprets as a request for a routine from a library. When the linker links the program to the library file, it looks for all the external references and satisfies them by searching either default or user defined libraries. If any of these refer-ences cannot be found, an error message appears and the process aborts. This also happens with symbols where data types and variables have been used but not specified. As with references, the use of undefined symbols is not detected until the linker stage, when any unresolved or multiply defined symbols cause an error message. This situation is similar to a partially complete jigsaw, where there are pieces missing which represent the object file produced by the assembler. The linker supplies the missing pieces, fits them and makes sure that the jigsaw is complete.
The linker does not stop there. It also calculates all the addresses which the program needs to jump or branch to. Again, until the linker stage, these addresses are not calculated because the sizes of the library routines are not known and any calculations performed prior to this stage would be incorrect. What is done is to allocate enough storage space to allow the addresses to be inserted. Although the linker normally locates the program at $00000000 in memory, it can be instructed to relocate either the whole or part of the code to a different memory location. It also generates symbol tables and maps which can be used for debugging.
As can be seen, the linker stage is not only complicated but can also be extremely complex. For most compilations, the defaults used by the compiler are more than adequate.
As explained earlier, an object file generated by the assembler contains the basic program code but is not complete and cannot be executed. The command ld takes the object file and searches library files to find the routines it calls. It calcu-lates all the address references and incorporates any symbolic information. Its final task is to create a COFF (common object format file) file which can be executed. This stage is often referred to as linking or loading and ld is often called the linker or loader. ld gives the final control to the programmer concern-ing where sections are located in memory, which routines are used (and from which libraries) and how unresolved refer-ences are reconciled. Normally, three sections are used — .text for the actual code, and .data and .bss for data. Again, there are several options:
-a Produces an absolute file and gives warnings for undefined references. Relocation information is stripped from the output object file unless the option is given. This is the default if no option is specified.
-e epsym Sets the start address for the output file to epsym.
-f fill Sets the default fill pattern for holes within an output section. This is space that has not been used within blocks or between blocks of memory. The argument fill is a 2 byte constant.
-lx Searches library libx.a, where x contains up to seven characters. By default, libraries are located in /lib and /usr/lib. The placement of this option is important because the libraries are searched in the same order as they are encountered on the command line. To ensure that an object file can extract routines from a library, the library must be searched after the file is given to the linker. Common values for x are c, which searches the standard C library and m, which accesses the maths library.
-m Produces a map or listing of the input/output sections on the standard output. This is useful when debug- ging.
-o outfile Produces an output object file called outfile. The name of default object file is a.out.
-r Retains relocation entries in the output object file. Relocation entries must be saved if the output file is to become an input file in a subsequent ld session.
-s Strips line number entries and symbol table informa- tion from the output file — normally to save space.
-t Turns off the warning about multiply-defined symbols that are not of the same size.
-usymname Enters symname as an undefined symbol in the symbol table.
-x Does not preserve local symbols in the output symbol table. This option reduces the output file size.
-Ldir Changes the library search order so libx.a looks in dir before /lib and /usr/lib. This option needs to be in front of the -l option to work!
-N Puts the data section immediately after the text in the output file.
-V Outputs a message detailing the version of ld used.
-VS num Uses num as a decimal version stamp to identify the output file produced.
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