Additional components: preprocessors, assemblers, loaders/link-editors

Several other programs may be required to produce an EXECUTABLE target program.

• A PREPROCESSOR produces an input to a compiler. To do so it may perform the following tasks.

  Collecting files.

  A source program may be divided into modules stored in different files. The PREPROCESSOR will collect these modules (for instance inclusion of header files in C) and generate the source program file.

  Macro processing.

  The preprocessor will expand shorthands, called MACROS.

  Language extension.

  Some language facilities (data structures, case statements) may be added to the language by BUILT-IN macros expanded by the he preprocessor.

• An ASSEMBLER translates
  • ASSEMBLY CODE that is a mnemonic version of machine code where names are used instead of binary codes for
    • operations,
    • memory addresses
  • into RELOCATABLE MACHINE CODE that is a machine code that can be loaded from any location $\ell$ of the memory (hence, if $\ell$ is added to all addresses in the code, all references will remain correct).
• A LOADER/LINK-EDITOR performs the following tasks.
  • The link-editor makes a single file from several files of relocatable machine code.
  • The loader
    • translates a relocatable machine code into an absolute machine code,
    • places it in memory at the proper locations.

We consider now an example. The above file addAndPrint.c is a C program. We compile it with the cc compiler using the -save-temps option which allows to save temporary files. In our example these files are

• addAndPrint.i which is the output of the preprocessor,
• addAndPrint.s which is the assembly code generated by cc,
• addAndPrint.o which is the relocatable machine code generated by the assembler of cc from addAndPrint.s,
• a.out which is the executable machine code generated by the linker from addAndPrint.o.

DIV ALIGN="CENTER">

[moreno@iguanodon tmp]$ more addAndPrint.c

#define ADD +
#define PRINT__floats__x__and__y printf("x = % d y = % d\n", x, y);

main()
{
  int x,y;
  x=20; y=3;
  x = x ADD y*10;
  PRINT__floats__x__and__y;
}

[moreno@iguanodon tmp]$ cc -save-temps addAndPrint.c 
[moreno@iguanodon tmp]$ ls
addAndPrint.c  addAndPrint.i  addAndPrint.o  addAndPrint.s  a.out
[moreno@iguanodon tmp]$ more addAndPrint.i

# 4 "addAndPrint.c"
main()
{
  int x,y;
  x=20; y=3;
  x = x + y*10;
  printf("x = % d y = % d\n", x, y);;
}

[moreno@iguanodon tmp]$ file addAndPrint.s 
addAndPrint.s: ASCII assembler program text
[moreno@iguanodon tmp]$ more addAndPrint.s

	.file	"addAndPrint.c"
	.version	"01.01"
gcc2_compiled.:
		.section	.rodata
.LC0:
	.string	"x = % d y = % d\n"
.text
	.align 4
.globl main
	.type	 main,@function
main:
	pushl	%ebp
	movl	%esp, %ebp
	subl	$8, %esp
	movl	$20, -4(%ebp)
	movl	$3, -8(%ebp)
	movl	-8(%ebp), %edx
	movl	%edx, %eax
	sall	$2, %eax
	addl	%edx, %eax
	leal	(%eax,%eax), %edx
	leal	-4(%ebp), %eax
	addl	%edx, (%eax)
	subl	$4, %esp
	pushl	-8(%ebp)
	pushl	-4(%ebp)
	pushl	$.LC0
	call	printf
	addl	$16, %esp
	leave
	ret
.Lfe1:
	.size	 main,.Lfe1-main
	.ident	"GCC: (GNU) 2.96 20000731 (Red Hat Linux 7.3 2.96-110)"

[moreno@iguanodon tmp]$ file addAndPrint.o 
addAndPrint.o: ELF 32-bit LSB relocatable, Intel 80386, version 1 (SYSV), not stripped
[moreno@iguanodon tmp]$ file a.out
a.out: ELF 32-bit LSB executable, Intel 80386, version 1 (SYSV), dynamically linked (uses shared libs), not stripped