compiler construction - Preprocessing vs Linking -

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everywhere linker (besides relocation, seem put under "loading") people give examples of 2 (say) c modules call functions each other.

as far understand , should taken care of preprocessing (#include etc). can understand if want assemble 2 modules @ different times need link them later. when use gcc, directly outputs executable , in context don't see point of linking except when preprocessing (which recursive i'd guess) final hits object code.

note : i'm referring static linking here. can see point when comes dynamic linking.

gcc using lot of tools create executable, compiler 1 of them , linker, preprocessor etc, see it's doing pass -v option. if want see intermediate files use -save-temps ie once you've created files below try this... 

gcc-5 -save-temps -v -std=c11 -wall -pedantic-errors main.c foo.c

if in directory you'll see several files... 

a.out // executable when no name specified foo.c // original foo.c foo.h // original foo.h foo.i // preprocessed ouput of foo.c , foo.h foo.o // foo object file foo.s // foo assembler main.c // original main.c main.i // main preprocessed main.o // main object file main.s // main assembler

if in main.i you'll notice contains this

# 1 "foo.h" 1   int foo(int i);

but doesn't know function foo does. that's in foo.i. means when object files created main.o knows needs foo function. connecting symbol foo in main.o foo in foo.o linker doing.

here's example, create following 3 files

foo.h

#ifndef foo_h #define foo_h int foo(int i);  #endif

foo.c

#include "foo.h" int foo(int i) {   return - 1729; }

main.c

#include "foo.h" #include <assert.h> #include <stdio.h> #include <stdlib.h> int main(void) {   printf("%d\n", foo(1));   return 0; }

create object files ie not executable follows, note i've added verbose flags this..

gcc-5 -v -std=c11 -wall -pedantic-errors main.c foo.c -c

if had used -o flag see actual linker command gcc called link objects executable ie

gcc-5 -v -std=c11 -wall -pedantic-errors main.c foo.c -o fooer

at point find 2 object files in directory. can create executable using ld follows (note mac specific, use output gcc -v find correct command machine).

ld -dynamic -arch x86_64 -macosx_version_min 10.11.3 -lsystem main.o foo.o -o fooer

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