CODEWITHSUNDEEP
cinclude
mr mojo risin
mr mojo risin

Reputation: 555

C - How are c source files incuded without an include statement

I've taken the plunge and am learning C. It's been a pretty good but manageable learning curve coming from a scripting (php, perl) background with only a little bit of C#.

I've used the web-site "Learn C The Hard Way" and am so far grasping reasonably well (I think) but I can't understand this part of one of the exercises:

http://c.learncodethehardway.org/book/ex19.html

He created four source files - object.h, object.c, ex19.h, ex19.c

But I don't understand how the object.c file is included.

The main function is located in ex19.c, and it has the line

#include "ex19.h"

File ex19.h has the line

#include "object.h"

But object.h makes no reference to including object.c. Interestingly object.c contains the line

#include "object.h"

Is there some sort of implied include where if you include the header file, it will automatically include the c source code of the same name?

Upvotes: 3

Views: 247

Answers (3)

Anshul
Anshul

Reputation: 1

The entire logic lies in the makefile and an intelligent compiler. The final binary created has an object file named as object.o which will ideally contain all the function definitions defined in object.h file. It is the linker which links the functions declared in .h file with the definition which are available in .o file.

Upvotes: 0

Preet Sangha
Preet Sangha

Reputation: 65556

Object.c is not included.

It's compiled as it's own unit and it includes object.h

See the make file:

CFLAGS=-Wall -g

all: ex19

ex19: object.o

clean:
  rm -f ex19

ex19: object.o tells you that object.o must be created before ex19 can be built, and this is picked up by default from make file as an object.c exists.

So this make file says

to build all you need ex19, to get ex19 you need object.o, and the to create object.o the makefile picks up object.o built from the object.c

From the page you reference:

make can't see anything in the file for object.o, but it does see an object.c file, and it knows how to turn a .c into a .o, so it does that.

Upvotes: 1

templatetypedef
templatetypedef

Reputation: 373112

This is the job of a separate program called the linker. In C, source files need access to the header files of other C source files so that they can see information about what functions, types, and variables are defined by that second C file that the first file might want to use. Each C file is then compiled independently of the rest. The output of the compiler is an object file.

To build a final program, a second program called the linker comes in and combines all the object files together into the overall executable. This program is tasked with taking the implementations of all the different C files and cross-referencing them against one another so that each time one C file references a function or variable in a different C file, that reference can actually be made to the appropriate object.

This is why you don't need to include .c files. Once a source file has a header, it knows enough about the other file in order to use the functions it provides for the compiler to verify that it's using them correctly. The linker then handles of the job of actually making the cross-references. You can think of the compiler as a program that checks to see that if the functions are defined, then the program would work. The linker then actually checks to make sure that those functions are defined in the first place and sets up the appropriate links in the executable.

Hope this helps!

Upvotes: 6

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