Using function from another c file by including its header

Question

I am writing C code (not C++) on a Linux system. I am trying to compile one executable from 2 .c files and 1 header file: main.c, sort.c, and sort.h.

main.c's first line reads: #include "sort.h"

inside sort.h, each function in sort.c is defined like this example: extern void aFunct(int param);

However, when I try to call a function in sort.c from main.c, I get the following error upon compilation: "undefined reference to 'aFunct'".

If i replace #include "sort.h" with #include "sort.c" my program works without issue. However, as I understand it, this is bad form and I would prefer to avoid this. Thanks for any help.

edit: I am compiling this with a makefile containing the following code:

all: index sort.o

sort.o: sort.c sort.h
    gcc -Wall -g -c sort.c

index: main.c sort.o
    gcc -Wall -g -o index main.c

clean:
    rm index
    rm sort.o

edit: I have fixed the problem. The problem did not stem from a misunderstanding of C files and how they link, but rather a misunderstanding of the makefile/gcc commands themself. My program works with the following makefile:

all: index sort.o

sort.o: sort.c
    gcc -Wall -g -c sort.c

index: main.c sort.o
    gcc -Wall -g -o index main.c sort.o

clean:
    rm sort.o
    rm index

I added my makefile to my question so you know exactly how it is being compiled/linked — jnel899
– jnel899, Commented Mar 15, 2015 at 20:41
SleuthEye, turns out my problem was I THOUGHT I was linking sort.o to the executable, but I had a syntax error my makefile that stemmed from a fundamental misunderstanding of gcc syntax. — jnel899
– jnel899, Commented Mar 15, 2015 at 21:19

George · Accepted Answer · 2015-03-15 20:29:07Z

1

You should include #include "sort.h" in sort.c as well. You might be doing this already.

The important point is, you need to make sure that you are building both the .c files [main.c and sort.c]

Now both these obj files [main.o and sort.o] should be the input to linker.

As per my guess, you are not compiling sort.c , so linker is not able to see the implementation of the functions.

answered Mar 15, 2015 at 20:29

George

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2 Comments

jnel899 Over a year ago

sort.h is already included in sort.c. And unless I am having some crisis of understanding (very possible), I am compiling sort.c into an object file. If you want you can check my question again as it now contains the makefile

jnel899 Over a year ago

The issue was I was incorrectly linking sort.o. I figured it out on my own but this answer was pretty close so I decided to best answer it. Thanks man!

petrovich · Accepted Answer · 2015-03-15 20:29:52Z

0

When you include one source code file to other (e.g. #include "sort.c") you can have a troubles if you include sort.c file to more than one c-file of the same project. But as I understand you just compile one c-file in which you include sort.c... so it works. But, better read some tutorials, for example How to Create Multi-Module Programs

answered Mar 15, 2015 at 20:29

petrovich

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1 Comment

jnel899 Over a year ago

I believe I am already doing everything that the link mentions

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