Undefined reference error while compiling

Question

I'm trying to compile a project that has multiple *.c files and *.h file when I type the following command:

$gcc -c -I ../hdr main.c gestic.c menu.c

the hdr folder is where the *.h files are located, the .o files are created but when I try to link them using the command:

$gcc -o main.o gestic.o menu.o

I see errors

gestic.c:(.text+0x..): undefined reference to functions that are declared in *.h files

Answer 1

Try:

$ gcc main.o gestic.o menu.o

-o filename: Place output in filename

Answer 2

$gcc -Wall -c -I../hdr -o main.o main.c
$gcc -Wall -c -I../hdr -o menu.o menu.c
$gcc -Wall -c -I../hdr -o gestic.o gestic.c
$gcc -Wall -o myprogram main.o menu.o gestic.o

Using a Makefile is very common for this task

example (untested) Makefile:

CC=gcc
CFLAGS=-Wall -I../hdr 

all: myprogram

myprogram: main.o menu.o gestic.o
 $(CC) -o $@ $^

gestic.o: gestic.c
 $(CC) $(CFLAGS) -c -o $@ $<

main.o: main.c
 $(CC) $(CFLAGS) -c -o $@ $<

menu.o: menu.c
 $(CC) $(CFLAGS) -c -o $@ $<

Answer 3

Firstly header files are not included, secondly you might use the function that doesn't contain the definition.

The message undefined reference to 'function_name' implies that of all the object files you're giving to the linker, none of them has a definition for function_name. That means that either

You're not linking with *.o *.c (as compiled) does not contain a definition for function_name -- by 'as compiled' I mean with all of the various preprocessor options you use on it.

here - Iinclude/

try,

gcc -c file.c -I<include_dir> 

If you compile more files better to have makefile to create the objects of those files, link those objects along with header.

Sample makefile,

CC = gcc
CFLAGS = -Wall
INCLUDE = sample.h
OBJ = samople1.o sample2.o

%.o: %.c $(INCLUDE)
        $(CC) $(CFLAGS) -c -o $@ $<

go: $(OBJ)
        gcc $(CFLAGS) -o $@ $^