Undefined reference error while compiling - linux

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

$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 2

Try:
$ gcc main.o gestic.o menu.o
-o filename: Place output in filename

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 $# $^