gcc makefile undefine 오류

# 
# gcc echoserver.c
/tmp/ccbAX8SF.o: In function `main':
echoserver.c:(.text+0x6c): undefined reference to `open_listenfd'
/tmp/ccbAX8SF.o: In function `echo':
echoserver.c:(.text+0x100): undefined reference to `readline'
collect2: ld returned 1 exit status
# 

CC = gcc
CFLAGS = -g -c
LDFLAGS = -lpthread -lrt
 
OBJS = nethelp.o echoserver.o echoservert.o echoclient.o
 
all: echoserver echoservert echoclient
 
nethelp.o: nethelp.c
        $(CC) $(CFLAGS) -c nethelp.c
 
echoserver.o: echoserver.c
        $(CC) $(CFLAGS) -c echoserver.c
 
echoservert.o: echoservert.c
        $(CC) $(CFLAGS) -c echoservert.c
 
echoclient.o: echoclient.c
        $(CC) $(CFLAGS) -c echoclient.c
 
echoserver: echoserver.o nethelp.o
        $(CC) $(LDFLAGS) -o $@ echoserver.o nethelp.o
 
echoservert: echoservert.o nethelp.o
        $(CC) $(LDFLAGS) -o $@ echoservert.o nethelp.o
 
echoclient: echoclient.o nethelp.o
        $(CC) $(LDFLAGS) -o $@ echoclient.o nethelp.o
 
clean:
        rm -f echoserver echoservert echoclient *.o *~ core
~