Undefined Symbol for Architecture x86_64 - C

I'm beginning to write a program in C but I have the following error and I can't figure out why:

Undefined symbols for architecture x86_64: "_receiveByte", referenced from: _main in main_prog.o ld: symbol(s) not found for architecture x86_64 clang: error: linker command failed with exit code 1 (use -v to see invocation) make: *** [q1] Error 1 

As far as I understand, I have included everything properly in the appropriate files and in my makefile, yet I still get this error. This is what my code looks like:

main_prog.c

#include <stdio.h> #include <string.h> #include <stdlib.h> #include "physical.h" #include "main_prog.h" int main(int argc, char *argv[]) { PhysicalState * initPhysical(); const Direction blah = L2R; unsigned char new_char; new_char = receiveByte(blah); printf("Done.\n"); } 

main_prog.h

#ifndef _MAIN_PROG_H #define _MAIN_PROG_H // nothing yet #endif 

physical.h

#ifndef _PHYSICAL_H #define _PHYSICAL_H #include <pthread.h> #define MAX_FRAME_SIZE 1024 typedef enum { false, true } boolean; typedef enum { L2R, R2L } Direction; struct PHYSICAL_STATE { pthread_cond_t L2RTxSignal; pthread_cond_t L2RRxSignal; boolean L2RReady; pthread_mutex_t L2RLock; pthread_cond_t R2LTxSignal; pthread_cond_t R2LRxSignal; boolean R2LReady; pthread_mutex_t R2LLock; }; typedef struct PHYSICAL_STATE PhysicalState; PhysicalState * initPhysical(); unsigned char receiveByte(const Direction dir); #endif 

and finally my

makefile

PROG = main_prog HDRS = physical.h main_prog.h SRCS = main_prog.c OBJDIR = object OBJS = $(patsubst %.c, $(OBJDIR)/%.o, $(SRCS)) CC = gcc CFLAGS = -Wall --std=c99 -L. LIBS = -lm all : $(OBJDIR) $(PROG) $(PROG) : $(OBJS) $(CC) $(CFLAGS) $^ -o $(PROG) $(LIBS) object/%.o : %.c $(HDRS) $(CC) -c $(CFLAGS) $< -o $@ $(LIBS) $(OBJDIR) : mkdir -p $@/ clean: rm -rf object/ rm -f $(PROG) 

There is also a physical.o file located in the same directory as all of these files, whereas the main_prog.o file gets located in object/ as specified by the makefile. I've tried moving the physical.o file around as well as the main_prog.o file around (and changing the makefile to correspond) but I still get this error. Any solution to this error would be greatly appreciated thanks.