Calling C functions in main with struct interfaces and additional C files - c

Question

I have implemented a header file, "msg_queue.h" into a non-main file called "msg_queue.c." From here I created a main file known as "mq_test.c" whereby I created a message queue and called functions that were implemented in "msg_queue.c." Yet, when I run the 'make' command I seem to get this error for each function called:
"enqueue", referenced from:
_main in test.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: *** [qmsgtest] Error 1
What seems to be the problem, have I not implemented the structs correctly?
The headerFile:
#ifndef MessageQueue_H
#define MessageQueue_H
typedef struct node{
int data;
struct node *next;
} Node;
typedef struct {
Node *front;
Node *rear;
} MessageQueue;
void enqueue(...); // enqueue
...
#endif //MessageQueue_H
The file implementing the header:
#include "MessageQueue_H.h"
#ifndef MessageQueue_H
#define MessageQueue_H
/* Interface for integer MQueue */
typedef struct node{
int data;
struct node *next;
} Node;
typedef struct {
Node *front;
Node *rear;
} MQueue;
void enqueue(... ){ // enqueue
....

Answer 1

In the file implementing the header, msg_queue.c, you have an include guard:
#ifndef MSG_QUEUE_H
#define MSG_QUEUE_H
You have an include guard with the same name in msg_queue.h and as the header is preprocessed first, the implementation code is removed by the preprocessor (because MSG_QUEUE_H is already defined, your include guard is removing the implementation code), which is why it can't find the symbols.
You can check this yourself by running gcc or clang with the -E option on your source code. It will show you what the preprocessor has done to your code (which in your case means that you get only the prototypes of the functions, not the implementations).
So you need to remove the include guard in your .c source file.
It's worth noting that include guards aren't normally needed in .c implementation files, as these normally aren't included from other files.
In your example, you've also defined your struct types twice (both in the header and in the C file). This violates the single definition rule, so if you want to define your interface with the data types in the header, you should remove them from the source file, otherwise it won't compile.
There are other errors in this code which prevent it from compiling (at least in C99 mode which is what I've tested it as), but these are unrelated to your problem with the linker not being able to find the function symbol(s).