Compile time error while running multiple c files

Question

#Edited

I am tested on simple two files now that are add.h and add.c.

I made a Makefile in order to compile my program. Here is my makefile.

# Make file for running the project
CC=gcc
CFLAGS= -Wall -g
LDFLAGS = -include
OBJFILES = add.o
LIB = add.h
TARGET = add

all: ${TARGET}

%.o: %.c
    ${CC} ${CFLAGS} -c -o $@ $<

${TARGET}: ${OBJFILES}
    ${CC} ${CFLAGS} -o ${TARGET} ${OBJFILES}

clean:
    rm -f $(OBJFILES) $(TARGET) *~

when I run

make add

I get the following error:

gcc -Wall -g -c -o add.o add.c
gcc -Wall -g -o add add.o
Undefined symbols for architecture x86_64:
  "_b", referenced from:
      _main in add.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: *** [add] Error 1

here are snippets of my code

add.c

#include <stdio.h>
#include <stdlib.h>
#include "add.h"

int main(void) {
    int a = 10, b=20;
    add(a, b);
    return 0;
}

add.h

#ifndef __ADD_H_
#define __ADD_H_

extern int a,b;

int add(a,b)
{
    return a+b;
}

#endif // __ADD_H_

Use remake to debug your Makefile but do read the documentation of GNU make and try make -p; also read documentation of GCC — Basile Starynkevitch
– Basile Starynkevitch, Commented Jun 28, 2020 at 12:30
1) Don't write this much code before testing it. 2) Do you give things names with leading underscores (e.g. _com_task)? Don't. 3) Read the page on minimal complete examples. — Beta
– Beta, Commented Jun 28, 2020 at 12:46
@BasileStarynkevitch will have a look at the sources you provided. Thanks — Owais Qayum
– Owais Qayum, Commented Jun 28, 2020 at 12:56
Read also Modern C and perhaps n1570, the C11 standard. See this C reference and learn to use GDB; there is no reason to have 3 header files in your case. Look for inspiration on some programs on github — Basile Starynkevitch
– Basile Starynkevitch, Commented Jun 28, 2020 at 12:57
@Beta actually yes, I have given all my functions name starting or including _com_tasks. Do you think that can be the issue ? — Owais Qayum
– Owais Qayum, Commented Jun 28, 2020 at 12:57

TsRoe · Accepted Answer · 2020-06-28 12:04:39Z

2

You are lacking a rule to actually build your object files. Right now you only have one to link them all together once they already exist. Try adding:

%.o: %.c
    ${CC} ${CFLAGS} -c -o $@ $<

This tells make how to build object files out of source files.

answered Jun 28, 2020 at 12:04

TsRoe

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4 Comments

Owais Qayum Over a year ago

I have tried adding and compiling it but it's still showing the same error.

TsRoe Over a year ago

You seem to be using _com_task in multiple functions, but you never defined it anywhere. Is there an implementation in any of the source files?

Owais Qayum Over a year ago

com_task is a struct variable and I have used it only in stack.h and nowhere else. do I need to provide my whole source code here?

Owais Qayum Over a year ago

I have compiled the program on the simplest version as you can see in the edited post.

Beta · Accepted Answer · 2020-06-28 13:50:46Z

1

You are misusing the extern keyword, losing track of your variables and making your code too complicated.

Let's try something very simple:

int main(void) {
  int a = 10; b=20;

  return 0;
}

This fails. The compiler complains about the statement b=20;, since it has never heard of this b. The semicolon that made this a separate statement was either a typo or a conceptual error caused by declaring extern int b elsewhere. There is no need for extern here, at least not yet.

This:

int main(void) {
  int a=10, b=20;

  return 0;
}

works.

Now for an add function.

int add(int a, int b)
{
  return a+b;
}

int main(void) {
  int a = 10, b=20;

  add(a,b);

  return 0;
}

Note that the a and b in add are not the same variables as the a and b in main. This is crucial; do not proceed until you understand it.

Now add a declaration of the add function:

int add(int a, int b);  // <- declaration

int add(int a, int b)   // <- definition
{
  return a+b;
}

The declaration can be moved into a header file (add.h); the definition belongs in a source file (add.c).

Finally, I would advise you to add a line to the makefile:

add.o: add.h

answered Jun 28, 2020 at 13:50

Beta

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1 Comment

Owais Qayum Over a year ago

waoooo they way you explained it cleared all my problems. I have read blogs but your explanation answered all my questions.

andreagalle · Accepted Answer · 2020-06-28 12:12:51Z

1

Aren't you missing an #endif at the end of the last header file queue.h ? This can be for sure an issue when compiling. Moreover, aren't you missing the main_application.h header file you are then including within the stack.h one for example?

Have a look at this: https://medium.com/@m.muizzsuddin_25037/error-ld-symbol-not-found-for-architecture-x86-64-a5e5b648ffc seems helpful here and point our attention to the header files again!..

edited Jun 28, 2020 at 12:12

answered Jun 28, 2020 at 11:58

andreagalle

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5 Comments

Owais Qayum Over a year ago

queue.h is having #endif

andreagalle Over a year ago

Ok @owais, I was pretty sure of this, thus just add it editing your code snippet here above ;)

Owais Qayum Over a year ago

I thought I might have done some mistake in declaring the headers

andreagalle Over a year ago

What @tsroe is saying is for definitely right!.. About the "mistake in declaring the headers" I was wondering if the main_application.c file should instead have been named main_application.h or is it just a typo? If not you need anyway an header file there!

Owais Qayum Over a year ago

Actually I have created a main_application.h just to define the variables that are used in other files such as queue and stack. I tried the solution provided by @tsroe but it's not working as well.

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