How to use a header file in c

Question

I defined a method in class MyStrLen.c and implemented it and i declared for it in the head file MyStrLen.h, what i wanted is to use a method from MyStrLen in another class MyStrCmp.c but it shows a compilation error in the shell when i try to make the o file.

MyStr.h

  int inputLen(char* myStr);

MyStr.c

int inputLen(char* myStr)
{
  ....
  ....
}

MyStrCmp.c

 #include "MyStr"
void method()
{
 inputLen(someinput)
}

and this is the compilation error

MyStrCmp.c:(.text+0x18): undefined reference to inputLen' MyStrCmp.c:(.text+0x29): undefined reference toinputLen' MyStrCmp.c:(.text+0x55): undefined reference to inputLen' MyStrCmp.c:(.text+0x77): undefined reference toinputLen'

it looks like your error has more to do with the fact that you aren't linking one of the files. — tay10r, Nov 13 '13 at 07:27

score 2 · Answer 1 · edited May 23 '17 at 10:31

Right, the basic checklist goes as follows:

Does MyStrCmp.c include the MyStr.h file: #include "MyStr.h" should be at the top of the file (along side #include <stdio.h> and #include <stdlib.h>)
Does MyStr.c do the same? By that I mean include its own header file (#include "MyStr.h")
Are the 3 files mentioned (MyStrCmp.c, MyStr.c and MyStr.h) in the same directory?
Are you passing both the MyStrCmp.c file and the MyStr.c file to gcc?

If the answer to all 4 of these questions is yes, then:

$ gcc -o MyStrCmp -Wall MyStrCmp.c MyStr.c -std=c99

Should work. Because of the way you've written the inputLen function (in MyStr.c), it's written as a file that can be compiled externally, or separatly (gcc -o MyStr.c, to produce an o-file). As a result, the linking has to be done explicitly, by passing both source files to the compiler. By the way, more details can be found in this duplicate question
Basically, open a terminal window, and enter the following commands:

$ mkdir test
$ cd test/
$ touch MyStr.c && touch MyStr.h && touch MyStrCmp.c
$ vim MyStr.c MyStr.h -O

I use Vim, you can use your preferred editor, but that's besides the point.
In the MyStr.h file, you type:

int inputLen(char* myStr);

Save and close it, then edit the MyStr.c file, and define your actual function:

#include "MyStr.h"
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
int inputLen(char *myStr)
{
    printf("%s\n", myStr);
    return strlen(myStr);
}

Save & close, then edit the MyStrCmp.c file, and write something like:

#include <stdio.h>
#include <stdlib.h>
#include "MyStr.h"
int main(int argc, char **argv )
{
    const char *test = "Some test-string";
    int l = 0;
    l = inputLen(test);
    printf("The printed string is %d long\n", l);
    return EXIT_SUCCESS;
}

Then compile with the command I provided above. This worked just fine for me...

+1 for showing the correct way to link files. – tay10r Nov 13 '13 at 07:29 — tay10r, Nov 13 '13 at 07:29

score 1 · Answer 2 · answered Nov 12 '13 at 11:26

1

In MyStrCmp.c, put this at the top:

#include "MyStr.h"

answered Nov 12 '13 at 11:26

Leigh

12,038
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i did that but still didn't work – KaramJaber Nov 12 '13 at 11:35
1

If those are the only files you've got: `gcc -Wall MyStr.c MyStrCmp.c -o programoutput`. It looks like you didn't compile the MyStr.c file yet. – Leigh Nov 12 '13 at 11:37
@somebody You did what? `#include "MyStr"` or `#include "MyStr.h"`? Also, are you developing on an OS where file names are case-sensitive? – Lundin Nov 12 '13 at 11:44
1

@somebody: Where are the files in respect to your `MyStrCmp.c` file? are both the `MyStr.h` and `MyStr.c` file in the same directory? – Elias Van Ootegem Nov 12 '13 at 12:06

score 1 · Answer 3 · answered Nov 12 '13 at 12:18

Your "MyStr.h" should have this: extern int inputLen(char* myStr);

and your "MyStr.c" should have #include<MyStr.h> and your MyStrCmp.c should also have #include<MyStr.h>

provided that, all the headers and sources are in the same directory!

To avoid multiple inclusion confusions: use header guards

#ifndef MYSTR_H
#define MYSTR_H

extern int inputLen(char* myStr);

#endif

score 0 · Answer 4 · answered Nov 12 '13 at 12:26

The answer may be that you have not used the right compilation command.

You must use the -c flag (if you are using gcc) to create a .o file, otherwise gcc will try to link the executable and without knowing about MyStr.c it will fail to find the inputLen function.

Try compiling Mystr.c and MyStrCmp.c separately using -c and then linking the .o files.

You mention in a comment that you "did compile it", but you must make sure the two files get combined by the linker.

How to use a header file in c

4 Answers4

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