Link Main error with library

Question

Recently, I tried to create a library and to do that I created two files: file-parser.c and array.c with their corresponded .h file. So, in the file-parser I am using a function from the array that calls a length and when I compile (without linking it) parser-file.c the commands

gcc -c -o file-parser.o file-parser.c
gcc -c -o array.o array.c
ar r libutils.a array.o file-parser.o 

don't tell me an error.

The problem is that when I compile the main.c file

gcc -o prova main.c -L. -lutils

gcc returns:

array/libutils.a(file-parser.o): in function "AnalizeRow":
file-parser.c:(.text+0x109): undefined reference to "length"
array/libutils.a(file-parser.o): in the function "GetWord":
file-parser.c:(.text+0x16a): undefined reference to "length"
collect2: error: ld returned 1 exit status

I'm not English, so I think the error message took to me by ld. Can anyone explain where was my error? Thanks.

Answer 1

A similar error may take place in anywhere in a development process!!!

Your question does not give a complete picture about current development process.

I see, after read your question, you has two files with source code - array.c and file-parser.c. Also, you mentioned the static library libutils.a, but you did not indicate - how are you created it and what contains it for now.

---

I try to recreate your current position:

The file file-parser.c

#include <stdlib.h>

int
AnalizeRow(char row[])
{
    size_t len = length(row);
    return 0;
}


char *
GetWord(char row[])
{
    size_t len = length(row);
    return NULL;
}

The file file-parser.h

int AnalizeRow(char row[]);
char* GetWord(char row[]);

The file array.c

#include <string.h>

size_t
length(char row[])
{
    return strlen(row);
}

The file array.h

size_t length(char row[]);

Create the empty static library libutils.a

$ ar cr libutils.a
$ ar t libutils.a (it empty)

Your code

$ gcc -c -o file-parser.o file-parser.c
$ gcc -c -o array.o array.c
$ ar r libutils.a array.o file-parser.o

Content the libutils.a

$ ar t libutils.a
array.o
file-parser.o

Build and run

$ gcc -o prova main.c -L. -lutils && ./prova

I have not errors at all..

---

Preliminary conclusions:

1. I have wrong development process
2. Your static library libutils.a is cause the problem
3. Your source code in files is cause the problem

---

Testing environment

$ gcc --version
gcc (Debian 4.9.2-10) 4.9.2
$ lsb_release -a
No LSB modules are available.
Distributor ID: Debian
Description:    Debian GNU/Linux 8.7 (jessie)
Release:    8.7
Codename:   jessie
$ uname -a
$ apt-cache policy binutils
binutils:
  Installed: 2.25-5+deb8u1
  Candidate: 2.25-5+deb8u1
  Version table:
 *** 2.25-5+deb8u1 0
        500 http://ftp.ru.debian.org/debian/ jessie-proposed-updates/main amd64 Packages
        100 /var/lib/dpkg/status
     2.25-5 0
        500 http://ftp.ru.debian.org/debian/ jessie/main amd64 Packages
        500 http://httpredir.debian.org/debian/ jessie/main amd64 Packages