[c] What does the "undefined reference to varName" in C mean?

I have 2 files: a.c and b.c

In a.c I am sending a signal to a function located in b.c

signal(SIGUSR1,doSomething);

On top of the a.c file, I have:

extern void doSomething (int    sig);

When I compile, however, I get an error:

/tmp/ccCw9Yun.o: In function main':
a.c:(.text+0xba): undefined reference todoSomething'
collect2: ld returned 1 exit status

The following headers are included:

#include <stdlib.h>
#include <stdio.h>
#include <signal.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/wait.h>

How do I fix this?

You need to compile and then link the object files like this:

gcc -c a.c  
gcc -c b.c  
gcc a.o b.o -o prog  

make sure your doSomething function is not static.

An initial reaction to this would be to ask and ensure that the two object files are being linked together. This is done at the compile stage by compiling both files at the same time:

gcc -o programName a.c b.c

Or if you want to compile separately, it would be:

gcc -c a.c
gcc -c b.c
gcc -o programName a.o b.o

It is very bad style to define external interfaces in .c files. .

You should do this

a.h

    extern void doSomething (int    sig);

a.c

    void doSomething (int    sig)
    {
       ... do stuff 
    }

b.c

#include "a.h"
.....
signal(SIGNAL, doSomething); 

.

You're getting a linker error, so your extern is working (the compiler compiled a.c without a problem), but when it went to link the object files together at the end it couldn't resolve your extern -- void doSomething(int); wasn't actually found anywhere. Did you mess up the extern? Make sure there's actually a doSomething defined in b.c that takes an int and returns void, and make sure you remembered to include b.c in your file list (i.e. you're doing something like gcc a.c b.c, not just gcc a.c)