"Undefined symbol <function> first referenced in file <file>" link error - c

Question

I'm writing my first program in C for a class; I've managed to even out most of the syntax errors, but I'm getting a strange error when gcc tries to link the object files together. It prints exactly like below:
gcc -o proj04.support.o proj04.driver.o
Undefined first referenced
symbol in file
convert proj04.driver.o
I've looked around for a few answers, but none really make sense to me. I'll post the files I'm using to make the program below, and if you've got the answer I would really appreciate the help. It seems to be a pretty basic error, so it's probably something silly I didn't do.
Makefile (posting this first because I suspect the issue is here)
# Comments
# Comments
proj04: proj04.support.o proj04.driver.o
gcc -o proj04.support.o proj04.driver.o
proj04.support.o: proj04.support.c
gcc -Wall -c proj04.support.c
proj04.driver.o: proj04.driver.c
gcc -Wall -c proj04.driver.c
Header file (provided by the professor, unchangeable, one line long):
int convert( int, unsigned, char[], int )
Implementation file
#include <stdio.h>
#include "/user/cse320/Projects/project04.support.h"
#include <string.h>
void formatdisplay( char[], int );
int convert( int I, unsigned base, char result[], int display )
{
int quotient, dividend, remainder;
const int divisor = base;
int count = 0;
char ending[] = " base ";
dividend = I;
remainder = 0;
quotient = 1;
while (quotient != 0)
{
if (count <= strlen(result))
{
quotient = (dividend / divisor);
remainder = (dividend % divisor);
//convert to ascii char
result[count] = remainder;
count++;
}
}
formatdisplay ( result, display );
strrev(result);
if ( I >= 0 ) { result[0] = '+'; }
if ( I < 0 ) { result[0] = '-'; }
printf( "%s" , strcat (result, ending));
}
void formatdisplay ( char str[], int disp )
{
if ( disp < 0 )
{
unsigned i = 0;
for ( i; i < strlen(str)-1; i++)
{
if ( str[i] = '\0') { str[i] = '0'; }
}
}
if ( disp >= 0 )
{
unsigned i = 0;
for ( i; i < strlen(str)-1; i++)
{
if ( str[i] = '\0') { str[i] = ' '; }
}
}
}
Driver file (not really implemented yet)
#include <stdio.h>
#include "/user/cse320/Projects/project04.support.h"
int main () {
char Result1[32];
int T = convert(10, 2, Result1, 1);
}

Answer 1

Yes, the problem is probably in the Makefile:
proj04: proj04.support.o proj04.driver.o
gcc -o proj04.support.o proj04.driver.o
The -o option to gcc takes an argument, the output filename. So this is asking gcc to link the file proj04.driver.o, producing an output file of proj04.support.o.
gcc -o proj04 proj04.support.o proj04.driver.o should work better.

Answer 2

I had the almost the same problem.
Undefined first referenced symbol in file isThreeOfAKind /var/tmp//ccIQWbaj.o
My problem was that I had missed on a letter in one of my functions, so the declaration and the function misaligned. Ex:
void isThreeOfAKind (void);
void isThreeOfAkind {}
Missed the uppercase K, and wrote lowercase k instead.
After I changed the k to a uppercase K it compiled fine.
I dont know if it helps, but it could be something as easy as that.

Answer 3

I am no expert but as Andreas Joensson (who, judging by function name is writing the exact same program I am) says above this problem seems to occur when there's some mismatch between declaration and use of a function.
Your function convert is declared as returning an int but I find no return value. That might be the problem.