UMBC CS 201, Spring 02
Linker errors
The compiler makes assumptions
In a previous lecture, we have seen that the compiler makes an
assumption for functions that have no prototype (i.e the function
returns an int).
When we compile with the -c switch as in
<![CDATA[
gcc -c -Wall -ansi hello.c
]]>
we are telling the compiler not to invoke the linker.
Therefore, the compiler assumes that any function which is used
in hello.c but not declared in hello.c will be found in some
other file.
What's the linker do
It's the linkers job to "resolve" the location of functions that
the compiler couldn't find.
But what if the linker can't find the function either?
If the linker can't find the function in any of the .o files or
libraries, it generates an error known as "undefined reference".
An executable file cannot be produced.
findstats.c revisited
Here's a slightly modified version of findstats.c.
The function call to Sum(a,b) has been changed to sum(a,b).
<![CDATA[
/********************************************
* File: findstats.c
* Author: S. Bogar
* Date: 8/3/99
* Section 101
* SSN: 123-45-6789
* EMail: bogar@cs.umbc.edu
*
* This file uses functions from a separately
* compiled object file.
*********************************************/
#include <stdio.h>
/* stats.h contains function prototypes */
#include "stats.h"
int main ()
{
int a, b ;
/* get two ints from user */
printf("Enter first number: ") ;
scanf ("%d", &a) ;
printf("Enter second number: ") ;
scanf ("%d", &b) ;
printf("\n") ;
/* call arithmetic functions and print results */
printf("The sum of %d and %d is: %d\n",
a, b, sum (a, b) ) ;
printf("The smaller of %d and %d is: %d\n",
a, b, Min (a, b) ) ;
printf("The larger of %d and %d is: %d\n",
a, b, Max (a, b) ) ;
printf("The average of %d and %d is: %f\n",
a, b, Avg (a, b) ) ;
printf("The distance between %d and %d is: %d\n",
a, b, Dist (a, b) ) ;
/* we're done !! */
printf("That's all folks.\n") ;
return 0;
}
]]>
The functions in stats.c haven't been changed.
<![CDATA[
/********************************************
* File: stats.c
* Author: S. Bogar
* Date: 8/3/99
* Section 101
* SSN: 123-45-6789
* EMail: bogar@cs.umbc.edu
*
* This file is compiled separately from findstats.c
*
* Functions in this file:
* Sum (x, y)
* Min (x, y)
* Max (x, y)
* Avg (x, y)
* Dist (x, y)
*
*********************************************/
/***********************************
* Header files needed by the code
* found in the functions defined
* in this file
************************************/
#include "stats.h"
/********************************************
* Function: Sum
* Usage: z = Sum (x, y)
*
* Input: two integers to be added
* Output: Returns the sum of x and y.
*********************************************/
int Sum (int x, int y)
{
int sum ;
sum = x + y ;
return sum ;
}
/*********************************************
* Function: Max
* Usage: z = Max (x, y)
*
* Inputs: two integers to be compared
* Output: Returns the larger of x and y.
*********************************************/
int Max (int x, int y)
{
int max ;
if (x > y) {
max = x ;
} else {
max = y ;
}
return max ;
}
/*********************************************
* Function: Min
* Usage: z = Min (x, y)
*
* Inputs: two integers to be compared
* Output: Returns the smaller of x and y.
*********************************************/
int Min (int x, int y)
{
int min ;
if (x < y) {
min = x ;
} else {
min = y ;
}
return min ;
}
/*********************************************
* Function: Avg
* Usage: z = Avg (x, y)
*
* Inputs: two integers to be compared
* Output: Returns average of x and y as a
* *** double ***
*
*********************************************/
double Avg (int x, int y)
{
double average ;
average = (x + y) / 2.0 ;
return average ;
}
/*********************************************
* Function: Dist
* Usage: z = Dist (x, y)
*
* Input: two integers for which distance is
* calulated
* Output:Returns the absolute value of x - y.
*********************************************/
int Dist (int x, int y)
{
int distance ;
distance = x - y ;
if (distance < 0) {
distance = -distance ;
}
return distance ;
}
]]>
Now let's compile and see what happens
<![CDATA[
linux2[113] % ls
big.c findstats.c stats.c stats.h
linux2[114] % gcc -c -Wall -ansi findstats.c
findstats.c: In function `main':
findstats.c:31: warning: implicit declaration of function `sum'
linux2[115] % gcc -c -Wall -ansi stats.c
linux2[116] % gcc -Wall -ansi findstats.o stats.o
findstats.o: In function `main':
findstats.o(.text+0x58): undefined reference to `sum'
collect2: ld returned 1 exit status
linux2[117] % ls
big.c findstats.c findstats.o stats.c stats.h stats.o
linux2[118] %
]]>
Notice that the compiler gave us a warning, but threw no errors. It assumed
that the function "sum" would be found by the linker in some other .o
or library file. When the linker didn't find the function "sum",
it reported the "undefined reference". So even though the two .o files were
produced the linker cannot make the executable file, a.out
The most frequent cause of this error is simply misspellling the name
of your function.
<![CDATA[
]]>
Last modified - Tuesday, 05-Feb-2002 15:58:30 EST
[an error occurred while processing this directive]