gcc-4 anomaly (sqrt function)

joeymac · 01-02-2006, 10:04 PM

I am experiencing a strange problem with gcc-4.0.1 under Mandriva-2006.0. I am trying to generate a file of small primes to use in a project. The relevant code is shown below:

#include <stdio.h>
#include <math.h>
int main(int argc, char *argv[])
{
FILE *output;
int i, j, limit, lowLimit, *theArray;
//...(snip)
lowLimit = (int) sqrt(limit) + 1;
//...(snip)
}

If I compile to object (using gcc -c), the code compiles without error. However, if I attempt to compile to an executable (using gcc -o ...), I get the following diagnostic:

/home/jmcdow/tmp/ccCBoony.o: In function `main':
findLowPrimes.c

.text+0x74): undefined reference to `sqrt'
collect2: ld returned 1 exit status

So I check /usr/include/math.h and, sure enough, there is no prototype for sqrt(); yet, if I replace the limit variable with a constant value, no error occurs!

Could anyone please give me some insight on whats up here? I get lost on the gcc forums.

randyding · 01-02-2006, 10:43 PM

If you are missing a prototype, like didn't include a header, the error you'll get is "warning: implicit declaration of function `sqrt'".. so that's not the problem.
If your error is "undefined reference" this means the function is probably prototyped but not defined.

sqrt() is actually defined in a library called the math library, and you have to explicitly link with the math library by puting "-lm" on the gcc command line.

joeymac · 01-02-2006, 11:45 PM

Thanks a bunch--solved the problem.

I've been using gcc/g++ for years and never knew that standard libraries needed to be explicitly linked.