GCC Bugzilla Bug 13734

The round() function outputs random numbers

#include <math.h>
main()
{
printf( "%f %f\n", round(1.0), round(1.0001));
}

Output
-2.000000 6.000000

However these values appear to be random, and can change on different 
machines and at different times on the same machine.

(Mandrake 9.0 and 9.1)
gcc -o test test.c -lm

Use warnings:
pr13734.c:4: warning: return type defaults to `int'
pr13734.c: In function `main':
pr13734.c:5: warning: implicit declaration of function `round'
pr13734.c:5: warning: double format, different type arg (arg 2)
pr13734.c:5: warning: double format, different type arg (arg 3)
pr13734.c:6: warning: control reaches end of non-void function

See how round is implicitly declared, use -std=c99 or -std=gnu99 which turns on the c99 
functions in math.h.