I think what happens is that the compiler (not glibc) computes that
exp10 for you and that the compiler happens to be more accurate.
Here's what I get for the loop:
for (int i = 1; i < 20; i++) {
printf ("%.20g\n", exp10 (i));
}
welinder@CarbonX1:~$ ./a.out
10
100
1000.0000000000001137
10000.000000000001819
100000
1000000
9999999.9999999981374
99999999.999999985099
999999999.99999988079
10000000000
100000000000
1000000000000
10000000000000
100000000000000
1000000000000000
10000000000000000
99999999999999984
1000000000000000000
10000000000000000000
Here's the bug report to go with this:
https://sourceware.org/bugzilla/show_bug.cgi?id=28472
Note comment 6. It is clearly not a high-priority item for glibc.
M.
