Can't Log information about a double

Hello all, I'm trying to log some information during the bird process execution. I'm doing it in the bgp section. I used the command: log(L_INFO "** some text **"); with success, i added to the log some information about an integer variable with success, here it is the code: int a = 5; log(L_INFO "** some text ** %d", a); Now i'm trying to make the same with a double variable but without success double a = 5.5; log(L_INFO "** some text ** %lf", a); the output was: 2018-03-30 18:09:12.270 <INFO> ** some text ** %f I had already tried with %f instead of %lf but nothing changed in the output How can i log some double value? i need to use another function instead of log()? Thanks, Mattia