[asterisk-users] Idiot's Question

[Venefax]

Believe it or not, I cannot find online a single piece of documentation for
the Asterisk function SPRINTF. This example does not work, for it changes
the caller id.
Set(CALLERID(num)=${SPRINTF(%010lld,0${CALLERID(num)})}),
For instance, if the incoming caller id is 17864335989, I get 0684466805 out
of that function, which is not intended one. To be precise, of the caller
has less than 10 chars, I want to complete it with a string of '0's. If the
caller id is nothing, or empty, I want to replace it with 10 zeroes. I guess
I can figure it out if a link to the documentation of SPRINTF is provided.