The convention is to show file descriptors as decimal. Add explicit entries to syscalls.c to avoid the default hex output.

Before:

fsync(0x12) = 0 (0x0)
fdatasync(0x12) = 0 (0x0)

After:

fsync(18) = 0 (0x0)
fdatasync(18) = 0 (0x0)

truss install -S foo /tmp/foo

See how the fsync() call (caused by -S) is displayed.