i call glibc function, for example, printf:

mov $some_string, %rdi
call printf   //<----crash

in some cases, such a call will crash. I add xor %rax, %rax this solves the problem.

I have two questions:

1. Why, in some cases, calling printf without xor %rax leads to a crash, and in some it does not?

2. Somewhere I saw such a call printf:

   lea  some_string(%rip), %rdi
   xor %eax, %eax
   call printf
   

Why this used rip register and how is this passing of an argument different from the usual?