c printf("%x"), what does it print?

Question

I type this code in c: __asm__("mov $10, %rsi"); printf("%x"), It print a.
I debug it in gdb, found that the result store int register esi.
QUESTION: why the result is esi?

Answer 1

printf("%x") tries to get a second argument, but it isn't there, so it just reads the memory where it should have been, finds whatever garbage is there, and prints it.

In short: it is undefined-behavior.

EDIT: the reason you see the same value as in the esi register, is because the System V AMD64 ABI (which is the spec most Unixes follow) passes the first few arguments through registers when possible. The second argument is passed through rsi, therefore that's where printf is reading from.

Answer 2

When calling printf("%x"), it happens, that the arguments which it needs are pushed to the stack (in reverse order) and eventually, the function is called using assembly command call. When providing your format string "%x", printf() expects at least ONE argument following, so it will read the value next on the stack, which could be anything...
So this behavior is actually undefined and often the cause for exploits, because you can corrupt the stack.

Maybe a little excourse on this topic.