问题
I'm in a gdb session to analyze a postmortem crash. I'm looking at disassemble output for a function and I see this:
=> 0x00007f8d354aed52 <+50>: callq *(%rax)
The => indicates that this was the instruction called at the time of the crash. So I got a seg fault calling the function at *(%rax). I'm pretty new to assembly. I see that parens around a register mean to deference (get the value at) that address. Thus (%rax) means to get the value of the pointer currently stored in %rax. What does the star decoration do on that? Does that further dereference that value (thus (%rax) is itself a pointer)? I'm having trouble googling *( assembly syntax.
This is x64 assembly generated from GCC 4.8 compiling C++ code.
回答1:
The asterisk indicates that the call is an indirect call. This is to distinguish call foo (call function foo) from call *foo (call function stored in variable foo). The instruction callq *(%rax) loads a quad word (64 bits) from the address stored in rax and calls the function beginning at that quad word.
Refer to the GNU assembler manual for details on the syntax.
来源：https://stackoverflow.com/questions/56979796/what-does-callq-rax-mean