在程序集中调用过程时是否需要使用括号？

Question

I am just starting to learn Assembly (x86 NASM) and I am currently going over function calls. Wherever I looked on the internet I saw everyone calling functions like this:

call power

Where power is the label where the function starts. But what I am trying to see is how to print something in Assembly, and interestingly enough, calling a function like in the above case doesn't seem to work. We'll use the printf function from C. Say I already used extern printf and import printf msvcrt.dll in my program (so I can actually use printf ), also say I already defined a symbol in my data segment msg db "Hello World", 0 and now I am trying to print this message. If I do this:

push dword msg
call printf

Nothing happens, it doesn't work. I have no idea why. However, if I do this:

push dword msg
call [printf]

The message is printed just as expected.

This doesn't make much sense to me after all the articles that I read used just the label, without brackets. It also made a lot of sense to me when using just the label as we're using the call instruction to perform a jump to that label, so we needed the address of the label. But here it doesn't make sense at all to me why we're using the brackets and what exactly happens. I mean, what is [printf] and what would [power] be, for the example I presented at the start of my question. However, despite my confusion, this is what works and the method I initially used doesn't work.

Can you please tell me exactly what is going on? (PS: I am using Olly Debugger if that makes any difference)

Answer 1

It depends on what is "printf" in your assembly. If it is a function pointer (aka, the address of some function is stored at the address named "printf"), then you need brackets []. If "printf" is a function, that is, if the machine code is stored at the address that your assembler calls "printf", then you must not put brackets (or else you will probably end up with a segmentation fault, as the first 32 of 64 bits of machine code of "printf" probably don't accidentally contain an address of an executable code).