Non null-terminated string reading standard input (Assembly Language)

Question

I'm working on making a standard library from scratch in assembly language using NASM. I'm not doing this to use it in huge project, but just to train myself with assembly language making a project that I can put on GitHub.

Anyway, I'm facing something strange with the SYS_READ syscall. When I type a string in the standard input and I try to create a file with it as filename, it creates the file with some strange characters after it.

As you can see, I'm trying to create a Hello.txt file. The program was able to do so, but it added a dollar sign and a line return. I don't know why. I reserved 10 bytes in the .bss section as my filename buffer, and I'm only using 8 bytes of it. I tried to use the full length and more to see what happened.

BUT, when I use GDB to see what's happening, there is not trace of a $ symbol. I don't know where it comes from :

When I'm only using the full length, the file is made without weird symbols. When I use more, it acts the same, but with an overflow on the console (obviously).

I'm working on a 64bits Windows Subsystem Linux. The repo is at this address : https://github.com/Ximaz/nasm-stdlib . The issue I'm facing is stored into the main.asm file, at the root of the project, and all functions used are into the stdlib folder. I'm trying to not use libc functions such as scanf or else, because I want to do it from scratch + this issue can help me to learn more that I think.

Thanks in advance.

Answer 1

Thanks to Jester :

I didn't notice that the stdin call returned the read bytes into rax, which is what I need to put a termination byte. Since the string contains the \n, I had to decrement it before puting the null byte at the \n position.