CODEWITHSUNDEEP
assemblynasm
BugHunterUK
BugHunterUK

Reputation: 8958

Create a directory with sys_mkdir. Directories end with?

I'm learning assembly (NASM) and using the Linux system calls.

I'm having a problem with the following code. I am attempting to call sys_mkdir. A directory "Hello World" should be created. But, instead it's creating the following Hello World?asmtest??. Why is this? How is the title being included and how do I remove the ?.

SECTION .data
msg:    db "Hello World", 10
len:    equ $-msg

title: db "asmtest", 7
mode:   dd 755

SECTION .text
global main

main:
    ; Make a directory            
    mov ecx,mode
    mov ebx,msg
    mov eax,39
    int 0x80

    ; Print Hello World to screen
    mov edx,len
    mov ecx,msg
    mov ebx,1
    mov eax,4
    int 0x80

    ; Exit
    mov ebx,0
    mov eax,1
    int 0x80

Upvotes: 1

Views: 1168

Answers (1)

David Hoelzer
David Hoelzer

Reputation: 16361

In this case, the function call that you are accessing is a system call. While it's not part of the C standard library, the handling of strings in this call (and many like it) are very "C like", meaning that byte strings are always null terminated.

In your case, since you define two strings (with some special characters in between, like "10" and "7"), it will use everything from the very first character found at the memory location that msg points to and keep going until it happens to find a null byte (00).

If you replace your , 10 with , 0 the sys_mkdir will function properly, but you will have to adjust that byte for your print function to work properly.

Upvotes: 5

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