Creating and Writing Files

CREATING AND WRITING FILES

Code Example

section .data
    pathname db "path/to/file.txt"
    msg db "Hello, World!", 0AH,0DH, "$"

section .text
    global main

main:
    ; open file
    mov eax, 5
    mov ebx, pathname

    ; create file if it doesn't exist
    mov ecx, 101o
    mov edx, 0700o
    int 80h

    ; write to file
    mov ebx, eax
    mov eax, 4
    mov ecx, msg
    mov edx, 16

    ; exit program
    mov eax, 1
    mov ebx, 0
    int 80h

Create and Read Flags

I need to combine flags using the or operator or use the combined value directly. For example, 101o is equivalent to O_CREAT | O_WRONLY.

    O_CREAT  = 0100
    O_WRONLY = 0001
 or ---------------
               0101

The way I declare 0101 in octal:

Exclude the leading zero 101;
Append a lowercase o to the number 101o to indicate octal format;

Visual Studio Code usually shows the value directly if I hover over the flag or I can check these values directly in stat.h.

File Permission Flags

    S_IRUSR = 0400
    S_IWUSR = 0200
    S_IXUSR = 0100
 or --------------
              0700

S_IRUSR is the read permission for the owner;
S_IWUSR is the write permission for the owner;
S_IXUSR is the execute permission for the owner;
0700 is the combination of these permissions;

To sum it up, or is adding the values together.

Open/Create File

eax register with the value 5 to indicate the open syscall;
ebx register with the file path;
ecx register with the value 101o for flags create and write;
edx register with the value 0700o for permissions like rwx;
int 80h to invoke the syscall;

Write to File

eax register with the value 4 to indicate the write syscall;
ebx register with the file descriptor eax obtained from the open syscall;
ecx register with the data to be written msg;
edx register with the size of the data to write 16;

Exit Program

eax register with the value 1 to indicate the exit syscall;
ebx register with the value 0 to indicate successful execution;