File Handling
In this example we will study how to create a new file, open an existing one, read from a file, write in a file and close a file.File Descriptor
A file descriptor acts as a file id. It is a 16-bit integer assigned when a new file is created or a file is opened. It is generated in eax register.file.asm and follow along -
global _start ; must be declared for global
section .data
file_name db "data.txt", 0 ; Defining file name
msg db "This is NASM tutorial.", 0x0a ; The message
len equ $ - msg ; len equals size of msg
section .bss
fd_out resb 1 ; Unitialized data variable, file descriptor for write
fd_in resb 1 ; Unitialized data variable, file descriptor for read
info resb 26 ; Unitialized data variable info
section .text
_start: ; start label
mov eax, 8 ; sys_creat system call
mov ebx, file_name ; file name as second parameter
mov ecx, 0777 ; Setting permission for read, write and execute by all in third parameter
int 0x80 ; Perform the system call
mov [fd_out], eax ; Reading the file descriptor in fd_out from eax
mov eax, 4 ; sys_write system call
mov ebx, [fd_out] ; file descriptor fd_out
mov ecx, msg ; ecx=msg
mov edx, len ; edx=len
int 0x80 ; Perform the system call
mov eax, 6 ; sys_close system call
mov ebx, [fd_out] ; file descriptor fd_out
mov eax, 5 ; sys_open system call
mov ebx, file_name ; Setting file name to be opened
mov ecx, 0 ; for read only access
mov edx, 0777 ; Setting permission for read, write and execute by all
int 0x80 ; Perform the system call
mov [fd_in], eax ; Reading file descriptor of opened file in fd_in
mov eax, 3 ; sys_read system call
mov ebx, [fd_in] ; file descriptor fd_in
mov ecx, info ; Reading in info
mov edx, 26
int 0x80 ; Perform the system call
mov eax, 6 ; sys_close system call
mov ebx, [fd_in] ; file descriptor fd_in
mov eax, 4 ; sys_write system call
mov ebx, 1 ; stdout file descriptor
mov ecx, info ; ecx=ans
mov edx, 26
int 0x80 ; Perform the system call
mov eax , 1 ; sys_exit system call
mov ebx , 0 ; setting exit status
int 0x80 ; Calling interrupt handler to exit program
>> nasm -f elf32 file.asm -o file.o
>> ld -m elf_i386 file.o -o file
>> ./file
>> This is NASM tutorial.