Assembly - File Management
File management in assembly language is used to organize data and files available on your device, modify them like create new files, edit, delete, modify, move, rename, read, write etc.
The system treats any input or output data as a stream of bytes. There are three standard file streams -
- Standard input (stdin),
- Standard output (stdout), and
- Standard error (stderr).
File descriptor and file pointer are used to write commands in file handler.
file descriptor - When a new file is created or an existing file is opened, the file descriptor is used to access the file. File descriptor is a 16-bit integer assigned to a file as a file ID.
The file descriptors of standard file streams - stdin, stdout and stderr are 0, 1 and 2 respectively.
file pointer - A file pointer specifies a location in a file for subsequent read/write operations, in terms of bytes. Each file is treated as a sequence of bytes. Each open file is associated with a file pointer that specifies an offset in bytes relative to the beginning of the file. When a file is opened, the file pointer is set to null.
File Handling System Calls
The following table briefly describes the system calls related to file handling −
| %eax | Name | %ebx | %ecx | %edx |
|---|---|---|---|---|
| 2 | sys_fork | struct pt_regs | - | - |
| 3 | sys_read | unsigned int | char * | size_t |
| 4 | sys_write | unsigned int | const char * | size_t |
| 5 | sys_open | const char * | int | int |
| 6 | sys_close | unsigned int | - | - |
| 8 | sys_creat | const char * | int | - |
| 19 | sys_lseek | unsigned int | off_t | unsigned int |
The steps required for using the system calls are same, as we discussed earlier −
- Put the system call number in the EAX register.
- Store the arguments to the system call in the registers EBX, ECX, etc.
- Call the relevant interrupt (80h).
- The result is usually returned in the EAX register.
Creating and Opening a File
For creating and opening a file, perform the following tasks −
- Put the system call sys_creat() number 8, in the EAX register.
- Put the filename in the EBX register.
- Put the file permissions in the ECX register.
The system call returns the file descriptor of the created file in the EAX register, in case of error, the error code is in the EAX register.
Opening an Existing File
For opening an existing file, perform the following tasks −- Put the system call sys_open() number 5, in the EAX register.
- Put the filename in the EBX register.
- Put the file access mode in the ECX register.
- Put the file permissions in the EDX register.
The system call returns the file descriptor of the created file in the EAX register, in case of error, the error code is in the EAX register.
Among the file access modes, most commonly used are: read-only (0), write-only (1), and read-write (2).
Reading from a File
For reading from a file, perform the following tasks −- Put the system call sys_read() number 3, in the EAX register.
- Put the file descriptor in the EBX register.
- Put the pointer to the input buffer in the ECX register.
- Put the buffer size, i.e., the number of bytes to read, in the EDX register.
The system call returns the number of bytes read in the EAX register, in case of error, the error code is in the EAX register.
Writing to a File
For writing to a file, perform the following tasks −- Put the system call sys_write() number 4, in the EAX register.
- Put the file descriptor in the EBX register.
- Put the pointer to the output buffer in the ECX register.
- Put the buffer size, i.e., the number of bytes to write, in the EDX register.
The system call returns the actual number of bytes written in the EAX register, in case of error, the error code is in the EAX register.
Closing a File
For closing a file, perform the following tasks −- Put the system call sys_close() number 6, in the EAX register.
- Put the file descriptor in the EBX register.
The system call returns, in case of error, the error code in the EAX register.
Updating a File
For updating a file, perform the following tasks −- Put the system call sys_lseek () number 19, in the EAX register.
- Put the file descriptor in the EBX register.
- Put the offset value in the ECX register.
- Put the reference position for the offset in the EDX register.
The reference position could be:
- Beginning of file - value 0
- Current position - value 1
- End of file - value 2
The system call returns, in case of error, the error code in the EAX register
Exampal
The following program creates and opens a file named myfile.txt, and writes a text 'Welcome to Dooseep.com' in this file. Next, the program reads from the file and stores the data into a buffer named info. Lastly, it displays the text as stored in info. −
section .text
global _start ;must be declared for using gcc
_start: ;tell linker entry point
;create the file
mov eax, 8
mov ebx, file_name
mov ecx, 0777 ;read, write and execute by all
int 0x80 ;call kernel
mov [fd_out], eax
; write into the file
mov edx,len ;number of bytes
mov ecx, msg ;message to write
mov ebx, [fd_out] ;file descriptor
mov eax,4 ;system call number (sys_write)
int 0x80 ;call kernel
; close the file
mov eax, 6
mov ebx, [fd_out]
; write the message indicating end of file write
mov eax, 4
mov ebx, 1
mov ecx, msg_done
mov edx, len_done
int 0x80
;open the file for reading
mov eax, 5
mov ebx, file_name
mov ecx, 0 ;for read only access
mov edx, 0777 ;read, write and execute by all
int 0x80
mov [fd_in], eax
;read from file
mov eax, 3
mov ebx, [fd_in]
mov ecx, info
mov edx, 26
int 0x80
; close the file
mov eax, 6
mov ebx, [fd_in]
int 0x80
; print the info
mov eax, 4
mov ebx, 1
mov ecx, info
mov edx, 26
int 0x80
mov eax,1 ;system call number (sys_exit)
int 0x80 ;call kernel
section .data
file_name db 'myfile.txt'
msg db 'Welcome to Dooseep.com'
len equ $-msg
msg_done db 'Written to file', 0xa
len_done equ $-msg_done
section .bss
fd_out resb 1
fd_in resb 1
info resb 26 result −
Written to file
Welcome to Dooseep.com 
