- Assembly - Memory Management
- Assembly - File Management
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- Assembly - Recursion
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- Assembly - Logical Instructions
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- Assembly - Constants
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- Assembly - Addressing Modes
- Assembly - System Calls
- Assembly - Registers
- Assembly - Memory Segments
- Assembly - Basic Syntax
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Assembly - File Management
The system considers any input or output data as stream of bytes. There are three standard file streams −
Standard input (stdin),
Standard output (stdout), and
Standard error (stderr).
File Descriptor
A file descriptor is a 16-bit integer assigned to a file as a file id. When a new file is created or an existing file is opened, the file descriptor is used for accessing the file.
File descriptor of the standard file streams - stdin, stdout and stderr are 0, 1 and 2, respectively.
File Pointer
A file pointer specifies the location for a subsequent read/write operation in the file in terms of bytes. Each file is considered 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 zero.
File Handpng System Calls
The following table briefly describes the system calls related to file handpng −
%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 earper −
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.
Example
The following program creates and opens a file named myfile.txt, and writes a text Welcome to Tutorials Point 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 pnker 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 Tutorials Point 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
When the above code is compiled and executed, it produces the following result −
Written to file Welcome to Tutorials PointAdvertisements