13.8: File Operations Examples
- Page ID
- 58404
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This section contains some simple example programs to demonstrate very basic file I/O operations. The more complex issues regarding file I/O buffering are addressed in a subsequent chapter.
Example, File Write
This example program writes a short message to a file. The file created contains a simple message, a URL in this example. The file name and message to be written to the file are hard-coded. This helps simplify the example, but is not realistic.
Since the open/create service is used, the file will be created (even if an old version must be overwritten).
; Example program to demonstrate file I/O. This example
; will open/create a file, write some information to the
; file, and close the file. Note, the file name and
; write message are hard-coded for the example.
section .data
; -----
; Define standard constants.
LF equ 10 ; line feed
NULL equ 0 ; end of string
TRUE equ 1
FALSE equ 0
EXIT_SUCCESS equ 0 ; success code
STDIN equ 0 ; standard input
STDOUT equ 1 ; standard output
STDERR equ 2 ; standard error
SYS_read equ 0 ; read
SYS_write equ 1 ; write
SYS_open equ 2 ; file open
SYS_close equ 3 ; file close
SYS_fork equ 57 ; fork
SYS_exit equ 60 ; terminate
SYS_creat equ 85 ; file open/create
SYS_time equ 201 ; get time
O_CREAT equ 0x40
O_TRUNC equ 0x200
O_APPEND equ 0x400
O_RDONLY equ 000000q ; read only
O_WRONLY equ 000001q ; write only
O_RDWR equ 000002q ; read and write
S_IRUSR equ 00400q
S_IWUSR equ 00200q
S_IXUSR equ 00100q
; -----
; Variables for main.
newLine db LF, NULL
header db LF, "File Write Example."
db LF, LF, NULL
fileName db "url.txt", NULL
url db "http://www.google.com"
db LF, NULL
len dq $-url-1
writeDone db "Write Completed.", LF, NULL
fileDesc dq 0
errMsgOpen db "Error opening file.", LF, NULL
errMsgWrite db "Error writing to file.", LF, NULL
;--------------------------------------------------------
section .text
global _start
_start:
; -----
; Display header line...
mov rdi, header
call printString
; -----
; Attempt to open file.
; Use system service for file open
; System Service - Open/Create
; rax = SYS_creat (file open/create)
; rdi = address of file name string
; rsi = attributes (i.e., read only, etc.)
; Returns:
; if error -> eax < 0
; if success -> eax = file descriptor number
; The file descriptor points to the File Control
; Block (FCB). The FCB is maintained by the OS.
; The file descriptor is used for all subsequent
; file operations (read, write, close).
openInputFile:
mov rax, SYS_creat ; file open/create
mov rdi, fileName ; file name string
mov rsi, S_IRUSR | S_IWUSR ; allow read/write
syscall ; call the kernel
cmp rax, 0 ; check for success
jl errorOnOpen
mov qword [fileDesc], rax ; save descriptor
; -----
; Write to file.
; In this example, the characters to write are in a
; predefined string containing a URL.
; System Service - write
; rax = SYS_write
; rdi = file descriptor
; rsi = address of characters to write
; rdx = count of characters to write ; Returns:
; if error -> rax < 0
; if success -> rax = count of characters actually read
mov rax, SYS_write
mov rdi, qword [fileDesc]
mov rsi, url
mov rdx, qword [len]
syscall
cmp rax, 0
jl errorOnWrite
mov rdi, writeDone
call printString
; -----
; Close the file.
; System Service - close
; rax = SYS_close
; rdi = file descriptor
mov rax, SYS_close
mov rdi, qword [fileDesc]
syscall
jmp exampleDone
; -----
; Error on open.
; note, rax contains an error code which is not used
; for this example.
errorOnOpen:
mov rdi, errMsgOpen
call printString
jmp exampleDone
; -----
; Error on write.
; note, rax contains an error code which is not used
; for this example.
errorOnWrite:
mov rdi, errMsgWrite
call printString
jmp exampleDone
; -----
; Example program done.
exampleDone:
mov rax, SYS_exit
mov rdi, EXIT_SUCCESS syscall
; **********************************************************
; Generic function to display a string to the screen.
; String must be NULL terminated.
; Algorithm:
; Count characters in string (excluding NULL)
; Use syscall to output characters
; Arguments:
; 1) address, string
; Returns: nothing
global printString
printString:
push rbp
mov rbp, rsp
push rbx
; Count characters in string.
mov rbx, rdi
mov rdx, 0
strCountLoop:
cmp byte [rbx], NULL
je strCountDone
inc rdx
inc rbx
jmp strCountLoop
strCountDone:
cmp rdx, 0
je prtDone
; Call OS to output string.
mov rax, SYS_write ; code for write ()
mov rsi, rdi ; addr of characters
mov rdi, STDOUT ; file descriptor
; count set above
syscall ; system call
; String printed, return to calling routine.
prtDone:
pop rbx
pop rbp
ret
; *******************************************************
This example creates the file which is read by the next example.
13.8.2 Example, File Read
This example will read a file. The file to be read contains a simple message, the URL from the previous example. The file name is hard-coded which helps simplify the example, but is not realistic. The file name used matches the previous file write example. If this example program is executed prior to the write example program being executed, it will generate an error since the file will not be found. After the file write example program is executed, this file read example program will read the file and display the contents.
; Example program to demonstrate file I/O.
; This example will open/create a file, write some
; information to the file, and close the file.
; Note, the file name is hard-coded for this example.
; This example program will open a file, read the
; contents, and write the contents to the screen.
; This routine also provides some very simple examples
; regarding handling various errors on system services.
; -------------------------------------------------------
section .data
; -----
; Define standard constants.
LF equ 10 ; line feed
NULL equ 0 ; end of string
TRUE equ 1
FALSE equ 0
EXIT_SUCCESS equ 0 ; success code
STDIN equ 0 ; standard input
STDOUT equ 1 ; standard output
STDERR equ 2 ; standard error
SYS_read equ 0 ; read
SYS_write equ 1 ; write
SYS_open equ 2 ; file open
SYS_close equ 3 ; file close
SYS_fork equ 57 ; fork
SYS_exit equ 60 ; terminate
SYS_creat equ 85 ; file open/create
SYS_time equ 201 ; get time
O_CREAT equ 0x40
O_TRUNC equ 0x200
O_APPEND equ 0x400
O_RDONLY equ 000000q ; read only
O_WRONLY equ 000001q ; write only
O_RDWR equ 000002q ; read and write
S_IRUSR equ 00400q
S_IWUSR equ 00200q
S_IXUSR equ 00100q
; -----
; Variables/constants for main.
BUFF_SIZE equ 255
newLine db LF, NULL
header db LF, "File Read Example."
db LF, LF, NULL
fileName db "url.txt", NULL
fileDes c dq 0
errMsgOpen db "Error opening the file.", LF, NULL
errMsgRead db "Error reading from the file.", LF, NULL
; -------------------------------------------------------
section .bss
readBuffer resb BUFF_SIZE
; -------------------------------------------------------
section .text
global _start
_start:
; -----
; Display header line...
mov rdi, header
call printString
; -----
; Attempt to open file - Use system service for file open
; System Service - Open
; rax = SYS_open
; rdi = address of file name string
; rsi = attributes (i.e., read only, etc.)
; Returns:
; if error -> eax < 0
; if success -> eax = file descriptor number
; The file descriptor points to the File Control
; Block (FCB). The FCB is maintained by the OS.
; The file descriptor is used for all subsequent file
; operations (read, write, close).
openInputFile:
mov rax, SYS_open ; file open
mov rdi, fileName ; file name string
mov rsi, O_RDONLY ; read only access
syscall ; call the kernel
cmp rax, 0 ; check for success
jl errorOnOpen
mov qword [fileDesc], rax ; save descriptor
; -----
; Read from file.
; For this example, we know that the file has only 1 line.
; System Service - Read
; rax = SYS_read
; rdi = file descriptor
; rsi = address of where to place data
; rdx = count of characters to read
; Returns:
; if error -> rax < 0
; if success -> rax = count of characters actually read
mov rax, SYS_read
mov rdi, qword [fileDesc]
mov rsi, readBuffer
mov rdx, BUFF_SIZE
syscall
cmp rax, 0
jl errorOnRead
; -----
; Print the buffer.
; add the NULL for the print string
mov rsi, readBuffer
mov byte [rsi+rax], NULL
mov rdi, readBuffer
call printString
printNewLine
; -----
; Close the file.
; System Service - close
; rax = SYS_close
; rdi = file descriptor
mov rax, SYS_close
mov rdi, qword [fileDesc]
syscall
jmp exampleDone
; -----
; Error on open.
; note, eax contains an error code which is not used
; for this example.
errorOnOpen:
mov rdi, errMsgOpen
call printString
jmp exampleDone
; -----
; Error on read.
; note, eax contains an error code which is not used ; for this example.
errorOnRead:
mov rdi, errMsgRead
call printString
jmp exampleDone
; -----
; Example program done.
exampleDone:
mov rax, SYS_exit
mov rdi, EXIT_SUCCESS
syscall
; **********************************************************
; Generic procedure to display a string to the screen.
; String must be NULL terminated.
; Algorithm:
; Count characters in string (excluding NULL)
; Use syscall to output characters
; Arguments:
; 1) address, string
; Returns:
; nothing
global printString
printString:
push rbp
mov rbp, rsp
push rbx
; -----
; Count characters in string.
mov rbx, rdi
mov rdx, 0
strCountLoop:
cmp byte [rbx], NULL
je strCountDone
inc rdx
inc rbx
jmp strCountLoop
strCountDone:
cmp rdx, 0
je prtDone
; -----
; Call OS to output string.
mov rax, SYS_write ; code for write ()
mov rsi, rdi ; addr of characters
mov rdi, STDOUT ; file descriptor
; count set above
syscall ; system call
; -----
; String printed, return to calling routine.
prtDone:
pop rbx
pop rbp
ret
The printString() function is the exact same in both examples and is only repeated to allow each program to be assembled and executed independently.