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13.8: File Operations Examples

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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.

    13.8: File Operations Examples is shared under a not declared license and was authored, remixed, and/or curated by LibreTexts.

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