Hi !
Yes ! That's what I wanted to know :-)
Thank you
Date: Thu, 22 May 2008 23:05:50 -0700
From: [EMAIL PROTECTED]
To: [EMAIL PROTECTED]
Subject: Re: [Pgubook-readers] Chapter 5, toupper-program: understanding the
read loop
CC: [email protected]
Hello,
If I understand you correctly, you are asking why isn't the same N (buffer
size) bytes of the file read over an over again, because there is no explicit
"move the file position pointer forward" in the program.
This is handled "under the covers" by the Linux system call "read."
.equ SYS_READ, 3
>From the documentation:
read()
attempts to read up to
count
bytes from file descriptor
fd
into the buffer starting at
buf.
"
On success, the number of bytes read is returned (zero indicates end of
file), and the file position is advanced by this number."
http://linux.about.com/od/commands/l/blcmdl2_read.htm
Justis
On Sat, May 17, 2008 at 6:10 AM, david helfgott <[EMAIL PROTECTED]> wrote:
Hi there !
I got some serious problems with understanding the read loop in the toupper
programm.
Where does the computer know to start again from, after the "jmp read_loop
begin" ?
Why doesn't the computer always reads the same first 500 bytes into the buffer ?
Where does he knows from, where he has to continue ?
after the "jmp read_loop begin" he starts again at
movl $SYS_READ, %eax
movl $ST_FD_IN(%ebp), %ebx
movl $BUFFER_DATA, %ecx
movl $BUFFER_SIZE, %edx
int $LINUX_SYSCALL
that's exactly the same information he got in the first place.
So if I have a file conatining 700 bytes.
Where does he know from to start reading at byte No. 501 in the secon run, and
not just read the first 500 bytes again ?
Where's the extra information, that tells him, he already red the first 500
bytes and has to read the next 500 byte ?
####################
read_loop_begin:
###READ IN A BLOCK FROM THE INPUT FILE###
movl $SYS_READ, %eax
#get the input file descriptor
movl ST_FD_IN(%ebp), %ebx
#the location to read into
movl $BUFFER_DATA, %ecx
#the size of the buffer
movl $BUFFER_SIZE, %edx
#Size of buffer read is returned in %eax
int $LINUX_SYSCALL
###EXIT IF WE'VE REACHED THE END###
#check for end of file marker
cmpl $END_OF_FILE, %eax
#if found or on error, go to the end
jle end_loop
continue_read_loop:
###CONVERT THE BLOCK TO UPPER CASE###
pushl $BUFFER_DATA #location of buffer
pushl %eax #size of the buffer
call convert_to_upper
popl %eax #get the size back
addl $4, %esp #restore %esp
###WRITE THE BLOCK OUT TO THE OUTPUT FILE###
#size of the buffer
movl %eax, %edx
movl $SYS_WRITE, %eax
#file to use
movl ST_FD_OUT(%ebp), %ebx
#location of the buffer
movl $BUFFER_DATA, %ecx
int $LINUX_SYSCALL
###CONTINUE THE LOOP###
jmp read_loop_begin
end_loop:
###CLOSE THE FILES###
#NOTE - we don't need to do error checking
# on these, because error conditions
# don't signify anything special here
movl $SYS_CLOSE, %eax
movl ST_FD_OUT(%ebp), %ebx
int $LINUX_SYSCALL
movl $SYS_CLOSE, %eax
movl ST_FD_IN(%ebp), %ebx
int $LINUX_SYSCALL
###EXIT###
movl $SYS_EXIT, %eax
movl $0, %ebx
int $LINUX_SYSCALL
#######################
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