As a student of Zen, I like the idea of a one-pointed mind: Do one thing at a time, and do it well.
This, indeed, is very much how UNIX® works as well. While a typical Windows® application is attempting to do everything imaginable (and is, therefore, riddled with bugs), a typical UNIX® program does only one thing, and it does it well.
The typical UNIX® user then essentially assembles his own applications by writing a shell script which combines the various existing programs by piping the output of one program to the input of another.
When writing your own UNIX® software, it is generally a good idea to see what parts of the problem you need to solve can be handled by existing programs, and only write your own programs for that part of the problem that you do not have an existing solution for.
I will illustrate this principle with a specific real-life example I was faced with recently:
I needed to extract the 11th field of each record from a database I downloaded from a web site. The database was a CSV file, i.e., a list of comma-separated values. That is quite a standard format for sharing data among people who may be using different database software.
The first line of the file contains the list of various fields separated by commas. The rest of the file contains the data listed line by line, with values separated by commas.
I tried awk, using the comma as a separator. But because several lines contained a quoted comma, awk was extracting the wrong field from those lines.
Therefore, I needed to write my own software to extract the 11th field from the CSV file. However, going with the UNIX® spirit, I only needed to write a simple filter that would do the following:
Remove the first line from the file;
Change all unquoted commas to a different character;
Remove all quotation marks.
Strictly speaking, I could use sed to remove the first line from the file, but doing so in my own program was very easy, so I decided to do it and reduce the size of the pipeline.
At any rate, writing a program like this took me about 20 minutes. Writing a program that extracts the 11th field from the CSV file would take a lot longer, and I could not reuse it to extract some other field from some other database.
This time I decided to let it do a little more work than a typical tutorial program would:
It parses its command line for options;
It displays proper usage if it finds wrong arguments;
It produces meaningful error messages.
Here is its usage message:
Usage: csv [-t<delim>] [-c<comma>] [-p] [-o <outfile>] [-i <infile>]
All parameters are optional, and can appear in any order.
The -t parameter declares what to replace
the commas with. The tab is the default here.
For example, -t; will replace all unquoted
commas with semicolons.
I did not need the -c option, but it may
come in handy in the future. It lets me declare that I want a
character other than a comma replaced with something else.
For example, -c@ will replace all at signs
(useful if you want to split a list of email addresses
to their user names and domains).
The -p option preserves the first line, i.e.,
it does not delete it. By default, we delete the first
line because in a CSV file it contains the field
names rather than data.
The -i and -o
options let me specify the input and the output files. Defaults
are stdin and stdout,
so this is a regular UNIX® filter.
I made sure that both -i filename and
-ifilename are accepted. I also made
sure that only one input and one output files may be
specified.
To get the 11th field of each record, I can now do:
%csv '-t;'data.csv| awk '-F;' '{print $11}'
The code stores the options (except for the file descriptors)
in EDX: The comma in DH, the new
separator in DL, and the flag for
the -p option in the highest bit of
EDX, so a check for its sign will give us a
quick decision what to do.
Here is the code:
;;;;;;; csv.asm ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;
; Convert a comma-separated file to a something-else separated file.
;
; Started: 31-May-2001
; Updated: 1-Jun-2001
;
; Copyright (c) 2001 G. Adam Stanislav
; All rights reserved.
;
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
%include 'system.inc'
%define BUFSIZE 2048
section .data
fd.in dd stdin
fd.out dd stdout
usg db 'Usage: csv [-t<delim>] [-c<comma>] [-p] [-o <outfile>] [-i <infile>]', 0Ah
usglen equ $-usg
iemsg db "csv: Can't open input file", 0Ah
iemlen equ $-iemsg
oemsg db "csv: Can't create output file", 0Ah
oemlen equ $-oemsg
section .bss
ibuffer resb BUFSIZE
obuffer resb BUFSIZE
section .text
align 4
ierr:
push dword iemlen
push dword iemsg
push dword stderr
sys.write
push dword 1 ; return failure
sys.exit
align 4
oerr:
push dword oemlen
push dword oemsg
push dword stderr
sys.write
push dword 2
sys.exit
align 4
usage:
push dword usglen
push dword usg
push dword stderr
sys.write
push dword 3
sys.exit
align 4
global _start
_start:
add esp, byte 8 ; discard argc and argv[0]
mov edx, (',' << 8) | 9
.arg:
pop ecx
or ecx, ecx
je near .init ; no more arguments
; ECX contains the pointer to an argument
cmp byte [ecx], '-'
jne usage
inc ecx
mov ax, [ecx]
.o:
cmp al, 'o'
jne .i
; Make sure we are not asked for the output file twice
cmp dword [fd.out], stdout
jne usage
; Find the path to output file - it is either at [ECX+1],
; i.e., -ofile --
; or in the next argument,
; i.e., -o file
inc ecx
or ah, ah
jne .openoutput
pop ecx
jecxz usage
.openoutput:
push dword 420 ; file mode (644 octal)
push dword 0200h | 0400h | 01h
; O_CREAT | O_TRUNC | O_WRONLY
push ecx
sys.open
jc near oerr
add esp, byte 12
mov [fd.out], eax
jmp short .arg
.i:
cmp al, 'i'
jne .p
; Make sure we are not asked twice
cmp dword [fd.in], stdin
jne near usage
; Find the path to the input file
inc ecx
or ah, ah
jne .openinput
pop ecx
or ecx, ecx
je near usage
.openinput:
push dword 0 ; O_RDONLY
push ecx
sys.open
jc near ierr ; open failed
add esp, byte 8
mov [fd.in], eax
jmp .arg
.p:
cmp al, 'p'
jne .t
or ah, ah
jne near usage
or edx, 1 << 31
jmp .arg
.t:
cmp al, 't' ; redefine output delimiter
jne .c
or ah, ah
je near usage
mov dl, ah
jmp .arg
.c:
cmp al, 'c'
jne near usage
or ah, ah
je near usage
mov dh, ah
jmp .arg
align 4
.init:
sub eax, eax
sub ebx, ebx
sub ecx, ecx
mov edi, obuffer
; See if we are to preserve the first line
or edx, edx
js .loop
.firstline:
; get rid of the first line
call getchar
cmp al, 0Ah
jne .firstline
.loop:
; read a byte from stdin
call getchar
; is it a comma (or whatever the user asked for)?
cmp al, dh
jne .quote
; Replace the comma with a tab (or whatever the user wants)
mov al, dl
.put:
call putchar
jmp short .loop
.quote:
cmp al, '"'
jne .put
; Print everything until you get another quote or EOL. If it
; is a quote, skip it. If it is EOL, print it.
.qloop:
call getchar
cmp al, '"'
je .loop
cmp al, 0Ah
je .put
call putchar
jmp short .qloop
align 4
getchar:
or ebx, ebx
jne .fetch
call read
.fetch:
lodsb
dec ebx
ret
read:
jecxz .read
call write
.read:
push dword BUFSIZE
mov esi, ibuffer
push esi
push dword [fd.in]
sys.read
add esp, byte 12
mov ebx, eax
or eax, eax
je .done
sub eax, eax
ret
align 4
.done:
call write ; flush output buffer
; close files
push dword [fd.in]
sys.close
push dword [fd.out]
sys.close
; return success
push dword 0
sys.exit
align 4
putchar:
stosb
inc ecx
cmp ecx, BUFSIZE
je write
ret
align 4
write:
jecxz .ret ; nothing to write
sub edi, ecx ; start of buffer
push ecx
push edi
push dword [fd.out]
sys.write
add esp, byte 12
sub eax, eax
sub ecx, ecx ; buffer is empty now
.ret:
ret
Much of it is taken from hex.asm above. But there
is one important difference: I no longer call write
whenever I am outputting a line feed. Yet, the code can be
used interactively.
I have found a better solution for the interactive problem since I first started writing this chapter. I wanted to make sure each line is printed out separately only when needed. After all, there is no need to flush out every line when used non-interactively.
The new solution I use now is to call write every
time I find the input buffer empty. That way, when running in
the interactive mode, the program reads one line from the user's
keyboard, processes it, and sees its input buffer is empty. It
flushes its output and reads the next line.
This change prevents a mysterious lockup in a very specific case. I refer to it as the dark side of buffering, mostly because it presents a danger that is not quite obvious.
It is unlikely to happen with a program like the csv above, so let us consider yet another filter: In this case we expect our input to be raw data representing color values, such as the red, green, and blue intensities of a pixel. Our output will be the negative of our input.
Such a filter would be very simple to write. Most of it would look just like all the other filters we have written so far, so I am only going to show you its inner loop:
.loop: call getchar not al ; Create a negative call putchar jmp short .loop
Because this filter works with raw data, it is unlikely to be used interactively.
But it could be called by image manipulation software.
And, unless it calls write before each call
to read, chances are it will lock up.
Here is what might happen:
The image editor will load our filter using the
C function popen().
It will read the first row of pixels from a bitmap or pixmap.
It will write the first row of pixels to
the pipe leading to
the fd.in of our filter.
Our filter will read each pixel from its input, turn it to a negative, and write it to its output buffer.
Our filter will call getchar
to fetch the next pixel.
getchar will find an empty
input buffer, so it will call
read.
read will call the
SYS_read system call.
The kernel will suspend our filter until the image editor sends more data to the pipe.
The image editor will read from the
other pipe, connected to the
fd.out of our filter so it can set the first row of the
output image before
it sends us the second row of the input.
The kernel suspends the image editor until it receives some output from our filter, so it can pass it on to the image editor.
At this point our filter waits for the image editor to send it more data to process, while the image editor is waiting for our filter to send it the result of the processing of the first row. But the result sits in our output buffer.
The filter and the image editor will continue waiting for each other forever (or, at least, until they are killed). Our software has just entered a race condition.
This problem does not exist if our filter flushes its output buffer before asking the kernel for more input data.
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