11.12 One-Pointed Mind

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.

11.12.1 CSV

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:

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:

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.

11.12.1.1 The Dark Side of Buffering

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:

  1. The image editor will load our filter using the C function popen().

  2. It will read the first row of pixels from a bitmap or pixmap.

  3. It will write the first row of pixels to the pipe leading to the fd.in of our filter.

  4. Our filter will read each pixel from its input, turn it to a negative, and write it to its output buffer.

  5. Our filter will call getchar to fetch the next pixel.

  6. getchar will find an empty input buffer, so it will call read.

  7. read will call the SYS_read system call.

  8. The kernel will suspend our filter until the image editor sends more data to the pipe.

  9. 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.

  10. 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.