We have already done some basic file work: We know how to open and close them, how to read and write them using buffers. But UNIX® offers much more functionality when it comes to files. We will examine some of it in this section, and end up with a nice file conversion utility.
Indeed, let us start at the end, that is, with the file conversion utility. It always makes programming easier when we know from the start what the end product is supposed to do.
One of the first programs I wrote for UNIX was tuc, a text-to-UNIX file converter. It converts a text file from other operating systems to a UNIX text file. In other words, it changes from different kind of line endings to the newline convention of UNIX. It saves the output in a different file. Optionally, it converts a UNIX text file to a DOS text file.
I have used tuc extensively, but always only to convert from some other OS to UNIX, never the other way. I have always wished it would just overwrite the file instead of me having to send the output to a different file. Most of the time, I end up using it like this:
% tuc myfile tempfile % mv tempfile myfile
It would be nice to have a ftuc, i.e., fast tuc, and use it like this:
% ftuc myfile
In this chapter, then, we will write ftuc in assembly language (the original tuc is in C), and study various file-oriented kernel services in the process.
At first sight, such a file conversion is very simple: All you have to do is strip the carriage returns, right?
If you answered yes, think again: That approach will work most of the time (at least with MS DOS text files), but will fail occasionally.
The problem is that not all non UNIX text files end their line with the carriage return / line feed sequence. Some use carriage returns without line feeds. Others combine several blank lines into a single carriage return followed by several line feeds. And so on.
A text file converter, then, must be able to handle any possible line endings:
carriage return / line feed
carriage return
line feed / carriage return
line feed
It should also handle files that use some kind of a combination of the above (e.g., carriage return followed by several line feeds).
The problem is easily solved by the use of a technique called finite state machine, originally developed by the designers of digital electronic circuits. A finite state machine is a digital circuit whose output is dependent not only on its input but on its previous input, i.e., on its state. The microprocessor is an example of a finite state machine: Our assembly language code is assembled to machine language in which some assembly language code produces a single byte of machine language, while others produce several bytes. As the microprocessor fetches the bytes from the memory one by one, some of them simply change its state rather than produce some output. When all the bytes of the op code are fetched, the microprocessor produces some output, or changes the value of a register, etc.
Because of that, all software is essentially a sequence of state instructions for the microprocessor. Nevertheless, the concept of finite state machine is useful in software design as well.
Our text file converter can be designed as a finite state machine with three possible states. We could call them states 0-2, but it will make our life easier if we give them symbolic names:
ordinary
cr
lf
Our program will start in the ordinary state. During
this state, the program action depends on its input as follows:
If the input is anything other than a carriage return or line feed, the input is simply passed on to the output. The state remains unchanged.
If the input is a carriage return, the state is changed to cr. The input is then discarded, i.e., no output is made.
If the input is a line feed, the state is changed to lf. The input is then discarded.
Whenever we are in the cr state, it is because the
last input was a carriage return, which was unprocessed. What our software does in this
state again depends on the current input:
If the input is anything other than a carriage return or line feed, output a line
feed, then output the input, then change the state to ordinary.
If the input is a carriage return, we have received two (or more) carriage returns in a row. We discard the input, we output a line feed, and leave the state unchanged.
If the input is a line feed, we output the line feed and change the state to
ordinary. Note that this is not the same as the first case
above – if we tried to combine them, we would be outputting two line feeds instead
of one.
Finally, we are in the lf state after we have
received a line feed that was not preceded by a carriage return. This will happen when
our file already is in UNIX format, or whenever several
lines in a row are expressed by a single carriage return followed by several line feeds,
or when line ends with a line feed / carriage return sequence. Here is how we need to
handle our input in this state:
If the input is anything other than a carriage return or line feed, we output a
line feed, then output the input, then change the state to ordinary. This is exactly the same action as in the cr state upon receiving the same kind of input.
If the input is a carriage return, we discard the input, we output a line feed,
then change the state to ordinary.
If the input is a line feed, we output the line feed, and leave the state unchanged.
The above finite state machine works for the entire file, but leaves the possibility that the final line end will be ignored. That will happen whenever the file ends with a single carriage return or a single line feed. I did not think of it when I wrote tuc, just to discover that occasionally it strips the last line ending.
This problem is easily fixed by checking the state after the entire file was
processed. If the state is not ordinary, we simply need to
output one last line feed.
Note: Now that we have expressed our algorithm as a finite state machine, we could easily design a dedicated digital electronic circuit (a "chip") to do the conversion for us. Of course, doing so would be considerably more expensive than writing an assembly language program.
Because our file conversion program may be combining two characters into one, we
need to use an output counter. We initialize it to 0, and
increase it every time we send a character to the output. At the end of the program, the
counter will tell us what size we need to set the file to.
The hardest part of working with a finite state machine is analyzing the problem and expressing it as a finite state machine. That accomplished, the software almost writes itself.
In a high-level language, such as C, there are several main approaches. One is to
use a switch statement which chooses what function should
be run. For example,
switch (state) {
default:
case REGULAR:
regular(inputchar);
break;
case CR:
cr(inputchar);
break;
case LF:
lf(inputchar);
break;
}
Another approach is by using an array of function pointers, something like this:
(output[state])(inputchar);
Yet another is to have state be a function pointer,
set to point at the appropriate function:
(*state)(inputchar);
This is the approach we will use in our program because it is very easy to do in
assembly language, and very fast, too. We will simply keep the address of the right
procedure in EBX, and then just issue:
call ebx
This is possibly faster than hardcoding the address in the code because the microprocessor does not have to fetch the address from the memory—it is already stored in one of its registers. I said possibly because with the caching modern microprocessors do, either way may be equally fast.
Because our program works on a single file, we cannot use the approach that worked for us before, i.e., to read from an input file and to write to an output file.
UNIX allows us to map a file, or a section of a
file, into memory. To do that, we first need to open the file with the appropriate
read/write flags. Then we use the mmap system call to map
it into the memory. One nice thing about mmap is that it
automatically works with virtual memory: We can map more of the file into the memory than
we have physical memory available, yet still access it through regular memory op codes,
such as mov, lods, and stos. Whatever changes we make to the memory image of the file
will be written to the file by the system. We do not even have to keep the file open: As
long as it stays mapped, we can read from it and write to it.
The 32-bit Intel microprocessors can access up to four gigabytes of memory – physical or virtual. The FreeBSD system allows us to use up to a half of it for file mapping.
For simplicity sake, in this tutorial we will only convert files that can be mapped into the memory in their entirety. There are probably not too many text files that exceed two gigabytes in size. If our program encounters one, it will simply display a message suggesting we use the original tuc instead.
If you examine your copy of syscalls.master, you will
find two separate syscalls named mmap. This is because of
evolution of UNIX: There was the traditional BSD mmap, syscall 71.
That one was superseded by the POSIX® mmap, syscall
197. The FreeBSD system supports both because older programs were written by using the
original BSD version. But new software uses
the POSIX
version, which is what we will use.
The syscalls.master file lists the POSIX version like this:
197 STD BSD { caddr_t mmap(caddr_t addr, size_t len, int prot, \
int flags, int fd, long pad, off_t pos); }
This differs slightly from what mmap(2) says. That is because mmap(2) describes the C version.
The difference is in the long pad argument, which is
not present in the C version. However, the FreeBSD syscalls add a 32-bit pad after pushing a 64-bit argument. In this case, off_t is a 64-bit value.
When we are finished working with a memory-mapped file, we unmap it with the
munmap syscall:
Tip: For an in-depth treatment of
mmap, see W. Richard Stevens' Unix Network Programming, Volume 2, Chapter 12.
Because we need to tell mmap how many bytes of the
file to map into the memory, and because we want to map the entire file, we need to
determine the size of the file.
We can use the fstat syscall to get all the
information about an open file that the system can give us. That includes the file
size.
Again, syscalls.master lists two versions of fstat, a traditional one (syscall 62), and a POSIX one (syscall
189). Naturally, we will use the POSIX version:
189 STD POSIX { int fstat(int fd, struct stat *sb); }
This is a very straightforward call: We pass to it the address of a stat structure and the descriptor of an open file. It will fill
out the contents of the stat structure.
I do, however, have to say that I tried to declare the stat structure in the .bss
section, and fstat did not like it: It set the carry flag
indicating an error. After I changed the code to allocate the structure on the stack,
everything was working fine.
Because our program may combine carriage return / line feed sequences into straight line feeds, our output may be smaller than our input. However, since we are placing our output into the same file we read the input from, we may have to change the size of the file.
The ftruncate system call allows us to do just
that. Despite its somewhat misleading name, the ftruncate
system call can be used to both truncate the file (make it smaller) and to grow it.
And yes, we will find two versions of ftruncate in
syscalls.master, an older one (130), and a newer one (201). We
will use the newer one:
201 STD BSD { int ftruncate(int fd, int pad, off_t length); }
Please note that this one contains a int pad
again.
We now know everything we need to write ftuc. We start by adding some new lines in system.inc. First, we define some constants and structures, somewhere at or near the beginning of the file:
;;;;;;; open flags %define O_RDONLY 0 %define O_WRONLY 1 %define O_RDWR 2 ;;;;;;; mmap flags %define PROT_NONE 0 %define PROT_READ 1 %define PROT_WRITE 2 %define PROT_EXEC 4 ;; %define MAP_SHARED 0001h %define MAP_PRIVATE 0002h ;;;;;;; stat structure struc stat st_dev resd 1 ; = 0 st_ino resd 1 ; = 4 st_mode resw 1 ; = 8, size is 16 bits st_nlink resw 1 ; = 10, ditto st_uid resd 1 ; = 12 st_gid resd 1 ; = 16 st_rdev resd 1 ; = 20 st_atime resd 1 ; = 24 st_atimensec resd 1 ; = 28 st_mtime resd 1 ; = 32 st_mtimensec resd 1 ; = 36 st_ctime resd 1 ; = 40 st_ctimensec resd 1 ; = 44 st_size resd 2 ; = 48, size is 64 bits st_blocks resd 2 ; = 56, ditto st_blksize resd 1 ; = 64 st_flags resd 1 ; = 68 st_gen resd 1 ; = 72 st_lspare resd 1 ; = 76 st_qspare resd 4 ; = 80 endstruc
We define the new syscalls:
%define SYS_mmap 197 %define SYS_munmap 73 %define SYS_fstat 189 %define SYS_ftruncate 201
We add the macros for their use:
%macro sys.mmap 0 system SYS_mmap %endmacro %macro sys.munmap 0 system SYS_munmap %endmacro %macro sys.ftruncate 0 system SYS_ftruncate %endmacro %macro sys.fstat 0 system SYS_fstat %endmacro
And here is our code:
;;;;;;; Fast Text-to-Unix Conversion (ftuc.asm) ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;; ;; ;; Started: 21-Dec-2000 ;; Updated: 22-Dec-2000 ;; ;; Copyright 2000 G. Adam Stanislav. ;; All rights reserved. ;; ;;;;;;; v.1 ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;; %include 'system.inc' section .data db 'Copyright 2000 G. Adam Stanislav.', 0Ah db 'All rights reserved.', 0Ah usg db 'Usage: ftuc filename', 0Ah usglen equ $-usg co db "ftuc: Can't open file.", 0Ah colen equ $-co fae db 'ftuc: File access error.', 0Ah faelen equ $-fae ftl db 'ftuc: File too long, use regular tuc instead.', 0Ah ftllen equ $-ftl mae db 'ftuc: Memory allocation error.', 0Ah maelen equ $-mae section .text align 4 memerr: push dword maelen push dword mae jmp short error align 4 toolong: push dword ftllen push dword ftl jmp short error align 4 facerr: push dword faelen push dword fae jmp short error align 4 cantopen: push dword colen push dword co jmp short error align 4 usage: push dword usglen push dword usg error: push dword stderr sys.write push dword 1 sys.exit align 4 global _start _start: pop eax ; argc pop eax ; program name pop ecx ; file to convert jecxz usage pop eax or eax, eax ; Too many arguments? jne usage ; Open the file push dword O_RDWR push ecx sys.open jc cantopen mov ebp, eax ; Save fd sub esp, byte stat_size mov ebx, esp ; Find file size push ebx push ebp ; fd sys.fstat jc facerr mov edx, [ebx + st_size + 4] ; File is too long if EDX != 0 ... or edx, edx jne near toolong mov ecx, [ebx + st_size] ; ... or if it is above 2 GB or ecx, ecx js near toolong ; Do nothing if the file is 0 bytes in size jecxz .quit ; Map the entire file in memory push edx push edx ; starting at offset 0 push edx ; pad push ebp ; fd push dword MAP_SHARED push dword PROT_READ | PROT_WRITE push ecx ; entire file size push edx ; let system decide on the address sys.mmap jc near memerr mov edi, eax mov esi, eax push ecx ; for SYS_munmap push edi ; Use EBX for state machine mov ebx, ordinary mov ah, 0Ah cld .loop: lodsb call ebx loop .loop cmp ebx, ordinary je .filesize ; Output final lf mov al, ah stosb inc edx .filesize: ; truncate file to new size push dword 0 ; high dword push edx ; low dword push eax ; pad push ebp sys.ftruncate ; close it (ebp still pushed) sys.close add esp, byte 16 sys.munmap .quit: push dword 0 sys.exit align 4 ordinary: cmp al, 0Dh je .cr cmp al, ah je .lf stosb inc edx ret align 4 .cr: mov ebx, cr ret align 4 .lf: mov ebx, lf ret align 4 cr: cmp al, 0Dh je .cr cmp al, ah je .lf xchg al, ah stosb inc edx xchg al, ah ; fall through .lf: stosb inc edx mov ebx, ordinary ret align 4 .cr: mov al, ah stosb inc edx ret align 4 lf: cmp al, ah je .lf cmp al, 0Dh je .cr xchg al, ah stosb inc edx xchg al, ah stosb inc edx mov ebx, ordinary ret align 4 .cr: mov ebx, ordinary mov al, ah ; fall through .lf: stosb inc edx ret
Warning: Do not use this program on files stored on a disk formatted by MS-DOS® or Windows®. There seems to be a subtle bug in the FreeBSD code when using
mmapon these drives mounted under FreeBSD: If the file is over a certain size,mmapwill just fill the memory with zeros, and then copy them to the file overwriting its contents.