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Modern Operating Systems by Herbert Bos and Andrew S. Tanenb...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf
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Modern Operating Systems by Herbert Bos and Andrew...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf-M ODERN O PERATING S YSTEMS
Modern Operating Systems by Herbert...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf-M ODERN O PERATING S YSTEMS
Page 300
SEC. 4.1
FILES
269
The great advantage of ASCII files is that they can be displayed and printed as
is, and they can be edited with any text editor. Furthermore, if large numbers of
programs use ASCII files for input and output, it is easy to connect the output of
one program to the input of another, as in shell pipelines.
(The interprocess
plumbing is not any easier, but interpreting the information certainly is if a stan-
dard convention, such as ASCII, is used for expressing it.)
Other files are binary, which just means that they are not ASCII files. Listing
them on the printer gives an incomprehensible listing full of random junk. Usually,
they have some internal structure known to programs that use them.
For example, in Fig. 4-3(a) we see a simple executable binary file taken from
an early version of UNIX.
Although technically the file is just a sequence of bytes,
the operating system will execute a file only if it has the proper format.
It has five
sections: header, text, data, relocation bits, and symbol table.
The header starts
with a so-called
magic number
, identifying the file as an executable file (to pre-
vent the accidental execution of a file not in this format).
Then come the sizes of
the various pieces of the file, the address at which execution starts, and some flag
bits. Following the header are the text and data of the program itself.
These are
loaded into memory and relocated using the relocation bits. The symbol table is
used for debugging.
Our second example of a binary file is an archive, also from UNIX.
It consists
of a collection of library procedures (modules) compiled but not linked. Each one
is prefaced by a header telling its name, creation date, owner, protection code, and
size. Just as with the executable file, the module headers are full of binary num-
bers. Copying them to the printer would produce complete gibberish.
Every operating system must recognize at least one file type: its own executa-
ble file; some recognize more. The old TOPS-20 system (for the DECsystem 20)
went so far as to examine the creation time of any file to be executed. Then it loca-
ted the source file and saw whether the source had been modified since the binary
was made. If it had been, it automatically recompiled the source.
In UNIX terms,
the
make
program had been built into the shell. The file extensions were manda-
tory, so it could tell which binary program was derived from which source.
Having strongly typed files like this causes problems whenever the user does
anything that the system designers did not expect. Consider, as an example, a sys-
tem in which program output files have extension
.dat
(data files). If a user writes
a program formatter that reads a
.c
file (C program), transforms it (e.g., by convert-
ing it to a standard indentation layout), and then writes the transformed file as out-
put, the output file will be of type
.dat
.
If the user tries to offer this to the C compi-
ler to compile it, the system will refuse because it has the wrong extension. At-
tempts to copy
file.dat
to
file.c
will be rejected by the system as invalid (to protect
the user against mistakes).
While this kind of ‘‘user friendliness’’ may help novices, it drives experienced
users up the wall since they have to devote considerable effort to circumventing the
operating system’s idea of what is reasonable and what is not.
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