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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 802
SEC. 10.5
INPUT/OUTPUT IN LINUX
771
functions into separate function calls primarily for terminal devices. In Linux and
modern UNIX systems, whether each one is a separate system call or they share a
single system call or something else is implementation dependent.
The first four calls listed in Fig. 10-20 are used to set and get the terminal
speed. Different calls are provided for input and output because some modems op-
erate at split speed. For example, old videotex systems allowed people to access
public databases with short requests from the home to the server at 75 bits/sec with
replies coming back at 1200 bits/sec. This standard was adopted at a time when
1200 bits/sec both ways was too expensive for home use. Times change in the net-
working world. This asymmetry still persists, with some telephone companies
offering inbound service at 20 Mbps and outbound service at 2 Mbps, often under
the name of
ADSL
(
Asymmetric Digital Subscriber Line
).
Function call
Description
s = cfsetospeed(&termios, speed)
Set the output speed
s = cfsetispeed(&termios, speed)
Set the input speed
s = cfgetospeed(&termios, speed)
Get the output speed
s = cfgtetispeed(&termios, speed)
Get the input speed
s = tcsetattr(fd, opt, &termios)
Set the attributes
s = tcgetattr(fd, &termios)
Get the attributes
Figure 10-20.
The main POSIX calls for managing the terminal.
The last two calls in the list are for setting and reading back all the special
characters used for erasing characters and lines, interrupting processes, and so on.
In addition, they enable and disable echoing, handle flow control, and perform
other related functions. Additional I/O function calls also exist, but they are some-
what specialized, so we will not discuss them further. In addition,
ioctl
is still avail-
able.
10.5.4 Implementation of Input/Output in Linux
I/O in Linux is implemented by a collection of device drivers, one per device
type. The function of the drivers is to isolate the rest of the system from the
idiosyncracies of the hardware. By providing standard interfaces between the driv-
ers and the rest of the operating system, most of the I/O system can be put into the
machine-independent part of the kernel.
When the user accesses a special file, the file system determines the major and
minor device numbers belonging to it and whether it is a block special file or a
character special file. The major device number is used to index into one of two in-
ternal hash tables containing data structures for character or block devices. The
structure thus located contains pointers to the procedures to call to open the device,
read the device, write the device, and so on. The minor device number is passed as
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