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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 766
SEC. 10.3
PROCESSES IN LINUX
735
pid = fork( );
/
*
if the fork succeeds, pid > 0 in the parent
*
/
if (pid < 0) {
handle
error( );
/
*
fork failed (e.g., memory or some table is full)
*
/
} else if (pid > 0) {
/
*
parent code goes here. /
*
/
} else {
/
*
child code goes here. /
*
/
}
Figure 10-4.
Process creation in Linux.
just finished. This can be important because a parent may have many children.
Since children may also have children, an original process can build up an entire
tree of children, grandchildren, and further descendants.
Processes in Linux can communicate with each other using a form of message
passing. It is possible to create a channel between two processes into which one
process can write a stream of bytes for the other to read. These channels are called
pipes
.
Synchronization is possible because when a process tries to read from an
empty pipe it is blocked until data are available.
Shell pipelines are implemented with pipes. When the shell sees a line like
sort <f | head
it creates two processes,
sort
and
head
, and sets up a pipe between them in such a
way that
sort
’s standard output is connected to
head
’s standard input.
In this way,
all the data that
sort
writes go directly to
head
, instead of going to a file. If the
pipe fills, the system stops running
sort
until
head
has removed some data from it.
Processes can also communicate in another way besides pipes: software inter-
rupts. A process can send what is called a
signal
to another process. Processes can
tell the system what they want to happen when an incoming signal arrives. The
choices available are to ignore it, to catch it, or to let the signal kill the process.
Terminatingthe process is the default for most signals.
If a process elects to catch
signals sent to it, it must specify a signal-handling procedure. When a signal ar-
rives, control will abruptly switch to the handler.
When the handler is finished and
returns, control goes back to where it came from, analogous to hardware I/O inter-
rupts. A process can send signals only to members of its
process group
, which
consists of its parent (and further ancestors), siblings, and children (and further
descendants). A process may also send a signal to all members of its process
group with a single system call.
Signals are also used for other purposes. For example, if a process is doing
floating-point arithmetic, and inadvertently divides by 0 (something that mathe-
maticians tend to frown upon), it gets a SIGFPE (floating-point exception) signal.
Some of the signals that are required by POSIX are listed in Fig. 10-5. Many
Linux systems have additional signals as well, but programs using them may not
be portable to other versions of Linux and UNIX in general.
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