Page 416 - DCAP103_Principle of operating system

Unit 14: Case Study of Linux Operating System

which consists of its parent (and further ancestors), siblings, and children (and further Notes
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, it gets a SIGFPE (floating-
point exception) signal. The signals that are required by POSIX are listed in Figure 14.5.
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.

Figure 14.5: The Signals Required by POSIX

Signal Cause
SIGABRT Sent to abort a process and force a core dump
SIGALRM The alarm clock has gone off

SIGFPE A floating-point error has occurred (e.g., division by 0)

SIGHUP The phone line the process was using has been hung up
SIGILL The user has hit the DEL key to interrupt the process

SIGQUIT The user has hit the key requesting a core dump

SIGKILL Sent to kill a process (cannot be caught or ignored)
SIGPIPE The process has written to a pipe which has no readers

SIGSEGV The process has referenced an invalid memory address

SIGTERM Used to request that a process terminate gracefully
SIGUSR1 Available for application-defined purposes

SIGUSR2 Available for application-defined purpose

14.2.2 Process Management System Calls in Linux
Let us now look at the Linux system calls dealing with process management. The main ones are
listed in Figure 14.6. Fork is a good place to start the discussion. The Fork system call, supported
also by other traditional UNIX systems, is the main way to create a new process in Linux systems.
It creates an exact duplicate of the original process, including all the file descriptors, registers
and everything else. After the fork, the original process and the copy (the parent and child) go
their separate ways. All the variables have identical values at the time of the fork, but since the
entire parent address space is copied to create the child, subsequent changes in one of them do
not affect the other one. The fork call returns a value, which is zero in the child, and equal to the
child’s PID in the parent. Using the returned PID, the two processes can see which is the parent
and which is the child. In most cases, after a fork, the child will need to execute different code
from the parent. Consider the case of the shell. It reads a command from the terminal, forks off
a child process, waits for the child to execute the command, and then reads the next command
when the child terminates. To wait for the child to finish, the parent executes a waitpid system
call, which just waits until the child terminates (any child if more than one exists).

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