Process API

Introduction

• fork()
• wait()
• exec()
• kill()
• pipe()

Fork 🍴

• creates a child process that is a clone of the parent
• The child is a near-exact copy of the parent. The main differences are its process id, its parent process id, and the value fork() returns (see man 2 fork for the full list)
• fork() is called once but it returns twice!

Process Hierarchy

Exercise

On success, the PID of the child process is returned in the parent, and 0 is returned in the child. On failure, -1 is returned in the parent, no child process is created, and errno is set to indicate the error.

/* process A */
/* ... */
if ( fork () == 0) {
   if ( fork () == 0) {
       if ( fork () == 0) {
           if ( fork () == 0) {
               /* do something */
           }
       }
   }
}

How many new processes are created by the code snippet?

Wait 🚏

Sometimes, as it turns out, it is quite useful for a parent to wait for a child process to finish what it has been doing. This task is accomplished with the wait() system call (or its more complete sibling waitpid()).

Exec

A final and important piece of the process creation API is the exec() system call. This system call is useful when you want to run a program that is different from the calling program

Linux vs Windows

Linux	Win32
fork(), exec()	CreateProcess() (fork exec combined)
exit()	ExitProcess()
wait(), waitpid()	WaitForSingleObject(), WaitForMultipleObjects()
kill()	TerminateProcess()

Signals

Linux/Unix signals are a type of event. Signals are asynchronous in nature and are used to inform processes of certain events happening (man 7 signal).

Common Signals

Signal	Default Action	Description
SIGINT	Terminate	Interrupt from keyboard (Ctrl+C)
SIGTERM	Terminate	Termination request (default signal from kill)
SIGKILL	Terminate	Forceful termination; cannot be caught or ignored
SIGCONT	Continue	Resume a stopped process
SIGSTOP	Stop	Pause process; cannot be caught or ignored
SIGSEGV	Core dump	Invalid memory reference (segmentation fault)
SIGABRT	Core dump	Abort signal from abort()
SIGALRM	Terminate	Timer expiration set by alarm()
SIGUSR1	Terminate	User-defined signal 1
SIGUSR2	Terminate	User-defined signal 2
SIGPIPE	Terminate	Broken pipe: write to pipe with no readers
SIGCHLD	Ignore	Child process stopped or terminated

SIGKILL and SIGSTOP cannot be caught, blocked, or ignored. The kernel handles them directly.

Full signal list