fork() and pipe()
fork() # create a new process
fork() is the UNIX system call that creates a new process.-
fork()creates a new process that is a copy of the calling process. -
After
fork()we refer to the caller as the parent and the newly-created process as the child. This relationship enables certain capabilities.
fork() Semantics
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Generally
fork()tries to make an exact copy of the calling process.-
Recent version of UNIX have relaxed this requirement and there are now many flavors of
fork()that copy different amounts of state and are suitable for different purposes. -
For the purposes of this class, ignore them.
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Threads are a notable exception!
fork() Against Threads
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Single-threaded
fork()has reliable semantics because the only thread the processes had is the one that calledfork().-
So nothing else is happening while we complete the system call.
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Multi-threaded
fork()creates a host of problems that many systems choose to ignore.-
Linux will only copy state for the thread that called
fork().
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Multi-Threaded fork()
fork()-
Another thread could be blocked in the middle of doing something (uniprocessor systems), or
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another thread could be actually doing something (multiprocessor systems).
fork()
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fork()copies one thread—the caller. -
fork()copies the address space. -
fork()copies the process file table.
After fork()
returnCode = fork();
if (returnCode == 0) {
# I am the child.
} else {
# I am the parent.
}-
The child thread returns executing at the exact same point that its parent called
fork().-
With one exception:
fork()returns twice, the PID to the parent and 0 to the child.
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All contents of memory in the parent and child are identical.
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Both child and parent have the same files open at the same position.
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But, since they are sharing file handles changes to the file offset made by the parent/child will be reflected in the child/parent!
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Pipes
pipe() system call.-
pipe()creates an anonymous pipe object and returns a two file descriptors: one for the read-only end, and the other for the write-only end. -
Anything written to the write-only end of the pipe is immediately available at the read-only end of the pipe.
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Pipe contents are buffered in memory.
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Why is this useful?
IPC Using fork() and pipe()
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Before calling
fork()the parent creates a pipe object by calling pipe(). -
Next, it calls
fork(). -
After
fork()the parent closes its copy of the read-only end and the child closes its copy of the write-only end. -
Now the parent can pass information to the child.
# pipeEnds[0] gets the read end; pipeEnds[1] gets the write end.
int pipeEnds[2];
pipe(pipeEnds);
int returnCode = fork();
if (returnCode == 0) {
# Don't need a loopback.
close(pipeEnds[1]);
# Read some data from the pipe.
char data[14];
read(pipeEnds[0], data, 14);
} else {
# Don't need a loopback.
close(pipeEnds[0]);
# Write some data to the pipe.
write(pipeEnds[1], "Hello, sweet child!\n", 14);
}Issues with fork()
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Especially when the next thing that a process frequently does is start load a new binary which destroys most of the state
fork()has carefully copied!
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Optimize existing semantics: through copy-on-write, a clever memory-management optimization we will discuss in several weeks.
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Change the semantics:
vfork(), which will fail if the child does anything other than immediately load a new executable.-
Does not copy the address space!
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fork()is now replaced byclone(), a more flexible primitive that enables more control:-
over sharing, including sharing memory, and signal handlers,
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and over child execution, which begins at a function pointer passed to the system call instead of resuming at the point where
fork()was called.
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Try
man clonein your CSE421 VM.
