fork() causes creation of a new process. The new process (child process) is an exact copy of the calling process (parent process) except for the following:
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The child process has a unique process ID.
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The child process has a different parent process ID (i.e., the process ID of the parent process).
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The child process has its own copy of the parent's descriptors. These descriptors reference the same underlying objects, so that, for instance, file pointers in file objects are shared between the child and the parent, so that an
lseek(2) on a descriptor in the child process can affect a subsequent
read(2) or
write(2) by the parent. This descriptor copying is also used by the shell to establish standard input and output for newly created processes as well as to set up pipes.
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The child process' resource utilizations are set to 0; see
setrlimit(2).
In general, the child process should call
_exit(2) rather than
exit(3). Otherwise, any stdio buffers that exist both in the parent and child will be flushed twice. Similarly,
_exit(2) should be used to prevent
atexit(3) routines from being called twice (once in the parent and once in the child).
In case of a threaded program, only the thread calling
fork() is still running in the child processes.
Child processes of a threaded program have additional restrictions, a child must only call functions that are async-signal-safe. Very few functions are asynchronously safe and applications should make sure they call
exec(3) as soon as possible.