fork, vfork, copy-on-write and clone

From man page of fork in linux
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fork creates a child process that differs from the parent process only in its PID and PPID, and in the fact that resource utilizations are set to 0. File locks and pending signals are not inherited.

Under Linux, fork is implemented using copy-on-write pages, so the only penalty incurred by fork is the time and memory required to duplicate the parent's page tables, and to create a unique task structure for the child.

RETURN VALUE
On success, the PID of the child process is returned in the parent's thread of execution, and a 0 is returned in the child's thread of execution.
On failure, a -1 will be returned in the parent's context, no child process will be created, and errno will be set appropriately.

ERRORS
EAGAIN fork cannot allocate sufficient memory to copy the parent's page tables and allocate a task structure for the child.

ENOMEM fork failed to allocate the necessary kernel structures because memory is tight.
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