Implement fork() syscall

[bmarcot]

NAME fork -- create a new process

LIBRARY Standard C Library (libc, -lc)

SYNOPSIS

include

 pid_t
 fork(void);

DESCRIPTION The fork() system call 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:

   +o   The child process has a unique process ID.

   +o   The child process has a different parent    process ID (i.e., the
       process ID of the parent process).

   +o   The child process has its own copy of the parent's descriptors,
       except for descriptors returned by kqueue(2), which are not
       inherited from the parent process.  These descriptors reference
       the same underlying objects, so that, for instance, file point-
       ers in file objects are shared between the child and the par-
       ent, 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.

   +o   The child process' resource utilizations    are set to 0; see
       setrlimit(2).

   +o   All interval timers are cleared;    see setitimer(2).

   +o   The child process has only one thread, corresponding to the
       calling thread in the parent process.  If the process has more
       than one thread, locks and other resources held by the other
       threads are not released and therefore only async-signal-safe
       functions (see sigaction(2)) are guaranteed to work in the
       child process until a call to execve(2) or a similar function.

RETURN VALUES Upon successful completion, fork() returns a value of 0 to the child process and returns the process ID of the child process to the parent process. Otherwise, a value of -1 is returned to the parent process, no child process is created, and the global variable errno is set to indi- cate the error.

ERRORS The fork() system call will fail and no child process will be created if:

 [EAGAIN]       The system-imposed limit on the total number of pro-
        cesses under execution would be exceeded.  The limit
        is given by the sysctl(3) MIB variable KERN_MAXPROC.
        (The limit is actually ten less than this except for
        the super user).

 [EAGAIN]       The user is not the super user, and the system-imposed
        limit on the total number of processes under execution
        by a single user would be exceeded.  The limit is
        given by the sysctl(3) MIB variable
        KERN_MAXPROCPERUID.

 [EAGAIN]       The user is not the super user, and the soft resource
        limit corresponding to the resource argument
        RLIMIT_NPROC would be exceeded (see getrlimit(2)).

 [ENOMEM]       There is insufficient swap space for the new process.

[bmarcot]

Partly implemented.