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RFORK(2)		    BSD System Calls Manual		      RFORK(2)

NAME
     rfork — manipulate process resources

LIBRARY
     Standard C Library (libc, -lc)

SYNOPSIS
     #include <unistd.h>

     pid_t
     rfork(int flags);

DESCRIPTION
     Forking, vforking or rforking are the only ways new processes are cre‐
     ated.  The flags argument to rfork() selects which resources of the
     invoking process (parent) are shared by the new process (child) or ini‐
     tialized to their default values.	The resources include the open file
     descriptor table (which, when shared, permits processes to open and close
     files for other processes), and open files.  The flags argument is the
     logical OR of some subset of:

     RFPROC	  If set a new process is created; otherwise changes affect
		  the current process.

     RFNOWAIT	  If set, the child process will be dissociated from the par‐
		  ent.	Upon exit the child will not leave a status for the
		  parent to collect.  See wait(2).

     RFFDG	  If set, the invoker's file descriptor table (see intro(2))
		  is copied; otherwise the two processes share a single table.

     RFCFDG	  If set, the new process starts with a clean file descriptor
		  table.  Is mutually exclusive with RFFDG.

     RFTHREAD	  If set, the new process shares file descriptor to process
		  leaders table with its parent.  Only applies when neither
		  RFFDG nor RFCFDG are set.

     RFMEM	  If set, the kernel will force sharing of the entire address
		  space, typically by sharing the hardware page table
		  directly.  The child will thus inherit and share all the
		  segments the parent process owns, whether they are normally
		  shareable or not.  The stack segment is not split (both the
		  parent and child return on the same stack) and thus rfork()
		  with the RFMEM flag may not generally be called directly
		  from high level languages including C.  May be set only with
		  RFPROC.  A helper function is provided to assist with this
		  problem and will cause the new process to run on the pro‐
		  vided stack.	See rfork_thread(3) for information.

     RFSIGSHARE	  If set, the kernel will force sharing the sigacts structure
		  between the child and the parent.

     RFLINUXTHPN  If set, the kernel will return SIGUSR1 instead of SIGCHILD
		  upon thread exit for the child.  This is intended to mimic
		  certain Linux clone behaviour.

     File descriptors in a shared file descriptor table are kept open until
     either they are explicitly closed or all processes sharing the table
     exit.

     If RFPROC is set, the value returned in the parent process is the process
     id of the child process; the value returned in the child is zero.	With‐
     out RFPROC, the return value is zero.  Process id's range from 1 to the
     maximum integer (int) value.  The rfork() system call will sleep, if nec‐
     essary, until required process resources are available.

     The fork() system call can be implemented as a call to rfork(RFFDG |
     RFPROC) but is not for backwards compatibility.

RETURN VALUES
     Upon successful completion, rfork() 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 rfork() 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_NOFILE would be exceeded (see getrlimit(2)).

     [EINVAL]		Both the RFFDG and the RFCFDG flags were specified.

     [EINVAL]		Any flags not listed above were specified.

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

SEE ALSO
     fork(2), intro(2), minherit(2), vfork(2), rfork_thread(3)

HISTORY
     The rfork() function first appeared in Plan9.

BUGS
     FreeBSD does not yet implement a native clone() library call, and the
     current pthreads implementation does not use rfork() with RFMEM.  A
     native port of the linux threads library, /usr/ports/devel/linuxthreads,
     contains a working clone() call that utilizes RFMEM.  The rfork_thread(3)
     function can often be used instead of clone().

BSD				 May 14, 2007				   BSD
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