freebsd-skq/lib/libc/sys/rfork.2

.\"
.\" This manual page is taken directly from Plan9, and modified to
.\" describe the actual BSD implementation. Permission for
.\" use of this page comes from Rob Pike <rob@plan9.att.com>.
.\"
.Dd Jan 12, 1996
.Dt RFORK 2
.Os
.Sh NAME
.Nm rfork
.Nd manipulate process resources
.Sh SYNOPSIS
.Fd #include <unistd.h>
.Ft int
.Fn rfork "int flags"
.Sh DESCRIPTION
Forking, vforking or rforking are the only ways new processes are created.
The
.Fa flags
argument to
.Fn rfork
selects which resources of the
invoking process (parent) are shared
by the new process (child) or initialized 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.
.Fa Flags
is the logical OR of some subset of:
.Bl -tag -width "RFCNAMEG" -compact -offset indent
.It RFPROC
If set a new process is created; otherwise changes affect the
current process.
The current implementation requires this flag to always be set.
.It RFNOWAIT
If set, the child process will be dissociated from the parent. Upon
exit the child will not leave a status for the parent to collect.
See 
.Xr wait 2 .
.It RFFDG
If set, the invoker's file descriptor table (see
.Xr intro 2
) is copied; otherwise the two processes share a
single table.
.It RFCFDG
If set, the new process starts with a clean file descriptor table.
Is mutually exclusive with
.Dv RFFDG .
.It RFMEM
If set, the kernel will force sharing of the entire address space.
The child
will then inherit all the shared segments the parent process owns. Other segment
types will be unaffected.  Subsequent forks by the parent will then
propagate the shared data and bss between children.  The stack segment
is always split.  May be set only with
.Dv RFPROC .
.El
.Pp
File descriptors in a shared file descriptor table are kept
open until either they are explicitly closed
or all processes sharing the table exit.
.Pp
If
.Dv 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.
Without
.Dv RFPROC ,
the return value is zero.
Process id's range from 1 to the maximum integer
.Ft ( int )
value.
.Fn Rfork
will sleep, if necessary, until required process resources are available.
.Pp
.Fn Fork
can be implemented as a call to
.Fn rfork "RFFDG|RFPROC"
but isn't for backwards compatibility.
.Sh RETURN VALUES
Upon successful completion,
.Fn 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
.Va errno
is set to indicate the error.
.Sh ERRORS
.Fn Rfork
will fail and no child process will be created if:
.Bl -tag -width [EAGAIN]
.It Bq Er EAGAIN
The system-imposed limit on the total
number of processes under execution would be exceeded.
The limit is given by the
.Xr sysctl 3
MIB variable
.Dv KERN_MAXPROC .
(The limit is actually one less than this
except for the super user).
.It Bq Er 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
.Xr sysctl 3
MIB variable
.Dv KERN_MAXPROCPERUID .
.It Bq Er EAGAIN
The user is not the super user, and
the soft resource limit corresponding to the resource parameter
.Dv RLIMIT_NOFILE
would be exceeded (see
.Xr getrlimit 2 ) .
.It Bq Er EINVAL
The RFPROC flag was not specified.
.It Bq Er EINVAL
Both the RFFDG and the RFCFDG flags were specified.
.It Bq Er ENOMEM
There is insufficient swap space for the new process.
.El
.Sh SEE ALSO
.Xr fork 2 ,
.Xr intro 2 ,
.Xr minherit 2 ,
.Xr vfork 2
.Sh HISTORY
The
.Fn rfork
function call first appeared in Plan9.