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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
     created.  The flags argument to 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.  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
                  parent.  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
                  provided 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.
     Without 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
     necessary, 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
     indicate 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
                        processes 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().