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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().