RFORK(2)                  FreeBSD 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
     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 either
     RFSPAWN or 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.  Note that a lot of
                      code will not run correctly in such an environment.

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

     RFTSIGZMB        If set, the kernel will deliver a specified signal to
                      the parent upon the child exit, instead of default
                      SIGCHLD.  The signal number signum is specified by oring
                      the RFTSIGFLAGS(signum) expression into flags.
                      Specifying signal number 0 disables signal delivery upon
                      the child exit.

     RFLINUXTHPN      If set, the kernel will deliver SIGUSR1 instead of
                      SIGCHLD 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 RFSPAWN is passed, rfork will use vfork(2) semantics but reset all
     signal actions in the child to default.  This flag is used by the
     posix_spawn(3) implementation in libc.

     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.

     [EINVAL]           An invalid signal number was specified.

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

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

HISTORY
     The rfork() function first appeared in Plan9.

FreeBSD 13.1-RELEASE-p6       September 25, 2019       FreeBSD 13.1-RELEASE-p6