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RFORK(2)                  FreeBSD System Calls Manual                 RFORK(2)

     rfork - manipulate process resources

     Standard C Library (libc, -lc)

     #include <unistd.h>

     rfork(int flags);

     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.  Flags is the logical OR of
     some subset of:
           RFPROC    If set a new process is created; otherwise changes affect
                     the current process.  The current implementation requires
                     this flag to always be set.
           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.
           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.
                     If set, the kernel will force sharing the sigacts
                     structure between the child and the parent.
                     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

     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.  Rfork() will sleep, if necessary, until
     required process resources are available.

     Fork() can be implemented as a call to rfork(RFFDG | RFPROC) but isn't
     for backwards compatibility.

     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.

     Rfork() 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

     [EAGAIN]           The user is not the super user, and the soft resource
                        limit corresponding to the resource parameter
                        RLIMIT_NOFILE would be exceeded (see getrlimit(2)).

     [EINVAL]           The RFPROC flag was not specified.

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

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

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

     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()
     library call can often be used instead of clone().

     The rfork() function call first appeared in Plan9.

FreeBSD 11.0-PRERELEASE        January 12, 1996        FreeBSD 11.0-PRERELEASE


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