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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.  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 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.