lib/libsys/pdfork.2

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.Dd January 20, 2026
.Dt PDFORK 2
.Os
.Sh NAME
.Nm pdfork ,
.Nm pdrfork ,
.Nm pdgetpid ,
.Nm pdkill ,
.Nm pdwait
.Nd System calls to manage process descriptors
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In sys/procdesc.h
.Ft pid_t
.Fn pdfork "int *fdp" "int pdflags"
.Ft pid_t
.Fn pdrfork "int *fdp" "int pdflags" "int rfflags"
.Ft int
.Fn pdgetpid "int fd" "pid_t *pidp"
.Ft int
.Fn pdkill "int fd" "int signum"
.Ft int
.Fo pdwait
.Fa "int fd"
.Fa "int *status"
.Fa "int options"
.Fa "struct __wrusage *wrusage"
.Fa "struct __siginfo *info"
.Fc
.Sh DESCRIPTION
Process descriptors are special file descriptors that represent processes,
and are created using
.Fn pdfork ,
a variant of
.Xr fork 2 ,
which, if successful, returns a process descriptor in the integer pointed to
by
.Fa fdp .
Processes created via
.Fn pdfork
will not cause
.Dv SIGCHLD
on termination.
.Fn pdfork
can accept the
.Fa pdflags:
.Bl -tag -width PD_CLOEXEC
.It Dv PD_DAEMON
Instead of the default terminate-on-close behaviour, allow the process to
live until it is explicitly killed with
.Xr kill 2 .
.Pp
This option is not permitted in
.Xr capsicum 4
capability mode (see
.Xr cap_enter 2 ) .
.El
.Bl -tag -width ".Dv PD_DAEMON"
.It Dv PD_CLOEXEC
Set close-on-exec on process descriptor.
.El
.Pp
The
.Fn pdrfork
system call is a variant of
.Fn pdfork
that also takes the
.Fa rfflags
argument to control sharing of process resources between the caller
and the new process.
Like
.Fn pdfork ,
the function writes the process descriptor referencing the created
process into the location pointed to by the
.Fa fdp
argument.
See
.Xr rfork 2
for a description of the possible
.Fa rfflag
flags.
The
.Fn pdrfork
system call requires that the
.Va RFPROC
or
.Va RFSPAWN
flag is specified.
.Pp
.Fn pdgetpid
queries the process ID (PID) in the process descriptor
.Fa fd .
.Pp
.Fn pdkill
is functionally identical to
.Xr kill 2 ,
except that it accepts a process descriptor,
.Fa fd ,
rather than a PID.
.Pp
The
.Fn pdwait
system call allows the calling thread to wait and retrieve
the status information on the process referenced by the
.Fa fd
process descriptor.
See the description of the
.Xr wait6
system call for the behavior specification.
.Pp
The following system calls also have effects specific to process descriptors:
.Pp
.Xr fstat 2
queries status of a process descriptor; currently only the
.Fa st_mode ,
.Fa st_birthtime ,
.Fa st_atime ,
.Fa st_ctime
and
.Fa st_mtime
fields are defined.
If the owner read, write, and execute bits are set then the
process represented by the process descriptor is still alive.
.Pp
.Xr poll 2
and
.Xr select 2
allow waiting for process state transitions; currently only
.Dv POLLHUP
is defined, and will be raised when the process dies.
Process state transitions can also be monitored using
.Xr kqueue 2
filter
.Dv EVFILT_PROCDESC ;
currently only
.Dv NOTE_EXIT
is implemented.
.Pp
.Xr close 2
will close the process descriptor unless
.Dv PD_DAEMON
is set; if the process is still alive and this is
the last reference to the process descriptor, the process will be terminated
with the signal
.Dv SIGKILL .
The PID of the referenced process is not reused until the process
descriptor is closed,
whether or not the zombie process is reaped by
.Fn pdwait ,
.Xr wait6 ,
or similar system calls.
.Sh RETURN VALUES
.Fn pdfork
and
.Fn pdrfork
return a PID, 0 or -1, as
.Xr fork 2
does.
.Pp
.Fn pdgetpid ,
.Fn pdkill ,
and
.Fn pdwait
return 0 on success and -1 on failure.
.Sh ERRORS
These functions may return the same error numbers as their PID-based equivalents
(e.g.
.Fn pdfork
may return the same error numbers as
.Xr fork 2 ) ,
with the following additions:
.Bl -tag -width Er
.It Bq Er EINVAL
The signal number given to
.Fn pdkill
is invalid.
.It Bq Er ENOTCAPABLE
The process descriptor being operated on has insufficient rights (e.g.
.Dv CAP_PDKILL
for
.Fn pdkill ) .
.El
.Sh SEE ALSO
.Xr close 2 ,
.Xr fork 2 ,
.Xr fstat 2 ,
.Xr kill 2 ,
.Xr kqueue 2 ,
.Xr poll 2 ,
.Xr wait4 2 ,
.Xr capsicum 4 ,
.Xr procdesc 4
.Sh HISTORY
The
.Fn pdfork ,
.Fn pdgetpid ,
and
.Fn pdkill
system calls first appeared in
.Fx 9.0 .
The
.Fn pdrfork
and
.Fn pdwait
system calls first appeared in
.Fx 16.0 .
.Pp
Support for process descriptors mode was developed as part of the
.Tn TrustedBSD
Project.
.Sh AUTHORS
.An -nosplit
These functions and the capability facility were created by
.An Robert N. M. Watson Aq Mt rwatson@FreeBSD.org
and
.An Jonathan Anderson Aq Mt jonathan@FreeBSD.org
at the University of Cambridge Computer Laboratory with support from a grant
from Google, Inc.
The
.Fn pdrfork
and
.Fn pdwait
functions were developed by
.An Konstantin Belousov Aq Mt kib@FreeBSD.org
with input from
.An Alan Somers Aq Mt asomers@FreeBSD.org .