freebsd/lib/libsys/pdfork.2

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.Dd April 19, 2026
.Dt PDFORK 2
.Os
.Sh NAME
.Nm pdfork ,
.Nm pdrfork ,
.Nm pdopenpid ,
.Nm pdgetpid ,
.Nm pdkill ,
.Nm pdwait ,
.Nm pddupfd
.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 pdopenpid "pid_t pid" "int pdflags"
.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
.Ft int
.Fn pddupfd "int fd" "int remotefd" "int flags"
.Ft int
.Fo pdptrace
.Fa "int req"
.Fa "int fd"
.Fa "int lwpid"
.Fa "void *addr"
.Fa "int data"
.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_PTRACE_CAP
.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 ) .
.Pp
Note: the option changes the behavior of
.Xr close 2
for the process descriptor returned by the operation.
If there is another process descriptor for the same process, created without
specifying the
.Dv PD_DAEMON
flag, closing that descriptor kills the process.
.It Dv PD_CLOEXEC
Set close-on-exec on process descriptor.
.It Dv PD_NOWAITPID
The parent cannot obtain the child's status with
.Xr waitpid 2 .
.Pp
A consequence of not setting the flag is that the child' zombie is
not reaped until the
.Xr waitpid 2
call is performed on the child' PID.
Setting the
.Dv
kern.pdfork_implicit_nowaitpid
sysctl MIB to
.Dv true
makes the system behave as if the flag is always passed.
.It Dv PD_PTRACE_CAP
Enable
.Xr pdptrace 2
requests on the resulting file descriptor.
Otherwise the descriptor cannot be used to debug the child process.
See
.Xr rights 4
for the description of the
.Dv CAP_PTRACE
capability.
.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 both the
.Va RFPROC
and
.Va RFPROCDESC
flags, or
.Va RFSPAWN
flag are specified.
.Pp
The
.Fn pdopenpid
function opens the process descriptor for the process
specified by the argument
.Fa pid .
It takes the same flags in the
.Fa pdflags
argument as
.Fn pdfork .
The caller must have permission to debug the target process in order for
.Fn pdopenpid
to succeed.
Zombie processes cannot be opened.
.Pp
There might be more that one file descriptor referencing the process.
After the zombie is reaped, calls to
.Fn pdwait
specifying any file descriptors for the same process fail with the
.Er ESRCH
error.
.Pp
The
.Fn pdopenpid
system call is allowed in the capability mode
.Pq see Xr capsicum 4
when the target process is the child of the calling process, or
when the calling process is the debugger of the target process.
The debugger is attached to its target by
.Xr ptrace 2 ,
.Xr pdptrace 2 ,
or by other means, e.g., by debugging the target process' parent with
the follow-on-fork mode enabled.
.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 2
system call for the behavior specification.
.Pp
The
.Fn pddupfd
function allows the caller to duplicate a file descriptor
across the process boundaries.
The function returns the new file descriptor that points to the same file
as the file descriptor
.Fa remotefd
in the process specified by the
.Fa fd
process descriptor.
The returned file descriptor has the
.Va O_CLOEXEC
flag set.
The
.Fa flags
argument is reserved and must be zero.
Certain file descriptor types cannot be copied this way, namely
kqueues.
.Pp
The
.Fn pdptrace
function enables execution of
.Xr ptrace 2
requests on the process specified by the process descriptor
.Fa fd .
.Pp
In addition to the arguments taken by the
.Xr ptrace 2 ,
system call, the
.Fa lwpid
thread identifier can designate the thread on which the request
must operate.
The
.Fa lwpid
argument can be specified as \-1 if the call is not thread-specific,
or kernel is allowed to select some thread on its own.
.Pp
Unlike the
.Xr ptrace 2
implementation,
.Fn pdptrace
does not clear the
.Dv errno
variable before executing the system call.
.Ss Interaction of Process Descriptors and Xr WAITPID 2
.Pp
The
.Fn pdwait
system call may be called on a process descriptor
an unlimited number of times.
In particular, it does not reap the target process,
even if that process has exited.
Each time, it returns the same status.
.Bl -dash
.It
If the process was forked with
.Fn pdfork ,
and the
.Dv PD_NOWAITPID
flag was specified, then the process is automatically reaped after the
last process descriptor referencing that process is closed.
No
.Xr waitpid 2
call
.Pq or a call from the same family of the wait functions which operate on PIDs
are needed to reap the zombie process.
.It
If the process was created by
.Fn pdfork ,
and the
.Dv PD_NOWAITPID
flag was not specified, then after exiting,
the process will not be reaped until the parent or reaper has called
.Xr waitpid 2
and all process descriptors referencing the process are closed.
.It
If the process was created by the
.Xr fork 2
system call
.Pq which does not allocate a process descriptor for the child ,
and later the process was opened by
.Fn pdopenpid ,
then a
.Xr waitpid 2
call from the parent is needed to reap the exited child.
.El
.Pp
In any case, the PID of the process is not reused until its zombie
is reaped, and all its process descriptors are closed.
.Pp
A debugger attached by
.Xr ptrace 2
can execute the
.Fn waitpid
calls against the alive target regardless of the way
the target process was forked.
.Ss Interaction of Process Descriptors with other System Calls
.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 pdopenpid ,
.Fn pdgetpid ,
.Fn pdkill ,
.Fn pdwait ,
and
.Fn pddupfd
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 EFAULT
The copyout of the resulting file descriptor value to the memory pointed
to by
.Fa fdp
failed.
.Pp
Note that the child process was already created when this condition
is detected,
and the child continues execution, same as the parent.
If this error must be handled, it is advisable to memoize the
.Fn getpid
result before the call to
.Fn pdfork
or
.Fn pdrfork ,
and compare it to the value returned by
.Fn getpid
after, to see if code is executing in parent or child.
.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 ) .
.It Bq Er EINVAL
The
.Fn pdwait
function is called with reserved bits set in
.Fa options .
.El
.Pp
The
.Fn pdopenpid
might return the same errors as
.Xr open 2 ,
related to the file descriptor allocation problems, as well as the
following specific errors:
.Bl -tag -width Er
.It Bq Er EINVAL
The
.Fa flags
argument has reserved bits set.
.It Bq Er ECAPMODE
.Fn pdopenpid
is called by the process in capability mode.
.It Bq Er EBUSY
The process specified by the
.Fa pid
argument already terminated.
.It Bq Er ESRCH
The process specified by the
.Fa pid
argument does not exist, or the caller does not have enough privileges
to open the process.
.It Bq Er EMFILE
The calling process already reached its limit for open file descriptors.
.Pp
Current implementation installs the opened process descriptor into the
calling process's file descriptors table.
If the descriptor cannot be installed, the process descriptor is closed,
which executes all actions performed by
.Xr close 2
on it.
.El
.Pp
The
.Fn pddupfd
returns the following errors:
.Bl -tag -width Er
.It Bq Er EINVAL
The
.Fa flags
argument is not zero.
.It Bq Er EINVAL
The file descriptor
.Fa fd
is not a process file descriptor.
.It Bq Er ESRCH
The process specified by the file descriptor
.Fa fd
exited.
.It Bq Er EBADF
The file descriptor
.Fa remotefd
is not a valid file descriptor in the specified process.
.It Bq Er ENOENT
The specified process does not have a file descriptor table.
.It Bq Er EOPNOTSUPP
.Fa remotefd
refers to a file that cannot be duplicated across a process boundary,
such as a kqueue.
.El
.Pp
The
.Fn pdptrace
system call returns the same errors as
.Xr ptrace 2 ,
as well as the following specific errors:
.Bl -tag -width Er
.It Bq Er ECAPMODE
The process issuing the
.Fn pdptrace
call is in capability mode,
and the
.Dv security.bsd.allow_ptrace_in_cap_mode
tunable is set to
.Dv false .
.It Bq Er ENOTCAPABLE
The process called
.Fn pdptrace
on the process descriptor that does not have the
.Dv CAP_PTRACE
capability enabled.
.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 15.1 .
The
.Fn pdopenpid
and
.Fn pddupfd
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 [email protected]
and
.An Jonathan Anderson Aq Mt [email protected]
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 [email protected]
with input from
.An Alan Somers Aq Mt [email protected] .
The
.Fn pdopenpid ,
.Fn pddupfd ,
and
.Fn pdptrace
functions were developed by
.An Konstantin Belousov Aq Mt [email protected] .