pdgetpid(2) System calls to manage process descriptors

Other Alias

pdfork, pdkill, pdwait4


Lb libc


In sys/procdesc.h Ft int Fn pdfork int *fdp int flags Ft int Fn pdgetpid int fd pid_t *pidp Ft int Fn pdkill int fd int signum Ft int Fn pdwait4 int fd int *status int options struct rusage *rusage


Process descriptors are special file descriptors that represent processes, and are created using Fn pdfork , a variant of 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 SIGCHLD on termination. Fn pdfork can accept the flags:

Instead of the default terminate-on-close behaviour, allow the process to live until it is explicitly killed with kill(2).

This option is not permitted in capsicum(4) capability mode (see cap_enter2).

Fn pdgetpid queries the process ID (PID) in the process descriptor Fa fd .

Fn pdkill is functionally identical to kill(2), except that it accepts a process descriptor, Fa fd , rather than a PID.

Fn pdwait4 behaves identically to wait4(2), but operates with respect to a process descriptor argument rather than a PID.

The following system calls also have effects specific to process descriptors:

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.

poll(2) and select(2) allow waiting for process state transitions; currently only POLLHUP is defined, and will be raised when the process dies.

close(2) will close the process descriptor unless 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 SIGKILL

and associated functions depend on options PROCDESC described in procdesc(4).


Fn pdfork returns a PID, 0 or -1, as fork(2) does.

Fn pdgetpid and Fn pdkill return 0 on success and -1 on failure.

Fn pdwait4 returns a PID on success and -1 on failure.


These functions may return the same error numbers as their PID-based equivalents (e.g. Fn pdfork may return the same error numbers as fork(2)), with the following additions:

The signal number given to Fn pdkill is invalid.
The process descriptor being operated on has insufficient rights (e.g. CAP_PDKILL for Fn pdkill ) .


The Fn pdfork , Fn pdgetpid , Fn pdkill and Fn pdwait4 system calls first appeared in Fx 9.0 .

Support for process descriptors mode was developed as part of the TrustedBSD Project.


An -nosplit These functions and the capability facility were created by An Robert N. M. Watson Aq [email protected] and An Jonathan Anderson Aq [email protected] at the University of Cambridge Computer Laboratory with support from a grant from Google, Inc.


Fn pdwait4 has not yet been implemented.