This documentation is intended for internal Performance Co-Pilot
(PCP) developer use.
These interfaces are not part of the PCP APIs that are guaranteed to
remain fixed across releases, and they may not work, or may provide
different semantics at some point in the future.
The routines implement an event queue and callback framework that
supports periodic evaluation of a series of events with varying
frequencies for Performance Co-Pilot (PCP) applications.
The pmlogger(1) application, the pmdatrace(1) PMDA and the
pmdahotproc(1) PMDA are the principal users of these services.
An event is created by calling __pmAFsetup or __pmAFregister and on
success the return value is an event number greater than zero. The
event has associated event data identified by the opaque pointer
data. The event will occur with frequency delta and each time the
event occurs the function func will be called with the event number
and the event data as arguments.
If __pmAFsetup is used then the first event is scheduled for the
current time plus start, else if __pmAFregister is used then the
first event is scheduled for the current time plus delta.
func is called in a SIGALRM signal handler context and so the
routines that may be safely called from func are restricted to the
so-called async-signal-safe set. In particular there must be no
Standard I/O calls nor calls to any of the malloc(3) routines to
modify the state of the heap. Refer to the Pointer to a Function
Section of the POSIX.1-2013 document at
for a fuller description.
The safest and simplest class of func routines are those that do
minimal processing, set some global state and return. The real work
associated with the event is done subsequently from the application's
main loop when the global state change is detected.
Once the event occurs and the callback has been executed, the event
will be rescheduled for delta into the future, except if all the
fields of delta are zero, in which case the event will not be
rescheduled (a ``one trip'' event).
Internally, events are processed serially so there is no possibility
of nested callbacks or re-entrant callbacks from the event management
Given an event number afid, __pmAFunregister will permanently remove
the corresponding entry from the event queue.
To control the event queue processing, __pmAFblock and __pmAFunblock
may be used to explicitly block and unblock the dispatch of events.
This is most useful when the caller wishes to set up a number of
events via __pmAFsetup or __pmAFregister and complete the
registration phase before the first event callback occurs.
A call to __pmAFisempty returns 1 or 0 depending on whether the event
queue is empty or not.
__pmAFsetup, __pmAFregister and __pmAFunregister return values less
than zero in the case of an error. These values are PCP error codes,
and may be used to produce error messages via pmErrStr(3).
The routines support the standard PCP debug tracing, and the af
option (or -D af on the command line) will produce diagnostics on
standard error that trace the enqueuing and execution of events.
These routines rely on setitimer(2) and manipulate the handling of
SIGALRM signals, and hence are probably ill-suited for applications
that require direct and concurrent access to these services and
If the callback functions are slow, or delayed, it is possible that
the event scheduling could fall behind and never catchup. When this
begins to happen, events are silently skipped and rescheduled at the
earliest possible time in the future according to the fixed schedule
defined by the time of the call to __pmAFsetup and the value of the
start and delta arguments (or defined by the time of the call to
__pmAFregister and the value of the delta argument).
In addition, the semantics of the interval timer(s) and the global
state needed to support these services demand that applications
calling these routines must do so from a single thread. This
restriction is enforced at the PMAPI(3), where routines may return
the error code PM_ERR_THREAD if the library detects calls from more
than one thread.
This page is part of the PCP (Performance Co-Pilot) project.
Information about the project can be found at ⟨http://www.pcp.io/⟩.
If you have a bug report for this manual page, send it to
email@example.com. This page was obtained from the project's upstream
Git repository ⟨https://github.com/performancecopilot/pcp.git⟩ on
2020-06-09. (At that time, the date of the most recent commit that
was found in the repository was 2020-06-09.) If you discover any
rendering problems in this HTML version of the page, or you believe
there is a better or more up-to-date source for the page, or you have
corrections or improvements to the information in this COLOPHON
(which is not part of the original manual page), send a mail to
Performance Co-Pilot PCP PMAF(3)