The pcp_trace library functions provide a mechanism for identifying
sections of a program as transactions or events for use by the trace
Performance Metrics Domain Agent (refer to pmdatrace(1) and PMDA(3)).
The monitoring of transactions using the Performance Co-Pilot (PCP)
infrastructure is initiated through a call to pmtracebegin. Time
will be recorded from the end of each pmtracebegin call to the start
of the following call to pmtraceend, where the same tag identifier is
used in both calls. Following from this, no visible recording will
occur until at least one call to pmtraceend is made referencing a tag
previously used in a call to pmtracebegin.
A transaction which is currently in progress can be cancelled by
calling pmtraceabort. No transaction data gathered for that
particular transaction will be exported, although data from previous
and subsequent successful transactions with that tag name are still
exported. This is most useful when an error condition arises during
transaction processing and the transaction does not run to
The tag argument to pmtracebegin, pmtraceend and pmtraceabort is used
to uniquely identify each transaction within the pcp_trace library
and later by the trace PMDA as the instance domain identifiers for
the transaction performance metrics which it exports. These routines
are most useful when used around blocks of code which are likely to
be executed a number of times over some relatively long time period
(in a daemon process, for example).
pmtracebegin has two distinct roles - firstly as the initiator of a
new transaction, and secondly as a mechanism for setting a new start
time. Similarly, pmtraceend is used to register a new tag and its
initial state with the trace PMDA, or alternatively to update the
statistics which the PMDA currently associates with the given tag.
A second form of program instrumentation can be obtained from
pmtracepoint. This is a simpler form of monitoring as it exports
only the number of times that a particular point in a program has
been passed. This differs to the transaction monitoring offered by
pmtracebegin and pmtraceend, which exports a running count of
successful transaction completions as well as statistics on the time
interval between the start and end points of each transaction. This
function is most useful when start and end points are not well
defined. Examples of this would be when the code branches in such a
way that a transaction cannot be clearly identified, or when
processing does not follow a transactional model, or the desired
instrumentation is akin to event rates rather than event service
The pmtraceobs and pmtracecounter functions have similar semantics to
pmtracepoint, but also allow an arbitrary numeric value to be passed
to the trace PMDA. The most recent value for each tag is then
immediately available from the PMDA. The only difference between
pmtraceobs and pmtracecounter is that the value exported via
pmtracecounter is assumed to be a monotonically increasing counter
value (e.g. the number of bytes read from a socket), whereas the
value exported via pmtraceobs can be any value at all.
pmtracestate allows the application to set state flags which are
honoured by subsequent calls to the pcp_trace library routines.
There are currently two types of flag - debugging flags and the
asynchronous protocol flag. A single call may specify a number of
flags together, combined using a (bitwise) logical OR operation, and
overrides the previous state setting.
The debugging flags to pmtracestate cause pcp_trace to print
diagnostic messages on the standard output stream at important
processing points. The default protocol used between the trace PMDA
and individual pcp_trace client applications is a synchronous
protocol, which allows for dropped connections to be reestablished at
a later stage should this become possible. An asynchronous protocol
is also available which does not provide the reconnection capability,
but which does away with much of the overhead inherent in synchronous
communication. This behaviour can be toggled using the pmtracestate
call, but must be called before other calls to the library. This
differs to the debugging state behaviour, which can be altered at any
time. pmtracestate returns the previous state (setting prior to
The following table describes each of the pmtracestate flags -
examples of the use of these flags in each supported language are
given in the demo applications (refer to the ``FILES'' section
│State Flags │ Semantics │
│0 NONE │ Synchronous PDUs and no diagnostics (default) │
│1 API │ Shows processing just below the API (debug) │
│2 COMMS │ Shows network-related activity (debug) │
│4 PDU │ Shows app<->PMDA IPC traffic (debug) │
│8 PDUBUF │ Shows internal IPC buffer management (debug) │
│16 NOAGENT │ No PMDA communications at all (debug) │
│32 ASYNC │ Use the asynchronous PDU protocol (control) │
Should any of the pcp_trace library functions return a negative
value, an error has occurred. This can be diagnosed further using
the pmtraceerrstr routine, which takes the negative return value as
its code argument, and in the C-callable interface returns a pointer
to the associated error message. This points into a static error
table, and should therefore not be passed to free(3). The Fortran-
callable interface has a slightly different syntax, requiring the
destination character array to be passed in as the second argument.
The Java-callable interface returns a UTF-8 string, created using the
JNI (Java Native Interface) routine NewStringUTF.
The pcp_trace routines communicate with the trace PMDA via a socket
connection, which by default uses TCP/IP port number 4323. This can
be over-ridden by setting PCP_TRACE_PORT to a different port number
when the application is started. The host where the trace PMDA is
running is by default the localhost, but this can be changed using
PCP_TRACE_HOST. When attempting to connect to a remote trace PMDA,
after some specified time interval has elapsed, the connection
attempt will be aborted and an error status will be returned. The
default timeout interval is 3 seconds, and this can be modified by
setting PCP_TRACE_TIMEOUT in the environment to a real number of
seconds for the desired timeout. This is most useful in cases where
the remote host is at the end of a slow network, requiring longer
latencies to establish the connection correctly.
Sample C programs and source for pmtrace(1). Use make(1)
to build these programs.
Sample Fortran program. Call `make fortran77' or `make
fortran90' to build this program.
Sample Java program. `make java' builds the java class
Java trace class definition.
Environment variables with the prefix PCP_ are used to parameterize
the file and directory names used by PCP. On each installation, the
file /etc/pcp.conf contains the local values for these variables.
The $PCP_CONF variable may be used to specify an alternative
configuration file, as described in pcp.conf(5). Values for these
variables may be obtained programmatically using the pmGetConfig(3)
file:$PCP_DOC_DIR/Tutorial/trace.html, pcp.man.tutorial, Provided
the, make(1), pmcd(1), pmdatrace(1), pmprobe(1), pmtrace(1), Relevant
information is also available from the on-line PCP Tutorial,
subsystem from the PCP images has been installed, access the URL and
from your web browser.
A negative return value from a pcp_trace function indicates that an
error has occurred - if this is the case, the return value can be
passed to pmtraceerrstr to obtain the associated error message.
Success is indicated by a return value of zero.
pmtracestate also returns an integer representing the state flags
which were set prior to the call.
Applications that use gethostbyname(3) should exercise caution
because the static fields in struct hostent may not be preserved
across some pcp_trace calls. In particular, pmtracebegin,
pmtraceend, pmtracepoint, pmtracecounter, and pmtraceobs may all call
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
firstname.lastname@example.org. This page was obtained from the project's upstream
Git repository ⟨https://github.com/performancecopilot/pcp.git⟩ on
2020-07-14. (At that time, the date of the most recent commit that
was found in the repository was 2020-07-14.) 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 PMDATRACE(3)