CA - Perl 5 interface to EPICS Channel Access
use lib '/path/to/cap5/lib/perl';
use CA;
my $chan = CA->new('pvname');
CA->pend_io(1);
my @access = ('no ', '');
printf " PV name: %s\n", $chan->name;
printf " Data type: %s\n", $chan->field_type;
printf " Element count: %d\n", $chan->element_count;
printf " Host: %s\n", $chan->host_name;
printf " State: %s\n", $chan->state;
printf " Access: %sread, %swrite\n",
$access[$chan->read_access], $access[$chan->write_access];
die "PV not found!" unless $chan->is_connected;
$chan->get;
CA->pend_io(1);
printf " Value: %s\n", $chan->value;
$chan->create_subscription('v', \&callback, 'DBR_TIME_DOUBLE');
CA->pend_event(10);
sub callback {
my ($chan, $status, $data) = @_;
if ($status) {
printf "%-30s %s\n", $chan->name, $status;
} else {
printf " Value: %g\n", $data->{value};
printf " Severity: %s\n", $data->{severity};
printf " Timestamp: %.6f\n",
$data->{stamp} + $data->{stamp_fraction};
}
}
CA is an efficient interface to the EPICS Channel Access client library for
use by Perl 5 programs. It provides most of the functionality of the C library
(omitting Synchronous Groups) but only handles the three standard Perl data
types integer (long), floating point (double) and string (now including long
strings). Programmers who understand the C API will very quickly pick up how to
use this library since the calls and concepts are virtually identical.
Create a channel for the named PV. If given, SUB will be called whenever the connection state of the channel changes. The arguments passed to SUB are the channel object and a scalar value that is true if the channel is now up.
The underlying CA channel will be cleaned up properly when the channel object is garbage-collected by Perl.
The following methods are provided for channel objects returned by
CA->new().
The PV name provided when this channel was created.
Returns the native DBF type of the process variable as a string, or the string
TYPENOTCONN if unconnected.
The maximum array element count from the server. Zero if the channel is not connected.
A string containing the server's hostname and port number. If the channel is
disconnected it will report <disconnected>.
A true/false value that indicates whether the client has read or write access to the specified channel.
A string giving the current connection state of the channel, one of never
connected, previously connected, connected or closed.
Returns true if the channel is currently connected, else false. Use this
in preference to the equivalent code $chan->state eq 'connected'.
The get method makes a ca_get() request for a single element of the Perl
type closest to the channel's native data type; a DBF_ENUM field will be
fetched as a DBF_STRING, and a DBF_CHAR array with multiple elements will
converted into a Perl string. Once the server has returned the value (for which
see the pend_io function below) it can be retrieved using the channel's
value method. Note that the get method deliberately only provides limited
capabilities; the get_callback method must be used for more complex
requirements.
The get_callback method takes a subroutine reference or name and calls that
routine when the server returns the data requested. With no other arguments the
data type requested will be the widened form of the channel's native type
(widening is discussed below), and if the channel is an array the request will
fetch all available elements.
The element count can be overridden by providing an integer argument in the
range 0 .. element_count, where zero means use the current length from the
server. Note that the count argument must be an integer; add 0 to it if it is
necessary to convert it from a string.
The optional data type TYPE should be a string naming
the desired DBR_xxx_yyy type; the actual type used will have the yyy part
widened to one of STRING, CHAR, LONG or DOUBLE. The valid type
names are listed in the Channel Access Reference Manual under the
section titled Channel Access Data Types; look in the CA Type Code column of the
two tables.
The callback subroutine will be given three arguments: the channel object, a
status value from the server, and the returned data. If there were no errors
the status value will be undef and the data will be valid; if an error
occurred the data will be undef and the status a printable string giving more
information. The format of the data is described under Channel Data
below.
Callback subroutines should only call Perl's exit, die or similar
functions if they are expecting the program to exit at that time; attempts to
die with an exception object in the callback and catch that using eval in
the main thread are not likely to succeed and will probably result in a crash.
Callbacks should not perform any operations that would block for more than a
fraction of a second as this will hold up network communications with the
relevant server and could cause the Perl program and/or the Channel Access
server to crash. Calling CA->pend_event from within a callback is not
permitted by the underlying Channel Access library.
Register a state change subscription and specify a subroutine to be called
whenever the process variable undergoes a significant state change. MASK
must be a string containing one or more of the letters v, l, a and p
which indicate that this subscription is for Value, Log (Archive), Alarm and
Property changes. The subroutine SUB is called as described for the
get_callback method above, and the same optional TYPE and COUNT
arguments may be supplied to modify the data type and element count requested
from the server.
The create_subscription method returns a ca::subscription object which is
required to cancel that particular subscription. Either call the clear
method on that object directly, or pass it to the CA->clear_subscription
class method.
The put method makes a ca_put() or ca_array_put() call depending on the
number of elements given in its argument list. The data type used will be the
native type of the channel, widened to one of STRING, array of CHAR,
LONG or DOUBLE.
put_callback is similar to the put method with the addition of the
subroutine reference or name SUB which is called when the server reports that
all actions resulting from the put have completed. For some applications this
callback can be delayed by minutes, hours or possibly even longer. The data
type is chosen the same way as for put. The arguments to the subroutine will
be the channel object and the status value from the server, which is either
undef or a printable string if an error occurred. The same restrictions
apply to the callback subroutine as described in get_callback above.
Applications that need to acknowledge alarms by doing a ca_put() with type
DBR_PUT_ACKS can do so using the put_acks method. The severity argument
may be provided as an integer from zero through three or as a string containing
one of the corresponding EPICS severity names NO_ALARM, MINOR, MAJOR or
INVALID. If a subroutine reference is provided it will be called after the
operation has completed on the server as described in put_callback above.
This method is for applications that need to enable/disable transient alarms by
doing a ca_put() with type DBR_PUT_ACKT. The TRANS argument is a
true/false value, and an optional subroutine reference can be provided as
described above.
This method replaces, adds or cancels the connection handler subroutine for the
channel; see the new constructor for details. If SUB is undef any
existing handler is removed, otherwise the new subroutine will be used for all
future connection events on this channel.
The data provided to a callback function registered with either get_callback
or create_subscription can be a scalar value or a reference to an array or a
hash, depending on the data type that was used for the data transfer. If the
request was for a single item of one of the basic data types, the data argument
will be a Perl scalar that holds the value directly. If the request was for
multiple items of one of the basic types, the data argument will be a reference
to an array holding the data. There is one exception though; if the data type
requested was for an array of DBF_CHAR values that array will be represented
as a single Perl string containing all the characters before the first zero
byte.
If the request was for one of the compound data types, the data argument will be
a reference to a hash with keys as described below. Keys that are not classed
as metadata are named directly after the fields in the C struct dbr_xxx_yyy,
and are only included when the C structure contains that particular field.
These metadata will always be present in the hash:
The DBR_xxx_yyy name of the data type from the server. This might have been
widened from the original type used to request or subscribe for the data.
The number of elements in the data returned by the server. If the data type is
DBF_CHAR the value given for COUNT is the number of bytes (including any
trailing zeros) returned by the server, although the value field is given as a
Perl string containing all the characters before the first zero byte.
These fields are always present in the hash:
The actual process variable data, expressed as a Perl scalar or a reference to
an array of scalars, depending on the request. An array of DBF_CHAR elements
will be represented as a string; to access the array elements as numeric values
the request must be for the DBF_LONG equivalent data type.
If TYPE is DBR_GR_ENUM or DBR_CTRL_ENUM, value can be accessed both
as the integer choice value and (if within range) as the string associated with
that particular choice.
The alarm status of the PV as a printable string, or undef if not in alarm.
The alarm severity of the PV, or undef if not in alarm. A defined severity
can be used as a human readable string or as a number giving the numeric value
of the alarm severity (1 = MINOR, 2 = MAJOR, 3 = INVALID).
These fields are only present for some values of TYPE:
A reference to an array containing all the possible choice strings for an ENUM.
Present only when TYPE is DBR_GR_ENUM or DBR_CTRL_ENUM.
The number of choices defined for an ENUM.
Present only when TYPE is DBR_GR_ENUM or DBR_CTRL_ENUM.
The process variable timestamp, converted to a local time_t. This value is
suitable for passing to the Perl localtime or gmtime functions.
Present only when TYPE is DBR_TIME_yyy.
The fractional part of the process variable timestamp as a positive floating point number less than 1.0.
Present only when TYPE is DBR_TIME_yyy.
The value of the process variable's transient acknowledgment flag, an integer.
Present only when TYPE is DBR_STSACK_STRING.
The alarm severity of the highest unacknowledged alarm for this process
variable. As with the severity value, this scalar is both a string and
numeric severity.
Present only when TYPE is DBR_STSACK_STRING.
The process variable's display precision, an integer giving the number of decimal places to display.
Present only when TYPE is DBR_GR_DOUBLE or DBR_CTRL_DOUBLE.
The engineering units string for the process variable.
Present only when TYPE is DBR_GR_yyy or DBR_CTRL_yyy where yyy is
not STRING.
The display range for the process variable; graphical tools often provide a way to override these limits.
Present only when TYPE is DBR_GR_yyy or DBR_CTRL_yyy where yyy is
not STRING.
These items give the values at which the process variable should go into an alarm state, although in practice the alarm severity associated with each level is not provided.
Present only when TYPE is DBR_GR_yyy or DBR_CTRL_yyy where yyy is
not STRING.
The range over which a client can control the value of the process variable.
Present only when TYPE is DBR_CTRL_yyy where yyy is not STRING.
The following functions are not channel methods, and should be called using the
class method syntax, e.g. CA->pend_io(10).
Returns the EPICS_VERSION_STRING from the version of EPICS Base this software was built using.
Flush outstanding IO requests to the server. This routine is useful for users who need to flush requests prior to performing client side labor in parallel with labor performed in the server. Outstanding requests are also sent whenever the buffer which holds them becomes full. Note that the routine can return before all flush operations have completed.
This function tests to see if all get requests are complete and channels
created without a connection callback subroutine are connected. It will return
a true value if all such operations are complete, otherwise false.
This function flushes the send buffer and then blocks until all outstanding
get requests complete and all channels created without a connection callback
subroutine have connected for the first time. Unlike pend_event, this
routine does not process CA's background activities if no IO requests are
pending.
If any I/O or connection operations remain incomplete after TIMEOUT seconds,
the function will die with the error ECA_TIMEOUT; see ERROR HANDLING
below. A TIMEOUT interval of zero is taken to mean wait forever if
necessary. The TIMEOUT value should take into account worst case network
delays such as Ethernet collision exponential back off until retransmission
delays which can be quite long on overloaded networks.
Flush the send buffer and process CA's background activities for TIMEOUT seconds. This function always blocks for the full TIMEOUT period, and if a value of zero is used it will never return.
It is generally advisable to replace any uses of Perl's built-in function
sleep with calls to this routine, allowing Channel Access to make use of the
delay time to perform any necessary housekeeping operations.
Flush the send buffer and process any outstanding CA background activity.
Cancel a subscription. Note that for this to take effect immediately it is
necessary to call CA->flush_io or one of the other class methods that
flushes the send buffer.
Trap exception events and execute SUB whenever they occur. The subroutine is
provided with four arguments: The channel object (if applicable), the status
value from the server, a printable context string giving more information about
the error, and a hash reference containing some additional data. If the
exception is not specific to a particular channel the channel object will be
undef. The status value is a printable string. The hash may contain any of
the following members:
The operation in progress when the exception occurred. This scalar when used as
a string is one of GET, PUT, CREATE_CHANNEL, ADD_EVENT,
CLEAR_EVENT or OTHER but can also be accessed as an integer (0-5).
The DBR_xxx_yyy name of the data type involved.
The number of elements in the request.
These refer to the source file and line number inside the CA client library where the exception was noticed.
This function provides a method to trap error messages from the CA client
library and redirect them to somewhere other than the STDERR stream. The
subroutine provided will be called with a single string argument every time the
client library wishes to output an error or warning message. Note that a single
error or warning message may result in several calls to this subroutine.
To revert back to the original handler, call CA->replace_printf_handler()
passing undef as the subroutine reference.
Errors in using the library will be indicated by the module throwing an
exception, i.e. calling croak() with an appropriate error message. These
exceptions can be caught using the standard Perl eval {} statement and
testing the $@ variable afterwards; if not caught, they will cause the
running program to die with an appropriate error message pointing to the
program line that called the CA library.
Error messages reported by the underlying CA client library all start with the
string ECA_ and the remainder of the symbol for the associated CA error
number, and are followed after a space-hyphen-space by a human-readable message
describing the error. Errors that are detected by the Perl interface layer do
not follow this pattern, but are still printable strings.
Andrew Johnson, <anj@aps.anl.gov>
Copyright (C) 2008-2014 UChicago Argonne LLC, as Operator of Argonne National Laboratory.
This software is distributed under the terms of the EPICS Open License.