Provided by: libpcp3-dev_4.3.1-1_amd64 bug

NAME

       pmRegisterDerived,   pmRegisterDerivedMetric    -  register  a  derived  metric  name  and
       definition

C SYNOPSIS

       #include <pcp/pmapi.h>

       char *pmRegisterDerived(char *name, char *expr);
       int pmRegisterDerivedMetric(char *name, char *expr, char **errmsg);

       cc ... -lpcp

DESCRIPTION

       Derived metrics provide a way of extending the Performance Metrics Name Space (PMNS)  with
       new metrics defined at the PCP client-side using expressions over the existing performance
       metrics.

       Typical uses would be to aggregate a number of similar metrics to provide  a  higher-level
       summary  metric  or  to support the ``delta V over delta V'' class of metrics that are not
       possible in the base data semantics of PCP.  An example of the latter class would  be  the
       average I/O size, defined as
                          delta(disk.dev.total_bytes) / delta(disk.dev.total)
       where both of the disk.dev metrics are counters, and what is required is to to sample both
       metrics, compute the difference between the current and previous values and then calculate
       the ratio of these differences.

       The  arguments  to pmRegisterDerived are the name of the new derived metric and expr is an
       expression defining how the values of name should be computed.

       pmRegisterDerivedMetric is the exact functional  equivalent  to  pmRegisterDerived  except
       that  it  provides  a  simplified  model  of  error handling, where a formatted message is
       returned via the errmsg parameter.

       Syntactic checking is performed at the time  pmRegisterDerived  is  called,  but  semantic
       checking  is  deferred until each new PMAPI context is created with pmNewContext(3) or re-
       established with pmReconnectContext(3), at which time the PMNS and metadata  is  available
       to allow semantic checking and the metadata of the derived metrics to be determined.  This
       means pmRegisterDerived does not apply retrospectively to any open PMAPI contexts, nor  to
       any PMAPI contexts already open at the time pmRegisterDerived is called, so the normal use
       would be to make all calls to pmRegisterDerived (possibly via  pmLoadDerivedConfig(3))  or
       pmRegisterDerivedMetric and then call pmNewContext(3).

       name should follow the syntactic rules for the names of performance metrics, namely one or
       more components separated with a dot (``.''),  and  each  component  must  begin  with  an
       alphabetic  followed  by  zero or more characters drawn from the alphabetics, numerics and
       underscore (``_'').  For more details, refer to PCPIntro(1) and pmns(5).

       name must be unique across all derived metrics and  should  not  match  the  name  of  any
       regular  metric  in  the PMNS.  It is acceptable for name to share some part of its prefix
       with an existing subtree of the PMNS, e.g. the average I/O  size  metric  above  could  be
       named  disk.dev.avgsz which would place it amongst the other disk.dev metrics in the PMNS.
       Alternatively, derived metrics could populate their own subtree  of  the  PMNS,  e.g.  the
       average I/O size metric above could be named my.summary.disk.avgsz.

       The expression expr follows these syntactic rules:

       * Terminal  elements are either names of existing metrics or numeric constants.  Recursive
         definitions are not allowed, so only the names of regular  metrics  (not  other  derived
         metrics) may be used. Numeric constants are either integers constrained to the precision
         of 32-bit unsigned integers or double precision floating point numbers.

       * The  usual  binary  arithmetic  operators  are  supported,  namely   addition   (``+''),
         subtraction  (``-''),  multiplication  (``*'')  and  division  (``/'')  with  the normal
         precedence rules where multiplication and division have higher precedence than  addition
         and subtraction, so a+b*c is evaluated as a+(b*c).

       * Unary negation may be used, e.g.  -3*some.metric.

       * C-style  relational operators are supported, namely ``<'', ``<='', ``=='', ``>='', ``>''
         and ``!=''.  Relational expresssions return a value as a 32-bit unsigned number being  0
         for  false  and 1 for true.  The expected operator precedence rules apply, so arithmetic
         operators have higher precedence than relational operators, and a-b>c+d is evaluated  as
         (a-b)>(c+d).   All the relational operators have equal precedence, so the (slightly odd)
         expression involving consecutive relational operators a>b!=c is evaluated as (a>b)!=c.

       * C-style boolean operators are supported, namely and (``&&'') and or  (``||'').   Boolean
         expresssions  return  a  value  as  a 32-bit unsigned number being 0 for false and 1 for
         true.  The expected operator precedence rules apply, so relational operators have higher
         precedence    than    boolean    operators,    and   a>b*c&&d<=e+f   is   evaluated   as
         (a>(b*c))&&(d<=(e+f)).  Both  the  boolean  operators  have  equal  precedence,  so  the
         expression  involving consecutive boolean operators a>=b||b>c&&d!=e||f>g is evaluated as
         (((a>=b)||(b>c))&&(d!=e))||(f>g).

       * Additionally, the ``!''  operator  may  be  used  to  negate  a  boolean  or  relational
         expression,  returning  a  value as a 32-bit unsigned number being 0 for false and 1 for
         true.  The expected  operator  precedence  rules  apply,  so  boolean  (and  relational)
         operators  have  higher  precedence than boolean negation, and !a>b||c<d is evaluated as
         !((a>b)||(c<d)), while !a<b+c is evaluated as !(a<(b+c)).

       * C-style ternary conditional expressions are supported. In general terms  the  expression
         check ? foo : bar is evaluated as foo (the ``true'' operand) if check (the ``guard'') is
         true, else the expression evaluates  to  bar  (the  ``false''  operand).   Some  special
         semantic rules apply to the ``guard'' expression and the other two operand expressions:
         (a) Each expression may involve a singular value or a set of values (when the expression
             involves one or more metrics with an instance domain).
         (b) All expressions with a set of values must be defined over the same instance domain.
         (c) Both operand expressions must have the same  metadata,  so  the  same  metric  type,
             semantics and units (dimension and scale).
         (d) The  ``guard''  expression must have an aritmetic or relational or boolean value, so
             that it can be evaluated as 0 for false, else true.
         (e) If the ``guard'' expression has a singular value  and  one  or  more  of  the  other
             operand  expressions  involves  an  instance  domain,  the  ``guard'' applies to all
             instances.
         (f) If the ``guard'' expression has a set of values and one or more of the other operand
             expressions  involves  an  instance domain, the ``guard'' is evaluated once for each
             instance (there must be one instance domain as per rule (b) above).
         (g) If one of the operand expressions has a singular value and the other has  a  set  of
             values, and the singular value is selected based on the evaluation of the ``guard'',
             then the result is a set of values (all the same) with  instance  enumeration  being
             taken from the other operand expression. For example in the expression: foo ? scalar
             : set, if foo is true, then the result is a set  of  values  (all  having  the  same
             value, scalar) over the instance domain of set.

       * Selection  of  a  single  instance can be specified by the construct ``[instance_name]''
         which may be appended to a metric name or a parenthesized expression.  For example:
         fw.bytes = network.interface.in.bytes[eth1] + \
                    network.interface.out.bytes[eth1]
         or (equivalently):
         fw.bytes = (network.interface.in.bytes + \
                     network.interface.out.bytes)[eth1]

         All characters between the ``['' and ``]'' are considered to be part of  the  (external)
         instance name, so be careful to avoid any spurious white space.  A backslash may be used
         as an escape prefix in the (unlikely) event that the external instance name  contains  a
         ``]''.

       * Numeric  constants  can  also be specified using the mkconst() constructor which takes a
         number of arguments: the first is a numeric constant (either integer or floating point),
         then  follow  one  or  more  parameters  of the form tag=value or tag= where the allowed
         values of tag and value are as follows:

                       ┌──────────┬───────────────────────────────────────────────┐
                       │   tagvalue                     │
                       ├──────────┼───────────────────────────────────────────────┤
                       │type      │ one  of  the  numeric   metric   types   from │
                       │          │ <pcp/pmapi.h>,   stripped   of  the  PM_TYPE_ │
                       │          │ prefix, so 32, U32, 64, U64, FLOAT or DOUBLE. │
                       ├──────────┼───────────────────────────────────────────────┤
                       │semantics │ one of the semantic types from <pcp/pmapi.h>, │
                       │          │ stripped  of  the PM_SEM_ prefix, so COUNTER, │
                       │          │ INSTANT or DISCRETE.                          │
                       ├──────────┼───────────────────────────────────────────────┤
                       │units     │ a  specification  of  dimension   and   scale │
                       │          │ (together  forming  the units), in the syntax │
                       │          │ accepted by pmParseUnitsStr(3).               │
                       └──────────┴───────────────────────────────────────────────┘
         The value may optionally be enclosed in double quotes, and may  appear  in  any  mix  of
         upper and/or lower case.  The tag must be in lower case as shown in the table above.

         This is most useful when the expression semantics require matching type and/or semantics
         and/or units for operands, e.g.
         idle = mem.util.free > mkconst(10485760, units=Kbyte)
         avg_io_size = delta(disk.dev.total) == 0 ? \
             -mkconst(1.0, semantics=instant, units="kbyte / count") : \
             delta(disk.dev.total_bytes) / delta(disk.dev.total)

       * Expressions may be rescaled using the rescale function that takes  two  arguments.   The
         first  is  an  arithmetic expression to be rescaled, and the second is the desired units
         after rescaling that is a string value in the  syntax  accepted  by  pmParseUnitsStr(3).
         For example:
         rescale(network.interface.total.bytes, "Mbytes/hour")

         The  expression  and  the  desired units must both have the same dimension, e.g Space=1,
         Time=-1 and Count=0 in the example above.

       * The following unary functions operate on a single performance metric and return  one  or
         more  values.   For all functions (except count(), defined() and instant()), the type of
         the operand metric must be arithmetic (integer of various sizes and signedness, float or
         double).

                       ┌───────────┬───────────────────────────────────────────────┐
                       │ Function  │                     Value                     │
                       ├───────────┼───────────────────────────────────────────────┤
                       │avg(x)     │ A  singular  instance being the average value │
                       │           │ across all instances for the metric x.        │
                       ├───────────┼───────────────────────────────────────────────┤
                       │count(x)   │ A singular instance being the  count  of  the │
                       │           │ number  of  instances for the metric x.  As a │
                       │           │ special  case,  if  fetching  the  metric   x │
                       │           │ returns an error, then count(x) will be 0.    │
                       ├───────────┼───────────────────────────────────────────────┤
                       │defined(x) │ A  boolean  value that is true (``1'') if the │
                       │           │ metric x is defined in the PMNS,  else  false │
                       │           │ (``0'').   The  function  is evaluated when a │
                       │           │ new   PMAPI   context   is    created    with │
                       │           │ pmNewContext(3)    or   re-established   with │
                       │           │ pmReconnectContext(3).   So  any   subsequent │
                       │           │ changes  to  the PMNS after the PMAPI context │
                       │           │ has been  established  will  not  change  the │
                       │           │ value  of  this  function  in  the expression │
                       │           │ evaluation.                                   │
                       ├───────────┼───────────────────────────────────────────────┤
                       │delta(x)   │ Returns the  difference  in  values  for  the │
                       │           │ metric  x  between one call to pmFetch(3) and │
                       │           │ the next. There is one value  in  the  result │
                       │           │ for  each  instance  that appears in both the │
                       │           │ current and the previous sample.              │
                       ├───────────┼───────────────────────────────────────────────┤
                       │rate(x)    │ Returns the  difference  in  values  for  the │
                       │           │ metric  x  between one call to pmFetch(3) and │
                       │           │ the next divided by the elapsed time  between │
                       │           │ the  calls  to  pmFetch(3).  The semantics of │
                       │           │ the derived metric are based on the semantics │
                       │           │ of  the operand (x) with the dimension in the │
                       │           │ time domain decreased by one and  scaling  if │
                       │           │ required  in  the time utilization case where │
                       │           │ the operand is in  units  of  time,  and  the │
                       │           │ derived  metric is unitless.  This mimics the │
                       │           │ rate conversion applied to counter metrics by │
                       │           │ tools   such   as   pmval(1),   pmie(1)   and │
                       │           │ pmchart(1).  There is one value in the result │
                       │           │ for  each  instance  that appears in both the │
                       │           │ current and the previous sample.              │
                       ├───────────┼───────────────────────────────────────────────┤
                       │instant(x) │ Returns the current value of  the  metric  x, │
                       │           │ even  it has the semantics of a counter, i.e. │
                       │           │ PM_SEM_COUNTER.  The semantics of the derived │
                       │           │ metric  are  based  on  the  semantics of the │
                       │           │ operand   (x);    if    x    has    semantics │
                       │           │ PM_SEM_COUNTER,  the  semantics of instant(x) │
                       │           │ is PM_SEM_INSTANT, otherwise the semantics of │
                       │           │ the   derived  metric  is  the  same  as  the │
                       │           │ semantics of the metric x.                    │
                       ├───────────┼───────────────────────────────────────────────┤
                       │max(x)     │ A singular instance being the  maximum  value │
                       │           │ across all instances for the metric x.        │
                       ├───────────┼───────────────────────────────────────────────┤
                       │min(x)     │ A  singular  instance being the minimum value │
                       │           │ across all instances for the metric x.        │
                       ├───────────┼───────────────────────────────────────────────┤
                       │sum(x)     │ A singular instance  being  the  sum  of  the │
                       │           │ values across all instances for the metric x. │
                       └───────────┴───────────────────────────────────────────────┘
       * The  matchinst function may be used to select a subset of the instances from an instance
         domain for a metric or expression.  The function takes two arguments:
         (a) A instance filter that consists of an optional negation operator ``!'' followed by a
             regular  expression  delimited  by ``/'' characters.  The regular expression follows
             the POSIX Extended Regular Expression syntax as described in regex(3).   Backslashes
             may  be  used  as  escape  prefixes,  but double backslash is required to escape any
             regular expression special characters, e.g. for the  (extremely  unlikely)  case  of
             wanting  to match instance names like ``some*text/other[text]'' a regular expression
             of the  form  /some\\*text\/other\\[text]/  would  be  required.   If  present,  the
             negation operator reverses the sense of the filtering, so all instances not matching
             the regular expression will be selected.
         (b) A metric or expression that must be defined over an instance domain.

         For  example,   the   following   expression   will   have   values   for   the   metric
         network.interface.in.bytes  for  all  network interfaces except the loopback and virtual
         bridge devices:
         matchinst(!/^(lo)|(vbir)/, network.interface.in.bytes)

       * The scalar function may be used convert a metric or expression defined over an  instance
         domain into a scalar value that can be used in other expressions.  For example:
         net.in.bytes = scalar(network.interface.in.bytes[eth0]) + \
                    scalar(network.interface.in.bytes[eth1])

         The  instance  domain  is  removed  from  the  metadata  for the result and the instance
         identifier is removed from the value during fetching.

         If the metric or expression involves more than one instance then the result is formed by
         picking  the  first  instance - this is arbitrary and implies the scalar function should
         only be used for metrics or expressions  that  are  expected  to  contain  zero  or  one
         instances,  e.g.   the  construct  ``[instance_name]''  or the matchinst function with a
         pattern that matches at most one instance.

       * Parenthesis may be used for explicit grouping.

       * Lines beginning with ``#'' are treated as comments and ignored.

       * White space is ignored.

SEMANTIC CHECKS AND RULES

       There are a number of conversions required to determine the metadata for a derived  metric
       and to ensure the semantics of the expressions are sound.

       In  an arithmetic expression or a relational expression, if the semantics of both operands
       is not a counter (i.e. PM_SEM_INSTANT  or  PM_SEM_DISCRETE)  then  the  result  will  have
       semantics PM_SEM_INSTANT unless both operands are PM_SEM_DISCRETE in which case the result
       is also PM_SEM_DISCRETE.

       For an arithmetic expression, the dimension of each operand  must  be  the  same.   For  a
       relational expression, the dimension of each operand must be the same, except that numeric
       constants   (with   no    dimension)    are    allowed,    e.g.    in    the    expression
       network.interface.in.drops > 0 .

       To  prevent arbitrary and non-sensical combinations some restrictions apply to expressions
       that combine metrics with counter semantics to produce a result  with  counter  semantics.
       For  an arithmetic expression, if both operands have the semantics of a counter, then only
       addition or subraction is allowed, or if the left operand  is  a  counter  and  the  right
       operand  is  not, then only multiplication or division are allowed, or if the left operand
       is not a counter and the right operand is a counter, then only multiplication is allowed.

       Because relational expressions use the current value only and produce a result that is not
       a counter, either or both operands of a relational expression may be counters.

       The mapping of the pmUnits of the metadata uses the following rules:

       * If  both  operands  have  a  dimension of Count and the scales are not the same, use the
         larger scale and convert the values of the operand with the smaller scale.

       * If both operands have a dimension of Time and the scales  are  not  the  same,  use  the
         larger scale and convert the values of the operand with the smaller scale.

       * If  both  operands  have  a  dimension of Space and the scales are not the same, use the
         larger scale and convert the values of the operand with the smaller scale.

       * For addition and subtraction all dimensions for each of  the  operands  and  result  are
         identical.

       * For  multiplication,  the  dimensions of the result are the sum of the dimensions of the
         operands.

       * For division, the dimensions of the result are the difference of the dimensions  of  the
         operands.

       Scale conversion involves division if the dimension is positive else multiplication if the
       dimension is negative. If scale conversion is applied  to  either  of  the  operands,  the
       result is promoted to type PM_TYPE_DOUBLE.

       Putting  all  of  this  together  in  an  example,  consider the derived metric defined as
       follows:
                   x = network.interface.speed - delta(network.interface.in.bytes) /
                                       delta(sample.milliseconds)
       The type, dimension and scale settings would propagate up the expression tree as follows.

              ┌────────────────────────┬────────┬───────────────────┬────────────────────┐
              │      Expression        │  Type  │ Dimension & Scale │  Scale Factor(s)   │
              ├────────────────────────┼────────┼───────────────────┼────────────────────┤
              │sample.milliseconds     │ DOUBLE │ millisec          │                    │
              │delta(...)              │ DOUBLE │ millisec          │                    │
              │network...bytes         │ U64    │ byte              │                    │
              │delta(...)              │ U64    │ byte              │                    │
              │delta(...) / delta(...) │ DOUBLE │ byte/millisec     │ /1048576 and *1000 │
              │network...speed         │ FLOAT  │ Mbyte/sec         │                    │
              │x                       │ DOUBLE │ Mbyte/sec         │                    │
              └────────────────────────┴────────┴───────────────────┴────────────────────┘
       Expressions involving single instance selection or the matchinst function must be
       associated with underlying metrics that have an instance domain.  These constructors make
       no sense for singular metrics.

       Because semantic checking cannot be done at the time pmRegisterDerived is called, errors
       found during semantic checking (when any subsequent calls to pmNewContext(3) or
       pmReconnectContext(3) succeed) are reported using pmprintf(3).  These include:

       Error: derived metric <name1>: operand: <name2>: <reason>
              There was a problem calling pmLookupName(3) to identify the operand metric <name2>
              used in the definition of the derived metric <name1>.

       Error: derived metric <name1>: operand (<name2> [<pmid2>]): <reason>
              There was a problem calling pmLookupDesc(3) to identify the operand metric <name2>
              with PMID <pmid2> used in the definition of the derived metric <name1>.

       Semantic error: derived metric <name>: <operand> : <operand> Different <metadata> for
       ternary operands
              For a ternary expression, the ``true'' operand and the ``false'' operand must have
              exactly the same metadata, so type, semantics, instance domain, and units
              (dimension and scale).

       Semantic error: derived metric <name>: <operand> <op> <operand>: Dimensions are not the
       same
              Operands must have the same units (dimension and scale) for each of addition,
              subtraction, the relational operators and the boolean ``and'' or ``or'' operators.

       Semantic error: derived metric <name>: <operand> <op> <operand>: Illegal operator for
       counter and non-counter
              Only multiplication or division are allowed if the left operand has the semantics
              of a counter and the right operand is not a counter.

       Semantic error: derived metric <name>: <operand> <op> <operand>: Illegal operator for
       counters
              If both operands have the semantics of counter, only addition or subtraction make
              sense, so multiplication and division are not allowed.

       Semantic error: derived metric <name>: <operand> <op> <operand>: Illegal operator for non-
       counter and counter
              Only multiplication is allowed if the right operand has the semantics of a counter
              and the left operand is not a counter.

       Semantic error: derived metric <metric> <expr> RESCALE <units>: Incompatible dimensions
              The parameters <expr> and <units> to the rescale function must have the same
              dimension along the axes of Time, Space and Count.

       Semantic error: derived metric <name>: Incorrect time dimension for operand
              Rate conversion using the rate() function is only possible for operand metrics with
              a Time dimension of 0 or 1 (see pmLookupDesc(3)).  If the operand metric's Time
              dimension is 0, then the derived metrics has a value "per second" (Time dimension
              of -1).  If the operand metric's Time dimension is 1, then the derived metrics has
              a value of time utilization (Time dimension of 0).

       Semantic error: derived metric <name>: <function>(<operand>): Non-arithmetic operand for
       function
              The unary functions are only defined if the operand has arithmetic type.  Similarly
              the first argument to the rescale function must be of arithmetic type.

       Semantic error: derived metric <name>: <expr> ? ...: Non-arithmetic operand for ternary
       guard
              The first expression for a ternary operator must have an arithmetic type.

       Semantic error: derived metric <name>: ... - ...: Non-arithmetic operand for unary
       negation
              Unary negation only makes sense if the following expression has an arithmetic type.

       Semantic error: derived metric <name>: <operand> <op> <operand>: Non-arithmetic type for
       <left-or-right> operand
              The binary arithmetic operators are only allowed with operands with an arithmetic
              type (integer of various sizes and signedness, float or double).

       Semantic error: derived metric <name>: <operand> <op> <operand>: Non-counter and not
       dimensionless <left-or-right> operand
              For multiplication or division or any of the relational operators, if one of the
              operands has the semantics of a counter and the other has the semantics of a non-
              counter (instantaneous or discrete) then the non-counter operand must have no units
              (dimension and scale).

       Semantic error: derived metric <name>: <expr> ? <expr> : <expr>: Non-scalar ternary guard
       with scalar expressions
              If the ``true'' and ``false'' operands of a ternary expression have a scalar value,
              then the ``guard'' expression must also have a scalar value.

       Semantic error: derived metric <name>: <expr> <op> <expr>: Operands should have the same
       instance domain
              For all of the binary operators (arithmetic and relational), if both operands have
              non-scalar values, then they must be defined over the same instance domain.

EXPRESSION EVALUATION

       For the binary arithmetic operators, if either operand must be scaled (e.g. convert bytes
       to Kbytes) then the result is promoted to PM_TYPE_DOUBLE.  Otherwise the type of the
       result is determined by the types of the operands, as per the following table which is
       evaluated from top to bottom until a match is found.

                         ┌─────────────────────────┬──────────┬────────────────┐
                         │     Operand Types       │ Operator │  Result Type   │
                         ├─────────────────────────┼──────────┼────────────────┤
                         │either is PM_TYPE_DOUBLE │ any      │ PM_TYPE_DOUBLE │
                         ├─────────────────────────┼──────────┼────────────────┤
                         │any                      │ division │ PM_TYPE_DOUBLE │
                         ├─────────────────────────┼──────────┼────────────────┤
                         │either is PM_TYPE_FLOAT  │ any      │ PM_TYPE_FLOAT  │
                         ├─────────────────────────┼──────────┼────────────────┤
                         │either is PM_TYPE_U64    │ any      │ PM_TYPE_U64    │
                         ├─────────────────────────┼──────────┼────────────────┤
                         │either is PM_TYPE_64     │ any      │ PM_TYPE_64     │
                         ├─────────────────────────┼──────────┼────────────────┤
                         │either is PM_TYPE_U32    │ any      │ PM_TYPE_U32    │
                         ├─────────────────────────┼──────────┼────────────────┤
                         │otherwise (both are      │ any      │ PM_TYPE_32     │
                         │PM_TYPE_32)              │          │                │
                         └─────────────────────────┴──────────┴────────────────┘

CAVEATS

       Derived metrics are not available when using pmFetchArchive(3) as this routine does not
       use a target list of PMIDs that could be remapped (as is done for pmFetch(3)).

       There is no pmUnregisterDerived method, so once registered a derived metric persists for
       the life of the application.

DIAGNOSTICS

       On success, pmRegisterDerived returns NULL.

       If a syntactic error is found at the time of registration, the value returned by
       pmRegisterDerived is a pointer into expr indicating where the error was found.  To
       identify what the error was, the application should call pmDerivedErrStr(3) to retrieve
       the corresponding parser error message.

       pmRegisterDerivedMetric returns 0 and errmsg is undefined if the parsing is successful.

       If the given expr does not conform to the required syntax pmRegisterDerivedMetric returns
       -1 and a dynamically allocated error message string in errmsg.  The error message is
       terminated with a newline and includes both the input name and expr, along with an
       indicator of the position at which the error was detected.  e.g.
                 Error: pmRegisterDerivedMetric("my.disk.rates", ...) syntax error
                 4rat(disk.dev.read)
                     ^

       The position indicator line may be followed by an additional diagnostic line describing
       the nature of the error, when available.

       In the case of an error, the caller is responsible for calling free(3) to release the
       space allocated for errmsg.

SEE ALSO

       PCPIntro(1), free(3), PMAPI(3), pmDerivedErrStr(3), pmFetch(3), pmLoadDerivedConfig(3),
       pmNewContext(3), pmprintf(3) and pmReconnectContext(3).