# pmRegisterDerived - Man Page

register a global 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 value-a over delta value-b” 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 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.

If **pmRegisterDerived** is called after one or more PMAPI contexts has been opened, then the newly registered metrics will be available in those contexts, however the more normal use would be to make all calls to **pmRegisterDerived** (possibly via pmLoadDerivedConfig(3)) or **pmRegisterDerivedMetric** before calling pmNewContext(3).

All of the defined global derived metrics are available in all PMAPI contexts.

It is also possible to define per-context derived metrics once a PMAPI context has been established. These derived metrics are private to the context in which they are defined using the allied routines pmAddDerived(3) and pmAddDerivedMetric(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 expressions 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 expressions 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 arithmetic 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:

`check ? foo : bar`

, if`foo`

is an expression with a singular value and`bar`

is a set-valued expression, then if`check`

is true, then the result is a set of values (all having the same value,`foo`

) over the instance domain of`bar`

.- (h)
As a special case, either (but not both) of the expressions

`foo`

or`bar`

may be the special constructor`novalue()`

. If`novalue()`

needs to be evaluated, it will return no values. The most common use of`novalue()`

is in conjunction with`defined()`

as in the following example:`mumble = defined(abc.def) ? count(abc.def) : novalue()`

The metadata for

`novalue()`

is taken from the peer operand of the`:`

operator, making conditions (b) and (c) above trivially true; if the peer operand cannot be evaluated, then the default metadata for`novalue()`

is a singular discrete integer with no units, and conditions (b) and (c) are waived.An extension of

`novalue()`

is`novalue(<metadata>)`

where the`<metadata>`

can be specified using one or more parameters of the form*tag=value*or*tag=*"*value*" as described for the`mkconst()`

constructor below, for example:`novalue(type=float, semantics=instant, units="Kbytes/sec")`

- (i)
As an additional further case, if

`check`

is of the form`defined(somemetric)`

then references to undefined metrics are allowed in whichever of`foo`

or`bar`

is**not**required once the existence if`somemetric`

has been established. This allows uses of the form:`fumble = defined(new.metric) ? new.metric : old.metric`

which is valid when`new.metric`

is defined**and**when`new.metric`

is**not**defined, although this does mean rules (b) and (c) are relaxed in this case, which further means`novalue()`

may have no peer operand to provide metadata.A generalization of this construct is supported for any

`check`

that can be evaluated statically, so a boolean expression involving`defined()`

predicates, for example:`bar = !defined(a) || !defined(b) ? novalue() : a + b`

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=*"*value*" where the allowed values of*tag*and*value*are as follows:tag value 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). meta a metric name and that metric provides the base metadata that may be modified by other parameters 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()`

and`defined()`

), 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.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 following unary function returns the instantaneous value of an expression, not the rate-converted value that is the default for expressions with the semantics of PM_SEM_COUNTER.

Function Value instant(expr) Returns the current value of the expression, 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 expression `expr`

; if`expr`

has semantics PM_SEM_COUNTER, the semantics of`instant(expr)`

is PM_SEM_INSTANT, otherwise the semantics of the derived metric is the same as the semantics of`expr`

.The following unary functions return values computed from the value of an expression on consecutive samples, or pmFetch(3) calls. The expression (

`expr`

below) may involve one or more metrics but must have an arithmetic value (integer of various sizes and signedness, float or double) for all instances.If

`expr`

is a set-valued expression then only those instances that appear in**both**samples will appear in the result.Function Value delta(expr) Returns the difference in values for the expression 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. If the expression is unsigned, then the type of the result is converted to ensure as much precision as possible can be retained, so if the expression has type PM_TYPE_U32 then the result is of type PM_TYPE_64, else if the expression has type PM_TYPE_U64 then the result is of type PM_TYPE_DOUBLE. Otherwise the type of the result is the same as the type of the expression. rate(expr) Returns the difference in values for the expression 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 expression 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. There is one value in the result for each instance that appears in both the current and the previous sample, except in the case where the expression has the semantics of a counter, i.e. PM_SEM_COUNTER, and current value of an instance is smaller than the previous value of the same instance then no value is returned for this instance (this corresponds to a “counter wrap” or a “counter reset”). These rules mimic the rate conversion applied to counter metrics by tools such as pmval(1), pmie(1) and pmchart(1).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). A single backslash may be used to escape the regular expression delimiter “/”, but double backslashes are 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.
- A line ending with “\” is treated as “to be continued” and the following line is appended after stripping the “\” and the embedded newline.
- 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 subtraction 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 for <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.

- Semantic error: derived metric <name>: operand <dname>: Illegal nested derived metric
A derived metric (<dname>) cannot be nested (i.e. used) in the definition of another derived metric (<name>).

- Semantic error: derived metric <name>: <expr> <op> <expr>: Illegal operator for non-counters
Metrics with counter semantics may be added or subtracted, but <op> (division or multiplication) is not allowed.

- Semantic error: derived metric <name>: operand <bname>: Unknown metric for ternary expression
When a new context was established, the metric <bname> was not in the PMNS of the new context and <bname> is a required operand in the definition of the derived metric <name>.

## 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 PM_TYPE_32) | any | PM_TYPE_32 |

## PMIDs AND MASKING

Within PCP each metric is assigned a unique Performance Metric Identifier (PMID) and internally a PMID is constructed from 3 fields: the domain number (of the associated Performance Metrics Domain Agent, or PMDA), the cluster number and the item number. Derived metrics use the reserved domain number 511 and special PMIDs as described in the following table, where the PMID is shown as domain.cluster.item:

Derived Metric | Starting PMID | Description |

global | 511.0.1 | Metrics are assigned ascending PMIDs as they are registered via pmRegisterDerived, pmRegisterDerivedMetric or pmLoadDerivedConfig(3). |

per-context | 511.2047.1023 | Metrics are assigned descending PMIDs as they are registered via pmAddDerived(3) or pmAddDerivedMetric(3). |

remapped | 511.c.i | When a derived metric is recorded in a PCP archive by pmlogger(1) or one of the related archive creation tools, the PMID of the derived metric is remapped in the archive so that c is the cluster of derived metric plus 2048, and i is the item of the derived metric. For example, a derived metric with PMID 511.0.13 will have the remapped PMID 511.2048.13 in an archive. See pmlogger(1) for a discussion about adding derived metrics to a PCP archive. |

For base metrics from a host or archive context, there is a requirement that each metric name is unique. But derived metrics require this restriction to be relaxed, so that the same metric name could associated with a base metric, and/or a per-context derived metric, and/or a global metric. This means that one or more of the metrics with the same name may be masked by other metrics of the same name. The following table describes which metric will be used when one of these multiply-defined names is presented to the PMAPI.

base metric | derived metric | chosen metric | ||

remapped | per-context | global | ||

no | no | no | no | none; the metric is undefined |

no | no | no | yes | global derived |

no | no | yes | no | per-context derived |

no | no | yes | yes | per-context derived |

no | yes | no | no | remapped |

no | yes | no | yes | remapped |

no | yes | yes | no | remapped |

no | yes | yes | yes | remapped |

yes | no | no | no | base metric |

yes | no | no | yes | base metric |

yes | no | yes | no | base metric |

yes | no | yes | yes | base metric |

yes | yes | - | - | cannot happen; for any name there can be at most one of a base metric or a remapped metric |

When masking happens, it is done silently. Most PCP applications support **-Dderive** on the command line and this pmdbg(1) debug option will report derived metric operations (including masking) on stderr.

## 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 **pmRegisterDerivedMetric** caller is responsible for calling free(3) to release the space allocated for *errmsg*.

## See Also

PCPIntro(1), pmlogger(1), PMAPI(3), free(3), pmAddDerived(3), pmAddDerivedMetric(3), pmDerivedErrStr(3), pmFetch(3), pmLoadDerivedConfig(3), pmNewContext(3), pmReconnectContext(3), pmprintf(3) and PMNS(5).

## Referenced By

pcp2elasticsearch(1), pcp2graphite(1), pcp2influxdb(1), pcp2json(1), pcp2openmetrics(1), pcp2spark(1), pcp2xlsx(1), pcp2xml(1), pcp2zabbix(1), pcp-dstat(5), pcp-htop(5), pmAddDerived(3), pmAddDerivedText(3), pmCreateFetchGroup(3), pmDerivedControl(3), pmDerivedErrStr(3), pmiectl(1), pminfo(1), pmLoadDerivedConfig(3), pmlogctl(1), pmlogger(1), pmReconnectContext(3), pmrep(1), pmrep.conf(5).

The man page pmRegisterDerivedMetric(3) is an alias of pmRegisterDerived(3).