pmjsonget(3) — Linux manual page


PMJSONGET(3)            Library Functions Manual            PMJSONGET(3)

NAME         top

       pmjsonGet, pmjsonPrint, pmjsonInit, pmjsonInitIndom - JSON string
       helpers and metrics extraction

C SYNOPSIS         top

       #include <pcp/pmapi.h>
       #include <pcp/pmjson.h>

       typedef int (*json_get)(void *buffer, int length, void *data);
       int pmjsonGet(json_metric_desc *json_metrics, int nmetrics,
               pmInDom indom, json_get callback, void *data);
       int pmjsonPrint(FILE *fp, json_flag *output_flags, const char
               *json_pointer, json_get callback, void *data);

       int pmjsonInit(int fd, json_metric_desc *json_metrics, int
       int pmjsonInitIndom(int fd, json_metric_desc *json_metrics, int
               nmetrics, pmInDom indom);

       cc ... -lpcp_web

DESCRIPTION         top

       pmjsonGet performs metric value extraction from JSON strings for
       a Performance Metric Domain Agent (PMDA(3)).  Metrics and
       instances are defined by the json_metrics parameter.  The result
       of parsing is the extraction of values for the requested metrics
       (as defined in the json_pointer field) in the values field.

       A simple diagnostic utility for reporting JSON strings iss
       provided as a convenience as well.  Its output_flags argument
       allows it to support a variety of alternate output formats,
       including minimal JSON (no optional whitespace), pretty-printed,
       and a simple YAML subset.

       json_metrics is the array of json_metric_desc struct which
       contains the json pointer to the metric data, possible flags to
       check the data against, number of values (in the case of an
       array), pmAtomValue where the data is stored, as well as the
       instance domain in the event the PMDA has multiple instance
       domains.  The number of elements in the json_metrics array is
       indicated by the nmetrics parameter.

       Interfaces for common cases are provided - pmjsonInit specifies a
       call to the JSON parsing and metric value extraction
       functionality where there are no instance domains (or the PMDA
       wishes to perform instance domain operations itself) and where
       data can be read directly from an open file descriptor.
       pmjsonInitIndom is similar, but makes use of the pmdaCacheAdd(3)
       function where possible.

       The JSON string must be provided by the callback function.  This
       function is passed a memory buffer of length bytes and an opaque
       user data pointer which is unchanged from the original call to
       pmjsonGet or pmjsonPrint.  The returned result must be the number
       of bytes written into buffer, zero when no more data is available
       or less than zero on error conditions.

       indom is the instance domain to be passed to pmdaCacheAdd.

            typedef struct json_metrics_desc {
                char          *json_pointer;  /* json pointer to metric */
                int           flags;          /* flags to check if set */
                int           num_values;     /* number of values */
                pmAtomValue   values;         /* metric value */
                char          *dom;           /* instance domain */
            } json_metric_desc;

       The JSON Pointer syntax current follows RFC6901 in specifying a
       metrics location within the JSON document.

EXAMPLE         top

       For the sample JSON document:

           "Name": "/clever_almeida",
           "State": {
               "Dead": false,
               "Error": "",
               "ExitCode": 0,
               "FinishedAt": "2016-07-18T21:21:20.332488706Z",
               "OOMKilled": false,
               "Paused": false,
               "Pid": 0,
               "Restarting": false,
               "Running": false,
               "StartedAt": "2016-07-18T14:10:58.52487316Z"

       A possible corresponding json_metric array would be as follows
       for the Pid, Name, and metrics to indicate whether the container
       is Running, Paused, or Restarting.

       static json_metric_desc json_metrics[] = {
           { "State/Pid", 0, 1, {0}, ""},
           { "Name", 0, 1, {0}, ""},
           { "State/Running", CONTAINER_FLAG_RUNNING, 1, {0}, ""},
           { "State/Paused", CONTAINER_FLAG_PAUSED, 1, {0}, ""},
           { "State/Restarting", CONTAINER_FLAG_RESTARTING, 1, {0}, ""},

RETURN CODE         top

       The functions return zero on success, otherwise an error code
       suitable for passing to pmErrStr_r(3) is returned.

SEE ALSO         top

       PMDA(3), pmErrStr_r(3), pmdaCacheAdd(3), , , and .

COLOPHON         top

       This page is part of the PCP (Performance Co-Pilot) project.
       Information about the project can be found at 
       ⟨⟩.  If you have a bug report for this manual
       page, send it to  This page was obtained from the
       project's upstream Git repository
       ⟨⟩ on 2023-12-22.
       (At that time, the date of the most recent commit that was found
       in the repository was 2023-12-16.)  If you discover any rendering
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       (which is not part of the original manual page), send a mail to

Performance Co-Pilot               PCP                      PMJSONGET(3)

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