A lightweight and simple client library for exposing metrics to Prometheus from C applications, contained in a single header file. It handles formatting the metrics and exposing an HTTP endpoint for the Prometheus scraper to pick them up.

To write an exporter, start by creating a prom_metric_set and initializing it.

prom_metric_set metrics;
prom_init(&metrics);

Register metrics by creating instances of prom_metric_def.

prom_metric_def current_time = {"current_time",
    "The current system time", PROM_METRIC_TYPE_COUNTER};
prom_register(&metrics, &current_time);

Obtain an instance of prom_metric by passing any number of labels to the prom_get function. If a metric with the given combination of labels does not exist, it will be created. Multiple calls to prom_get with the same labels will return the same pointer, but the labels must always be passed in the same order.

prom_label foo = {"foo", "bar"};
prom_label bing = {"bing", "bong"};

prom_metric *m = prom_get(&metrics, &current_time, 2, foo, bing);

You can start the server in a forked process. Then, you can manipulate the metric values in the main thread and write them out using prom_flush.

int pid = fork();
if (pid) {
  prom_start_server(&metrics, 50950);
} else {
  while (1) {
    m->value = time(NULL);
    prom_flush(&metrics);
    sleep(5);
  }
}

When you are done, call prom_cleanup to free the memory used by the metric set.

prom_cleanup(&metrics);

There are some more practical examples in the examples directory.

TODO:

• Option to include time in a metric reading
• Handle complex types like summary and histogram