This is a first draft of a C API for generating PPMP messages. The API is designed to be used with constrained devices, i.e. it has a very modest footprint and does not use the heap. Buffering of messages is factored out into a callback.
To generate a message, a PPMP structure has to be initialized using ppmp_init():
#include <ppmp.h> ... PPMP ppmp; ppmp_init(&ppmp, NULL, NULL, PPMP_PRETTY); ...
The first argument to ppmp_init() is a point to a PPMP struct. The second is a void *user_data that is not interpreted by the library, but passed unmodified to the third argument, that is a callback to write pieces of the generated message somewhere. The signature of the callback looks like:
int callback(void *user_data, const char *buf, int len);
The callback is expected to return 0 when no error has occured. The library provides a default callback that simply writes the message to stdout. The default is used, when NULL is passed for the callback argument.
The fourth and last argument to ppmp_init() is a set of flags. The only flags available at this time is PPMP_PRETTY the formats to be easily readable for humans.
A message is then constructed by calling APIs in the right order. To start a measurement payload, the first call on the initialized PPMP structure is ppmp_measurement_payload(), and the payload is finished with ppmp_finish():
... ppmp_measurement_payload(&ppmp); ... /* message data comes here */ ppmp_finish(&ppmp); ...
(The measurement payload is the only one implemented for now)
For each JSON object inside a message, there is a set of calls to construct the data. All calls take the PPMP structure as their first argument.
The Device object is created by ppmp_device(). The Part object by ppmp_part(). Compare their declarations in ppmp.h for their arguments.
The "measurements" array, due to its more complex structure, requires a handful of different calls. See the example in test_measurement.c for reference.
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To generate the message into a memory buffer, one can use the cbuffer module in this repository. The callback function is then set up to write into cbuffer object. Compare the example in buffer_example.c.