Libraries from the Registry

Your own lib/ is for code only you need. For everything the world has already written — JSON parsers, sensor drivers, screen libraries — there is the PlatformIO Registry: a public catalogue of libraries with owners, versions and documentation, fetched by name.

Installing one, for real

The guide's desktop program ends by printing its summary as JSON — work nobody should hand-write when a good parser exists. The lab installed the best-known one (transcript t03):

pio pkg install -e native --library "bblanchon/ArduinoJson"

Library Manager: ArduinoJson@7.4.3 has been installed!

Three things happened, each worth seeing. The library was downloaded into this project's own .pio/libdeps/native/ — not a global pile. The project file gained a line, written by the tool:

lib_deps = bblanchon/ArduinoJson@^7.4.3

And the pin in that line is the real lesson. ^7.4.3 means "7.4.3 or a later 7.x": close enough to receive fixes, far enough to never wake up to a version 8 that changed the rules. A project file with pins, rebuilt on another machine, fetches the same code every time. A project file without them rebuilds with whatever "latest" means that week — chapter 15 counts that as a limit, because it is one.

You can also write the lib_deps line yourself and let the next pio run fetch it; pio pkg install simply does both steps at once and records the version it actually got.

Using it is just an include

With the dependency declared, the desktop program gained six lines:

#include <ArduinoJson.h>
...
JsonDocument doc;
doc["samples"] = samples;
doc["window"] = SAMPLE_COUNT;
doc["average"] = avg.average();
char line[128];
serializeJson(doc, line, sizeof(line));

and the rebuild's Dependency Graph shows the new arrival before any code compiles (t03): ArduinoJson @ 7.4.3 beside the private RunningAverage. The program's last line — shown as the final build prints it — is now:

{"samples":6,"window":3,"average":21.86666667}

(The window reads 3 because chapter 10's experiment later changed it from 4; the line as first produced, window 4 and all, is in transcript t03.)

Scoping: why the Uno never saw it

The install was aimed with -e native, so the lib_deps landed in [env:native] alone. The Uno build's graph (t04, t05) contains no ArduinoJson — its code would not have fit the story anyway (the board program prints plain text), and on a 2 KB-RAM chip, a library you do not ship is memory you still have. Rule of thumb: declare libraries in the environment that uses them; promote one to the shared [env] section only when every target truly needs it.

Finding and judging libraries

pio pkg search <words> searches the registry from the terminal, and pio pkg show OWNER/NAME prints an entry's versions and requirements. Judging what you find is a human task with three quick checks: is it maintained (a release in living memory), does it declare the platforms and frameworks you are building for (its own library.json says so, and the LDF will enforce it), and do you need its whole weight — on a desktop a library costs disk; on a Uno it costs the flash and RAM lines of chapter 6. The registry is a commons, with a commons' mixture of the excellent and the abandoned; the pin in your project file is what makes your choice reproducible either way.

Check: what is installed, exactly

pio pkg list

prints, per environment, the platform, packages and libraries the project currently resolves to — the definitive answer to "what will this build actually use," and the first thing to paste into any request for help.