Missing structure definition?

[ChuckM]

This seems like it is likely operator error, when I include pngle.h I get a compile time error from the statement

type struct _pngle_t pngle_t;

In the Arduino environment, the esp32 compiler is complaining that struct _pngle_t isn't defined (which it isn't yet as it is defined in the C code).

[kikuchan]

Thank you for trying to use pngle!

Sorry for confusing, but it's intended.
typedef an incomplete type of struct is a common technique in C.
Such object cannot be instanciated directly.

Instead, please use a pointer to hold a pngle instance, like:
pngle_t *pngle = pngle_new();
and, shouldn't be like:
pngle_t pngle;
or something.