The signature for that function is `char * zcert_public_txt (zcert_t *self)` (see [zcert.c#L139](https://github.com/zeromq/czmq/blob/master/src/zcert.c#L139)).

`zcert_public_txt` returns a pointer to the `public_txt` member of the `zcert_t` structure, which is initialised by calling `zcert_new` or `zcert_load` in `ZMQCert#__construct`. This member, along with the rest of the structure, are freed by calling `zcert_destroy` in `ZMQCert#__destruct`.