[gpgme] when the password is aked?

[micron]

I'm using gpgme library to add signature/encryption/decryption functionalities 
to one of my programs.
The only gpgme's documentation resource I've found is an info file. In this 
file, in the section regarding the crypto operations (signature,...) there's 
nothing about key's password.
How the password is retrieved by gpgme?
The only thing I've to do is to set the password callback function for my 
context?
If so anybody can explain (maybe with some code) how the function 
"gpgme_error_t (*gpgme_passphrase_cb_t)(void *HOOK, const  char *UID_HINT, 
const char *PASSPHRASE_INFO, int PREV_WAS_BAD, int FD)" works?

Thanks in advance
 micron
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