mbedtls_gcm_init() function
This function initializes the specified GCM context, to make references valid, and prepares the context for mbedtls_gcm_setkey() or mbedtls_gcm_free(). The function does not bind the GCM context to a particular cipher, nor set the key. For this purpose, use mbedtls_gcm_setkey().
Arguments
ctx
The GCM context to initialize. This must not be \c NULL.