BIO_s_null() returns the null sink BIO method. Data written to the null sink is discarded, reads return EOF.
A null sink BIO behaves in a similar manner to the Unix /dev/null device.
A null bio can be placed on the end of a chain to discard any data passed through it.
A null sink is useful if, for example, an application wishes to digest some data by writing through a digest bio but not send the digested data anywhere. Since a BIO chain must normally include a source/sink BIO this can be achieved by adding a null sink BIO to the end of the chain