BIO_push, BIO_pop - Add and remove BIOs from a chain.
BIO * BIO_push(
BIO *append ); BIO * BIO_pop(
BIO *b );
The BIO_push() function appends the BIO append to b. It returns b.
The BIO_pop() function removes the BIO b from a chain and returns the
next BIO in the chain. The return is NULL if there are no more BIOs in
the chain. The removed BIO then becomes a single BIO with no associa‐
tion to the original chain. It can be freed or attached to a different
The names of these functions are somewhat misleading. The BIO_push()
function joins two BIO chains, whereas the BIO_pop() function deletes a
single BIO from a chain. The deleted BIO does not need to be at the end
of a chain.
The process of calling the BIO_push() and BIO_pop() functions on a BIO
may have additional consequences (a control call is made to the
affected BIOs). Any effects are noted in the descriptions of individual
The BIO_push() function returns the end of the chain, b.
The BIO_pop() function returns the next BIO in the chain. If there are
no more BIOs in the chain, the return is NULL.
For these examples, md1 and md2 are digest BIOs, b64 is a base64 BIO,
and f is a file BIO.
If the call: BIO_push(b64, f);
is made then the new chain will be b64-chain. After making the follow‐
ing calls BIO_push(md2, b64); BIO_push(md1, md2);
the new chain is md1-md2-b64-f. Data written to md1 will be digested by
md1 and md2, base64 encoded and written to f.
Reading causes data to pass in the reverse direction. Data is read from
f, base64 decoded and digested by md1 and md2.
If the call:
is made, the call will return b64, and the new chain will be md1-b64-f.
Data can be written to md1 as before.