How to deal with application when SSL_write write partial data

Question

I really don't know how to do in following scene:

1:In Application, we call SSL_write(s, data, 100) want to send 100 bytes plaintext to peer.

2:In openssl, data will be encrypted and expanded to 116 bytes (maybe other size, I don't care)

3:In openssl, after encrypting the input, it call BIO_write/send(tcp layer interface) to send 116 bytes but only 10 bytes sent (may be caused by not enough socket buffer and send() return 10)` .Does OpenSSL cache the 100 ciphertext in it's own buffer ?

What's the SSL_write's return value when the 1 2 3 happen?

1: If SSL_write returns 10 (or again), the application will cache 90 (or 100) bytes plaintext and will call SSL_writeagain to write 90(or 100) bytes plaintext when fd is writable, but this will cause encryption state error because SSL_write has already encrypted all the plaintext.

2: if SSL_write return 100, the application think it has sent all data/plaintext, and may not call select/epoll_add to polling fd's writable event. ( I find Nginx does like this, maybe I'm wrong ) , so how can the application do to flush the remain data in openssl's left data ?

Answer 1

well, I have found how OpenSSL does.
SSL_write will return again even only 10 bytes are really sent and 106 bytes will be cached in is't buffer. And OpenSSL will record the current input size(100) and remain size(106). The second time SSL_write being called, it will send the remain data(106) first, and then remove the first 100 bytes of current input data .