#include <openssl/ssl.h> int SSL_pending(const SSL *ssl); int SSL_has_pending(const SSL *s);
Data is received in whole blocks known as records from the peer. A whole record is processed (e.g. decrypted) in one go and is buffered by OpenSSL until it is read by the application via a call to SSL_read_ex(3) or SSL_read(3).
SSL_pending() returns the number of bytes which have been processed, buffered and are available inside ssl for immediate read.
If the SSL object's read_ahead flag is set (see SSL_CTX_set_read_ahead(3)), additional protocol bytes (beyond the current record) may have been read containing more TLS/SSL records. This also applies to DTLS and pipelining (see SSL_CTX_set_split_send_fragment(3)). These additional bytes will be buffered by OpenSSL but will remain unprocessed until they are needed. As these bytes are still in an unprocessed state SSL_pending() will ignore them. Therefore, it is possible for no more bytes to be readable from the underlying BIO (because OpenSSL has already read them) and for SSL_pending() to return 0, even though readable application data bytes are available (because the data is in unprocessed buffered records).
SSL_has_pending() returns 1 if s has buffered data (whether processed or unprocessed) and 0 otherwise. Note that it is possible for SSL_has_pending() to return 1, and then a subsequent call to SSL_read_ex() or SSL_read() to return no data because the unprocessed buffered data when processed yielded no application data (for example this can happen during renegotiation). It is also possible in this scenario for SSL_has_pending() to continue to return 1 even after an SSL_read_ex() or SSL_read() call because the buffered and unprocessed data is not yet processable (e.g. because OpenSSL has only received a partial record so far).
SSL_pending() returns the number of buffered and processed application data bytes that are pending and are available for immediate read. SSL_has_pending() returns 1 if there is buffered record data in the SSL object and 0 otherwise.
SSL_read_ex(3), SSL_read(3), SSL_CTX_set_read_ahead(3), SSL_CTX_set_split_send_fragment(3), ssl(7)
The SSL_has_pending() function was added in OpenSSL 1.1.0.
Copyright 2000-2020 The OpenSSL Project Authors. All Rights Reserved.
Licensed under the Apache License 2.0 (the “License”). You may not use this file except in compliance with the License. You can obtain a copy in the file LICENSE in the source distribution or at <https://www.openssl.org/source/license.html>.
SSL_CTX_set_read_ahead.3ossl(3), SSL_CTX_set_split_send_fragment.3ossl(3), SSL_read.3ossl(3).
The man page SSL_has_pending.3ossl(3) is an alias of SSL_pending.3ossl(3).