obtain result code for TLS/SSL I/O
SSL *ssl, int
returns a result code (suitable for the C “switch” statement)
for a preceding call to
SSL_read(3), SSL_peek(3), or
SSL_write(3) on ssl. The value returned by that
TLS/SSL I/O function must be passed to
SSL_get_error() in parameter
In addition to ssl and
inspects the current thread's OpenSSL error queue. Thus,
SSL_get_error() must be used in the same thread that
performed the TLS/SSL I/O operation, and no other OpenSSL function calls
should appear in between. The current thread's error queue must be empty
before the TLS/SSL I/O operation is attempted, or
SSL_get_error() will not work reliably.
The following return values can currently occur:
- The TLS/SSL I/O operation completed. This result code is returned if and only if ret > 0.
- The TLS/SSL connection has been closed. If the protocol version is SSL 3.0
or TLS 1.0, this result code is returned only if a closure alert has
occurred in the protocol, i.e., if the connection has been closed cleanly.
Note that in this case
SSL_ERROR_ZERO_RETURNdoes not necessarily indicate that the underlying transport has been closed.
- The operation did not complete; the same TLS/SSL I/O function should be
called again later. If, by then, the underlying BIO
has data available for reading (if the result code is
SSL_ERROR_WANT_READ) or allows writing data (
SSL_ERROR_WANT_WRITE), then some TLS/SSL protocol progress will take place, i.e., at least part of a TLS/SSL record will be read or written. Note that the retry may again lead to a
SSL_ERROR_WANT_WRITEcondition. There is no fixed upper limit for the number of iterations that may be necessary until progress becomes visible at application protocol level.
For socket BIOs (e.g., when
SSL_set_fd() was used), select(2) or poll(2) on the underlying socket can be used to find out when the TLS/SSL I/O function should be retried.
Caveat: Any TLS/SSL I/O function can lead to either of
SSL_ERROR_WANT_WRITE. In particular, SSL_read(3) or SSL_peek(3) may want to write data and SSL_write(3) may want to read data. This is mainly because TLS/SSL handshakes may occur at any time during the protocol (initiated by either the client or the server); SSL_read(3), SSL_peek(3), and SSL_write(3) will handle any pending handshakes.
- The operation did not complete; the same TLS/SSL I/O function should be called again later. The underlying BIO was not connected yet to the peer and the call would block in connect(2)/accept(2). The SSL function should be called again when the connection is established. These messages can only appear with a BIO_s_connect(3) or BIO_s_accept(3) BIO, respectively. In order to find out when the connection has been successfully established, on many platforms select(2) or poll(2) for writing on the socket file descriptor can be used.
- The operation did not complete because an application callback set by SSL_CTX_set_client_cert_cb(3) has asked to be called again. The TLS/SSL I/O function should be called again later. Details depend on the application.
- Some I/O error occurred. The OpenSSL error queue may contain more
information on the error. If the error queue is empty (i.e.,
ERR_get_error() returns 0), ret can be used to find out more about the error: If ret == 0, an
EOFwas observed that violates the protocol. If ret == −1, the underlying BIO reported an I/O error (for socket I/O on Unix systems, consult
- A failure in the SSL library occurred, usually a protocol error. The OpenSSL error queue contains more information on the error.
SSL_get_error() first appeared in SSLeay
0.8.0 and have been available since OpenBSD 2.4.