SSL_read(3)

SSL_read(3)                  OpenSSL                  SSL_read(3)

NAME

     SSL_read - read bytes from a TLS/SSL connection.

SYNOPSIS

      #include <openssl/ssl.h>

      int SSL_read(SSL *ssl, void *buf, int num);

DESCRIPTION

     SSL_read() tries to read num bytes from the specified ssl
     into the buffer buf.

NOTES

     If necessary, SSL_read() will negotiate a TLS/SSL session,
     if not already explicitly performed by SSL_connect(3) or
     SSL_accept(3). If the peer requests a re-negotiation, it
     will be performed transparently during the SSL_read()
     operation. The behaviour of SSL_read() depends on the
     underlying BIO.

     For the transparent negotiation to succeed, the ssl must
     have been initialized to client or server mode. This is
     being done by calling SSL_set_connect_state(3) or
     SSL_set_accept_state() before the first call to an
     SSL_read() or SSL_write(3) function.

     SSL_read() works based on the SSL/TLS records. The data are
     received in records (with a maximum record size of 16kB for
     SSLv3/TLSv1). Only when a record has been completely
     received, it can be processed (decryption and check of
     integrity). Therefore data that was not retrieved at the
     last call of SSL_read() can still be buffered inside the SSL
     layer and will be retrieved on the next call to SSL_read().
     If num is higher than the number of bytes buffered,
     SSL_read() will return with the bytes buffered.  If no more
     bytes are in the buffer, SSL_read() will trigger the
     processing of the next record. Only when the record has been
     received and processed completely, SSL_read() will return
     reporting success. At most the contents of the record will
     be returned. As the size of an SSL/TLS record may exceed the
     maximum packet size of the underlying transport (e.g. TCP),
     it may be necessary to read several packets from the
     transport layer before the record is complete and SSL_read()
     can succeed.

     If the underlying BIO is blocking, SSL_read() will only
     return, once the read operation has been finished or an
     error occurred, except when a renegotiation take place, in
     which case a SSL_ERROR_WANT_READ may occur. This behaviour
     can be controlled with the SSL_MODE_AUTO_RETRY flag of the
     SSL_CTX_set_mode(3) call.

1.0.2t               Last change: 2019-09-10                    1

SSL_read(3)                  OpenSSL                  SSL_read(3)

     If the underlying BIO is non-blocking, SSL_read() will also
     return when the underlying BIO could not satisfy the needs
     of SSL_read() to continue the operation. In this case a call
     to SSL_get_error(3) with the return value of SSL_read() will
     yield SSL_ERROR_WANT_READ or SSL_ERROR_WANT_WRITE. As at any
     time a re-negotiation is possible, a call to SSL_read() can
     also cause write operations! The calling process then must
     repeat the call after taking appropriate action to satisfy
     the needs of SSL_read(). The action depends on the
     underlying BIO. When using a non-blocking socket, nothing is
     to be done, but select() can be used to check for the
     required condition. When using a buffering BIO, like a BIO
     pair, data must be written into or retrieved out of the BIO
     before being able to continue.

     SSL_pending(3) can be used to find out whether there are
     buffered bytes available for immediate retrieval. In this
     case SSL_read() can be called without blocking or actually
     receiving new data from the underlying socket.

WARNING

     When an SSL_read() operation has to be repeated because of
     SSL_ERROR_WANT_READ or SSL_ERROR_WANT_WRITE, it must be
     repeated with the same arguments.

RETURN VALUES

     The following return values can occur:

     > 0 The read operation was successful.  The return value is
         the number of bytes actually read from the TLS/SSL
         connection.

     <= 0
     <0  The read operation was not successful, because either
         the connection was closed, an error occurred or action
         must be taken by the calling process.  Call
         SSL_get_error(3) with the return value ret to find out
         the reason.

         SSLv2 (deprecated) does not support a shutdown alert
         protocol, so it can only be detected, whether the
         underlying connection was closed. It cannot be checked,
         whether the closure was initiated by the peer or by
         something else.

         Old documentation indicated a difference between 0 and
         -1, and that -1 was retryable.  You should instead call
         SSL_get_error() to find out if it's retryable.

SEE ALSO

     SSL_get_error(3), SSL_write(3), SSL_CTX_set_mode(3),
     SSL_CTX_new(3), SSL_connect(3), SSL_accept(3)

1.0.2t               Last change: 2019-09-10                    2

SSL_read(3)                  OpenSSL                  SSL_read(3)

     SSL_set_connect_state(3), SSL_pending(3), SSL_shutdown(3),
     SSL_set_shutdown(3), ssl(3), bio(3)

1.0.2t               Last change: 2019-09-10                    3