SSL_read(3)

     SSL_read(3)            0.9.6h (2001-09-13)            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.

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          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.

     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.

          o   The read operation was not successful. The reason may
              either be a clean shutdown due to a "close notify" alert
              sent by the peer (in which case the
              SSL_RECEIVED_SHUTDOWN flag in the ssl shutdown state is
              set (see SSL_shutdown(3), SSL_set_shutdown(3)). It is
              also possible, that the peer simply shut down the
              underlying transport and the shutdown is incomplete.
              Call SSL_get_error() with the return value ret to find
              out, whether an error occurred or the connection was
              shut down cleanly (SSL_ERROR_ZERO_RETURN).

              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.

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

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

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     SSL_read(3)            0.9.6h (2001-09-13)            SSL_read(3)

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

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