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SSL_READ(3)                         OpenSSL                        SSL_READ(3)
NAME
       SSL_read_ex, SSL_read, SSL_peek_ex, SSL_peek - read bytes from a
       TLS/SSL connection
SYNOPSIS
        #include <openssl/ssl.h>
        int SSL_read_ex(SSL *ssl, void *buf, size_t num, size_t *readbytes);
        int SSL_read(SSL *ssl, void *buf, int num);
        int SSL_peek_ex(SSL *ssl, void *buf, size_t num, size_t *readbytes);
        int SSL_peek(SSL *ssl, void *buf, int num);
DESCRIPTION
       SSL_read_ex() and SSL_read() try to read num bytes from the specified
       ssl into the buffer buf. On success SSL_read_ex() will store the number
       of bytes actually read in *readbytes.
       SSL_peek_ex() and SSL_peek() are identical to SSL_read_ex() and
       SSL_read() respectively except no bytes are actually removed from the
       underlying BIO during the read, so that a subsequent call to
       SSL_read_ex() or SSL_read() will yield at least the same bytes.
NOTES
       In the paragraphs below a "read function" is defined as one of
       SSL_read_ex(), SSL_read(), SSL_peek_ex() or SSL_peek().
       If necessary, a read function 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 read function operation. The behaviour of the read functions
       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
       invocation of a read function.
       The read functions work based on the SSL/TLS records. The data are
       received in records (with a maximum record size of 16kB). Only when a
       record has been completely received, can it be processed (decryption
       and check of integrity).  Therefore, data that was not retrieved at the
       last read call can still be buffered inside the SSL layer and will be
       retrieved on the next read call. If num is higher than the number of
       bytes buffered then the read functions will return with the bytes
       buffered. If no more bytes are in the buffer, the read functions will
       trigger the processing of the next record.  Only when the record has
       been received and processed completely will the read functions return
       reporting success. At most the contents of one 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 the read call can succeed.
       If SSL_MODE_AUTO_RETRY has been switched off and a non-application data
       record has been processed, the read function can return and set the
       error to SSL_ERROR_WANT_READ.  In this case there might still be
       unprocessed data available in the BIO.  If read ahead was set using
       SSL_CTX_set_read_ahead(3), there might also still be unprocessed data
       available in the SSL.  This behaviour can be controlled using the
       SSL_CTX_set_mode(3) call.
       If the underlying BIO is blocking, a read function will only return
       once the read operation has been finished or an error occurred, except
       when a non-application data record has been processed and
       SSL_MODE_AUTO_RETRY is not set.  Note that if SSL_MODE_AUTO_RETRY is
       set and only non-application data is available the call will hang.
       If the underlying BIO is nonblocking, a read function will also return
       when the underlying BIO could not satisfy the needs of the function to
       continue the operation.  In this case a call to SSL_get_error(3) with
       the return value of the read function will yield SSL_ERROR_WANT_READ or
       SSL_ERROR_WANT_WRITE.  As at any time it's possible that non-
       application data needs to be sent, a read function can also cause write
       operations.  The calling process then must repeat the call after taking
       appropriate action to satisfy the needs of the read function.  The
       action depends on the underlying BIO.  When using a nonblocking 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 the read function can
       be called without blocking or actually receiving new data from the
       underlying socket.
RETURN VALUES
       SSL_read_ex() and SSL_peek_ex() will return 1 for success or 0 for
       failure.  Success means that 1 or more application data bytes have been
       read from the SSL connection.  Failure means that no bytes could be
       read from the SSL connection.  Failures can be retryable (e.g. we are
       waiting for more bytes to be delivered by the network) or non-retryable
       (e.g. a fatal network error).  In the event of a failure call
       SSL_get_error(3) to find out the reason which indicates whether the
       call is retryable or not.
       For SSL_read() and SSL_peek() 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
           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.
           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_ex(3), SSL_CTX_set_mode(3), SSL_CTX_new(3),
       SSL_connect(3), SSL_accept(3) SSL_set_connect_state(3), SSL_pending(3),
       SSL_shutdown(3), SSL_set_shutdown(3), ssl(7), bio(7)
HISTORY
       The SSL_read_ex() and SSL_peek_ex() functions were added in OpenSSL
       1.1.1.
COPYRIGHT
       Copyright 2000-2020 The OpenSSL Project Authors. All Rights Reserved.
       Licensed under the OpenSSL license (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>;.
1.1.1k                            2021-03-25                       SSL_READ(3)