5 SSL_read - read bytes from a TLS/SSL connection.
9 #include <openssl/ssl.h>
11 int SSL_read(SSL *ssl, void *buf, int num);
15 SSL_read() tries to read B<num> bytes from the specified B<ssl> into the
20 If necessary, SSL_read() will negotiate a TLS/SSL session, if
21 not already explicitly performed by L<SSL_connect(3)|SSL_connect(3)> or
22 L<SSL_accept(3)|SSL_accept(3)>. If the
23 peer requests a re-negotiation, it will be performed transparently during
24 the SSL_read() operation. The behaviour of SSL_read() depends on the
27 For the transparent negotiation to succeed, the B<ssl> must have been
28 initialized to client or server mode. This is not the case if a generic
29 method is being used (see L<SSL_CTX_new(3)|SSL_CTX_new(3)>, so that
30 L<SSL_set_connect_state(3)|SSL_set_connect_state(3)> or SSL_set_accept_state()
31 must be used before the first call to an SSL_read() or
32 L<SSL_write(3)|SSL_write(3)> function.
34 SSL_read() works based on the SSL/TLS records. The data are received in
35 records (with a maximum record size of 16kB for SSLv3/TLSv1). Only when a
36 record has been completely received, it can be processed (decryption and
37 check of integrity). Therefore data that was not retrieved at the last
38 call of SSL_read() can still be buffered inside the SSL layer and will be
39 retrieved on the next call to SSL_read(). If B<num> is higher than the
40 number of bytes buffered, SSL_read() will return with the bytes buffered.
41 If no more bytes are in the buffer, SSL_read() will trigger the processing
42 of the next record. Only when the record has been received and processed
43 completely, SSL_read() will return reporting success. At most the contents
44 of the record will be returned.
46 If the underlying BIO is B<blocking>, SSL_read() will only return, once the
47 read operation has been finished or an error occurred, except when a
48 renegotiation take place, in which case a SSL_ERROR_WANT_READ may occur.
49 This behaviour can be controlled with the SSL_MODE_AUTO_RETRY flag of the
50 L<SSL_CTX_set_mode(3)|SSL_CTX_set_mode(3)> call.
52 If the underlying BIO is B<non-blocking>, SSL_read() will also return
53 when the underlying BIO could not satisfy the needs of SSL_read()
54 to continue the operation. In this case a call to
55 L<SSL_get_error(3)|SSL_get_error(3)> with the
56 return value of SSL_read() will yield B<SSL_ERROR_WANT_READ> or
57 B<SSL_ERROR_WANT_WRITE>. As at any time a re-negotiation is possible, a
58 call to SSL_read() can also cause write operations! The calling process
59 then must repeat the call after taking appropriate action to satisfy the
60 needs of SSL_read(). The action depends on the underlying BIO. When using a
61 non-blocking socket, nothing is to be done, but select() can be used to check
62 for the required condition. When using a buffering BIO, like a BIO pair, data
63 must be written into or retrieved out of the BIO before being able to continue.
67 When an SSL_read() operation has to be repeated because of
68 B<SSL_ERROR_WANT_READ> or B<SSL_ERROR_WANT_WRITE>, it must be repeated
69 with the same arguments.
73 The following return values can occur:
79 The read operation was successful; the return value is the number of
80 bytes actually read from the TLS/SSL connection.
84 The read operation was not successful, probably because no data was
85 available. Call SSL_get_error() with the return value B<ret> to find out,
86 whether an error occurred.
90 The read operation was not successful, because either an error occurred
91 or action must be taken by the calling process. Call SSL_get_error() with the
92 return value B<ret> to find out the reason.
98 L<SSL_get_error(3)|SSL_get_error(3)>, L<SSL_write(3)|SSL_write(3)>,
99 L<SSL_CTX_set_mode(3)|SSL_CTX_set_mode(3)>, L<SSL_CTX_new(3)|SSL_CTX_new(3)>,
100 L<SSL_connect(3)|SSL_connect(3)>, L<SSL_accept(3)|SSL_accept(3)>
101 L<SSL_set_connect_state(3)|SSL_set_connect_state(3)>,
102 L<ssl(3)|ssl(3)>, L<bio(3)|bio(3)>