1=pod
2
3=head1 NAME
4
5SSL_read - read bytes from a TLS/SSL connection.
6
7=head1 SYNOPSIS
8
9 #include <openssl/ssl.h>
10
11 int SSL_read(SSL *ssl, void *buf, int num);
12
13=head1 DESCRIPTION
14
15SSL_read() tries to read B<num> bytes from the specified B<ssl> into the
16buffer B<buf>.
17
18=head1 NOTES
19
20If necessary, SSL_read() will negotiate a TLS/SSL session, if
21not already explicitly performed by L<SSL_connect(3)|SSL_connect(3)> or
22L<SSL_accept(3)|SSL_accept(3)>. If the
23peer requests a re-negotiation, it will be performed transparently during
24the SSL_read() operation. The behaviour of SSL_read() depends on the
25underlying BIO.
26
27For the transparent negotiation to succeed, the B<ssl> must have been
| 1=pod
2
3=head1 NAME
4
5SSL_read - read bytes from a TLS/SSL connection.
6
7=head1 SYNOPSIS
8
9 #include <openssl/ssl.h>
10
11 int SSL_read(SSL *ssl, void *buf, int num);
12
13=head1 DESCRIPTION
14
15SSL_read() tries to read B<num> bytes from the specified B<ssl> into the
16buffer B<buf>.
17
18=head1 NOTES
19
20If necessary, SSL_read() will negotiate a TLS/SSL session, if
21not already explicitly performed by L<SSL_connect(3)|SSL_connect(3)> or
22L<SSL_accept(3)|SSL_accept(3)>. If the
23peer requests a re-negotiation, it will be performed transparently during
24the SSL_read() operation. The behaviour of SSL_read() depends on the
25underlying BIO.
26
27For the transparent negotiation to succeed, the B<ssl> must have been
|
28initialized to client or server mode. This is not the case if a generic
29method is being used (see L<SSL_CTX_new(3)|SSL_CTX_new(3)>, so that
| 28initialized to client or server mode. This is being done by calling
|
30L<SSL_set_connect_state(3)|SSL_set_connect_state(3)> or SSL_set_accept_state()
| 29L<SSL_set_connect_state(3)|SSL_set_connect_state(3)> or SSL_set_accept_state()
|
31must be used before the first call to an SSL_read() or
32L<SSL_write(3)|SSL_write(3)> function).
| 30before the first call to an SSL_read() or L<SSL_write(3)|SSL_write(3)>
31function.
|
33
34SSL_read() works based on the SSL/TLS records. The data are received in
35records (with a maximum record size of 16kB for SSLv3/TLSv1). Only when a
36record has been completely received, it can be processed (decryption and
37check of integrity). Therefore data that was not retrieved at the last
38call of SSL_read() can still be buffered inside the SSL layer and will be
39retrieved on the next call to SSL_read(). If B<num> is higher than the
40number of bytes buffered, SSL_read() will return with the bytes buffered.
41If no more bytes are in the buffer, SSL_read() will trigger the processing
42of the next record. Only when the record has been received and processed
43completely, SSL_read() will return reporting success. At most the contents
44of the record will be returned. As the size of an SSL/TLS record may exceed
45the maximum packet size of the underlying transport (e.g. TCP), it may
46be necessary to read several packets from the transport layer before the
47record is complete and SSL_read() can succeed.
48
49If the underlying BIO is B<blocking>, SSL_read() will only return, once the
50read operation has been finished or an error occurred, except when a
51renegotiation take place, in which case a SSL_ERROR_WANT_READ may occur.
52This behaviour can be controlled with the SSL_MODE_AUTO_RETRY flag of the
53L<SSL_CTX_set_mode(3)|SSL_CTX_set_mode(3)> call.
54
55If the underlying BIO is B<non-blocking>, SSL_read() will also return
56when the underlying BIO could not satisfy the needs of SSL_read()
57to continue the operation. In this case a call to
58L<SSL_get_error(3)|SSL_get_error(3)> with the
59return value of SSL_read() will yield B<SSL_ERROR_WANT_READ> or
60B<SSL_ERROR_WANT_WRITE>. As at any time a re-negotiation is possible, a
61call to SSL_read() can also cause write operations! The calling process
62then must repeat the call after taking appropriate action to satisfy the
63needs of SSL_read(). The action depends on the underlying BIO. When using a
64non-blocking socket, nothing is to be done, but select() can be used to check
65for the required condition. When using a buffering BIO, like a BIO pair, data
66must be written into or retrieved out of the BIO before being able to continue.
67
68=head1 WARNING
69
70When an SSL_read() operation has to be repeated because of
71B<SSL_ERROR_WANT_READ> or B<SSL_ERROR_WANT_WRITE>, it must be repeated
72with the same arguments.
73
74=head1 RETURN VALUES
75
76The following return values can occur:
77
78=over 4
79
80=item E<gt>0
81
82The read operation was successful; the return value is the number of
83bytes actually read from the TLS/SSL connection.
84
85=item 0
86
| 32
33SSL_read() works based on the SSL/TLS records. The data are received in
34records (with a maximum record size of 16kB for SSLv3/TLSv1). Only when a
35record has been completely received, it can be processed (decryption and
36check of integrity). Therefore data that was not retrieved at the last
37call of SSL_read() can still be buffered inside the SSL layer and will be
38retrieved on the next call to SSL_read(). If B<num> is higher than the
39number of bytes buffered, SSL_read() will return with the bytes buffered.
40If no more bytes are in the buffer, SSL_read() will trigger the processing
41of the next record. Only when the record has been received and processed
42completely, SSL_read() will return reporting success. At most the contents
43of the record will be returned. As the size of an SSL/TLS record may exceed
44the maximum packet size of the underlying transport (e.g. TCP), it may
45be necessary to read several packets from the transport layer before the
46record is complete and SSL_read() can succeed.
47
48If the underlying BIO is B<blocking>, SSL_read() will only return, once the
49read operation has been finished or an error occurred, except when a
50renegotiation take place, in which case a SSL_ERROR_WANT_READ may occur.
51This behaviour can be controlled with the SSL_MODE_AUTO_RETRY flag of the
52L<SSL_CTX_set_mode(3)|SSL_CTX_set_mode(3)> call.
53
54If the underlying BIO is B<non-blocking>, SSL_read() will also return
55when the underlying BIO could not satisfy the needs of SSL_read()
56to continue the operation. In this case a call to
57L<SSL_get_error(3)|SSL_get_error(3)> with the
58return value of SSL_read() will yield B<SSL_ERROR_WANT_READ> or
59B<SSL_ERROR_WANT_WRITE>. As at any time a re-negotiation is possible, a
60call to SSL_read() can also cause write operations! The calling process
61then must repeat the call after taking appropriate action to satisfy the
62needs of SSL_read(). The action depends on the underlying BIO. When using a
63non-blocking socket, nothing is to be done, but select() can be used to check
64for the required condition. When using a buffering BIO, like a BIO pair, data
65must be written into or retrieved out of the BIO before being able to continue.
66
67=head1 WARNING
68
69When an SSL_read() operation has to be repeated because of
70B<SSL_ERROR_WANT_READ> or B<SSL_ERROR_WANT_WRITE>, it must be repeated
71with the same arguments.
72
73=head1 RETURN VALUES
74
75The following return values can occur:
76
77=over 4
78
79=item E<gt>0
80
81The read operation was successful; the return value is the number of
82bytes actually read from the TLS/SSL connection.
83
84=item 0
85
|
87The read operation was not successful, probably because no data was
88available. Call SSL_get_error() with the return value B<ret> to find out,
89whether an error occurred.
| 86The read operation was not successful. The reason may either be a clean
87shutdown due to a "close notify" alert sent by the peer (in which case
88the SSL_RECEIVED_SHUTDOWN flag in the ssl shutdown state is set
89(see L<SSL_shutdown(3)|SSL_shutdown(3)>,
90L<SSL_set_shutdown(3)|SSL_set_shutdown(3)>). It is also possible, that
91the peer simply shut down the underlying transport and the shutdown is
92incomplete. Call SSL_get_error() with the return value B<ret> to find out,
93whether an error occurred or the connection was shut down cleanly
94(SSL_ERROR_ZERO_RETURN).
|
90
| 95
|
| 96SSLv2 (deprecated) does not support a shutdown alert protocol, so it can
97only be detected, whether the underlying connection was closed. It cannot
98be checked, whether the closure was initiated by the peer or by something
99else.
100
|
91=item E<lt>0
92
93The read operation was not successful, because either an error occurred
94or action must be taken by the calling process. Call SSL_get_error() with the
95return value B<ret> to find out the reason.
96
97=back
98
99=head1 SEE ALSO
100
101L<SSL_get_error(3)|SSL_get_error(3)>, L<SSL_write(3)|SSL_write(3)>,
102L<SSL_CTX_set_mode(3)|SSL_CTX_set_mode(3)>, L<SSL_CTX_new(3)|SSL_CTX_new(3)>,
103L<SSL_connect(3)|SSL_connect(3)>, L<SSL_accept(3)|SSL_accept(3)>
104L<SSL_set_connect_state(3)|SSL_set_connect_state(3)>,
| 101=item E<lt>0
102
103The read operation was not successful, because either an error occurred
104or action must be taken by the calling process. Call SSL_get_error() with the
105return value B<ret> to find out the reason.
106
107=back
108
109=head1 SEE ALSO
110
111L<SSL_get_error(3)|SSL_get_error(3)>, L<SSL_write(3)|SSL_write(3)>,
112L<SSL_CTX_set_mode(3)|SSL_CTX_set_mode(3)>, L<SSL_CTX_new(3)|SSL_CTX_new(3)>,
113L<SSL_connect(3)|SSL_connect(3)>, L<SSL_accept(3)|SSL_accept(3)>
114L<SSL_set_connect_state(3)|SSL_set_connect_state(3)>,
|
| 115L<SSL_shutdown(3)|SSL_shutdown(3)>, L<SSL_set_shutdown(3)|SSL_set_shutdown(3)>,
|
105L<ssl(3)|ssl(3)>, L<bio(3)|bio(3)>
106
107=cut
| 116L<ssl(3)|ssl(3)>, L<bio(3)|bio(3)>
117
118=cut
|