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[dragonfly.git] / secure / lib / libssl / man / SSL_read.3
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129.\" ========================================================================
130.\"
131.IX Title "SSL_read 3"
132.TH SSL_read 3 "2004-12-22" "0.9.7e" "OpenSSL"
984263bc 133.SH "NAME"
a7d27d5a 134SSL_read \- read bytes from a TLS/SSL connection.
984263bc 135.SH "SYNOPSIS"
e056f0e0 136.IX Header "SYNOPSIS"
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137.Vb 1
138\& #include <openssl/ssl.h>
139.Ve
e056f0e0 140.PP
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141.Vb 1
142\& int SSL_read(SSL *ssl, void *buf, int num);
143.Ve
144.SH "DESCRIPTION"
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145.IX Header "DESCRIPTION"
146\&\fISSL_read()\fR tries to read \fBnum\fR bytes from the specified \fBssl\fR into the
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147buffer \fBbuf\fR.
148.SH "NOTES"
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149.IX Header "NOTES"
150If necessary, \fISSL_read()\fR will negotiate a \s-1TLS/SSL\s0 session, if
151not already explicitly performed by \fISSL_connect\fR\|(3) or
152\&\fISSL_accept\fR\|(3). If the
153peer requests a re\-negotiation, it will be performed transparently during
984263bc 154the \fISSL_read()\fR operation. The behaviour of \fISSL_read()\fR depends on the
e056f0e0 155underlying \s-1BIO\s0.
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156.PP
157For the transparent negotiation to succeed, the \fBssl\fR must have been
158initialized to client or server mode. This is being done by calling
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159\&\fISSL_set_connect_state\fR\|(3) or \fISSL_set_accept_state()\fR
160before the first call to an \fISSL_read()\fR or \fISSL_write\fR\|(3)
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161function.
162.PP
e056f0e0 163\&\fISSL_read()\fR works based on the \s-1SSL/TLS\s0 records. The data are received in
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164records (with a maximum record size of 16kB for SSLv3/TLSv1). Only when a
165record has been completely received, it can be processed (decryption and
166check of integrity). Therefore data that was not retrieved at the last
e056f0e0 167call of \fISSL_read()\fR can still be buffered inside the \s-1SSL\s0 layer and will be
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168retrieved on the next call to \fISSL_read()\fR. If \fBnum\fR is higher than the
169number of bytes buffered, \fISSL_read()\fR will return with the bytes buffered.
170If no more bytes are in the buffer, \fISSL_read()\fR will trigger the processing
171of the next record. Only when the record has been received and processed
172completely, \fISSL_read()\fR will return reporting success. At most the contents
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173of the record will be returned. As the size of an \s-1SSL/TLS\s0 record may exceed
174the maximum packet size of the underlying transport (e.g. \s-1TCP\s0), it may
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175be necessary to read several packets from the transport layer before the
176record is complete and \fISSL_read()\fR can succeed.
177.PP
e056f0e0 178If the underlying \s-1BIO\s0 is \fBblocking\fR, \fISSL_read()\fR will only return, once the
984263bc 179read operation has been finished or an error occurred, except when a
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180renegotiation take place, in which case a \s-1SSL_ERROR_WANT_READ\s0 may occur.
181This behaviour can be controlled with the \s-1SSL_MODE_AUTO_RETRY\s0 flag of the
182\&\fISSL_CTX_set_mode\fR\|(3) call.
984263bc 183.PP
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184If the underlying \s-1BIO\s0 is \fBnon-blocking\fR, \fISSL_read()\fR will also return
185when the underlying \s-1BIO\s0 could not satisfy the needs of \fISSL_read()\fR
984263bc 186to continue the operation. In this case a call to
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187\&\fISSL_get_error\fR\|(3) with the
188return value of \fISSL_read()\fR will yield \fB\s-1SSL_ERROR_WANT_READ\s0\fR or
189\&\fB\s-1SSL_ERROR_WANT_WRITE\s0\fR. As at any time a re-negotiation is possible, a
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190call to \fISSL_read()\fR can also cause write operations! The calling process
191then must repeat the call after taking appropriate action to satisfy the
e056f0e0 192needs of \fISSL_read()\fR. The action depends on the underlying \s-1BIO\s0. When using a
984263bc 193non-blocking socket, nothing is to be done, but \fIselect()\fR can be used to check
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194for the required condition. When using a buffering \s-1BIO\s0, like a \s-1BIO\s0 pair, data
195must be written into or retrieved out of the \s-1BIO\s0 before being able to continue.
984263bc 196.SH "WARNING"
e056f0e0 197.IX Header "WARNING"
984263bc 198When an \fISSL_read()\fR operation has to be repeated because of
e056f0e0 199\&\fB\s-1SSL_ERROR_WANT_READ\s0\fR or \fB\s-1SSL_ERROR_WANT_WRITE\s0\fR, it must be repeated
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200with the same arguments.
201.SH "RETURN VALUES"
e056f0e0 202.IX Header "RETURN VALUES"
984263bc 203The following return values can occur:
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204.IP ">0" 4
205.IX Item ">0"
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206The read operation was successful; the return value is the number of
207bytes actually read from the \s-1TLS/SSL\s0 connection.
e056f0e0 208.IP "0" 4
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209The read operation was not successful. The reason may either be a clean
210shutdown due to a \*(L"close notify\*(R" alert sent by the peer (in which case
211the \s-1SSL_RECEIVED_SHUTDOWN\s0 flag in the ssl shutdown state is set
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212(see \fISSL_shutdown\fR\|(3),
213\&\fISSL_set_shutdown\fR\|(3)). It is also possible, that
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214the peer simply shut down the underlying transport and the shutdown is
215incomplete. Call \fISSL_get_error()\fR with the return value \fBret\fR to find out,
216whether an error occurred or the connection was shut down cleanly
217(\s-1SSL_ERROR_ZERO_RETURN\s0).
218.Sp
219SSLv2 (deprecated) does not support a shutdown alert protocol, so it can
220only be detected, whether the underlying connection was closed. It cannot
221be checked, whether the closure was initiated by the peer or by something
222else.
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223.IP "<0" 4
224.IX Item "<0"
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225The read operation was not successful, because either an error occurred
226or action must be taken by the calling process. Call \fISSL_get_error()\fR with the
227return value \fBret\fR to find out the reason.
228.SH "SEE ALSO"
a7d27d5a 229.IX Header "SEE ALSO"
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230\&\fISSL_get_error\fR\|(3), \fISSL_write\fR\|(3),
231\&\fISSL_CTX_set_mode\fR\|(3), \fISSL_CTX_new\fR\|(3),
232\&\fISSL_connect\fR\|(3), \fISSL_accept\fR\|(3)
233\&\fISSL_set_connect_state\fR\|(3),
234\&\fISSL_shutdown\fR\|(3), \fISSL_set_shutdown\fR\|(3),
235\&\fIssl\fR\|(3), \fIbio\fR\|(3)