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129 .\" ========================================================================
131 .IX Title "SSL_write 3"
132 .TH SSL_write 3 "2017-12-07" "1.0.2n" "OpenSSL"
133 .\" For nroff, turn off justification. Always turn off hyphenation; it makes
134 .\" way too many mistakes in technical documents.
138 SSL_write \- write bytes to a TLS/SSL connection.
140 .IX Header "SYNOPSIS"
142 \& #include <openssl/ssl.h>
144 \& int SSL_write(SSL *ssl, const void *buf, int num);
147 .IX Header "DESCRIPTION"
148 \&\fISSL_write()\fR writes \fBnum\fR bytes from the buffer \fBbuf\fR into the specified
149 \&\fBssl\fR connection.
152 If necessary, \fISSL_write()\fR will negotiate a \s-1TLS/SSL\s0 session, if
153 not already explicitly performed by \fISSL_connect\fR\|(3) or
154 \&\fISSL_accept\fR\|(3). If the
155 peer requests a re-negotiation, it will be performed transparently during
156 the \fISSL_write()\fR operation. The behaviour of \fISSL_write()\fR depends on the
157 underlying \s-1BIO. \s0
159 For the transparent negotiation to succeed, the \fBssl\fR must have been
160 initialized to client or server mode. This is being done by calling
161 \&\fISSL_set_connect_state\fR\|(3) or \fISSL_set_accept_state()\fR
162 before the first call to an \fISSL_read\fR\|(3) or \fISSL_write()\fR function.
164 If the underlying \s-1BIO\s0 is \fBblocking\fR, \fISSL_write()\fR will only return, once the
165 write operation has been finished or an error occurred, except when a
166 renegotiation take place, in which case a \s-1SSL_ERROR_WANT_READ\s0 may occur.
167 This behaviour can be controlled with the \s-1SSL_MODE_AUTO_RETRY\s0 flag of the
168 \&\fISSL_CTX_set_mode\fR\|(3) call.
170 If the underlying \s-1BIO\s0 is \fBnon-blocking\fR, \fISSL_write()\fR will also return,
171 when the underlying \s-1BIO\s0 could not satisfy the needs of \fISSL_write()\fR
172 to continue the operation. In this case a call to
173 \&\fISSL_get_error\fR\|(3) with the
174 return value of \fISSL_write()\fR will yield \fB\s-1SSL_ERROR_WANT_READ\s0\fR or
175 \&\fB\s-1SSL_ERROR_WANT_WRITE\s0\fR. As at any time a re-negotiation is possible, a
176 call to \fISSL_write()\fR can also cause read operations! The calling process
177 then must repeat the call after taking appropriate action to satisfy the
178 needs of \fISSL_write()\fR. The action depends on the underlying \s-1BIO.\s0 When using a
179 non-blocking socket, nothing is to be done, but \fIselect()\fR can be used to check
180 for the required condition. When using a buffering \s-1BIO,\s0 like a \s-1BIO\s0 pair, data
181 must be written into or retrieved out of the \s-1BIO\s0 before being able to continue.
183 \&\fISSL_write()\fR will only return with success, when the complete contents
184 of \fBbuf\fR of length \fBnum\fR has been written. This default behaviour
185 can be changed with the \s-1SSL_MODE_ENABLE_PARTIAL_WRITE\s0 option of
186 \&\fISSL_CTX_set_mode\fR\|(3). When this flag is set,
187 \&\fISSL_write()\fR will also return with success, when a partial write has been
188 successfully completed. In this case the \fISSL_write()\fR operation is considered
189 completed. The bytes are sent and a new \fISSL_write()\fR operation with a new
190 buffer (with the already sent bytes removed) must be started.
191 A partial write is performed with the size of a message block, which is
192 16kB for SSLv3/TLSv1.
195 When an \fISSL_write()\fR operation has to be repeated because of
196 \&\fB\s-1SSL_ERROR_WANT_READ\s0\fR or \fB\s-1SSL_ERROR_WANT_WRITE\s0\fR, it must be repeated
197 with the same arguments.
199 When calling \fISSL_write()\fR with num=0 bytes to be sent the behaviour is
202 .IX Header "RETURN VALUES"
203 The following return values can occur:
206 The write operation was successful, the return value is the number of
207 bytes actually written to the \s-1TLS/SSL\s0 connection.
210 The write operation was not successful, because either the connection was
211 closed, an error occurred or action must be taken by the calling process.
212 Call \fISSL_get_error()\fR with the return value \fBret\fR to find out the reason.
214 SSLv2 (deprecated) does not support a shutdown alert protocol, so it can
215 only be detected, whether the underlying connection was closed. It cannot
216 be checked, why the closure happened.
218 Old documentation indicated a difference between 0 and \-1, and that \-1 was
220 You should instead call \fISSL_get_error()\fR to find out if it's retryable.
222 .IX Header "SEE ALSO"
223 \&\fISSL_get_error\fR\|(3), \fISSL_read\fR\|(3),
224 \&\fISSL_CTX_set_mode\fR\|(3), \fISSL_CTX_new\fR\|(3),
225 \&\fISSL_connect\fR\|(3), \fISSL_accept\fR\|(3)
226 \&\fISSL_set_connect_state\fR\|(3),
227 \&\fIssl\fR\|(3), \fIbio\fR\|(3)