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.\" ========================================================================
.\"
.IX Title "SSL_write 3"
.TH SSL_write 3 "2019-12-20" "1.0.2u" "OpenSSL"
.\" For nroff, turn off justification.  Always turn off hyphenation; it makes
.\" way too many mistakes in technical documents.
.if n .ad l
.nh
.SH "NAME"
SSL_write \- write bytes to a TLS/SSL connection.
.SH "SYNOPSIS"
.IX Header "SYNOPSIS"
.Vb 1
\& #include <openssl/ssl.h>
\&
\& int SSL_write(SSL *ssl, const void *buf, int num);
.Ve
.SH "DESCRIPTION"
.IX Header "DESCRIPTION"
\&\fBSSL_write()\fR writes \fBnum\fR bytes from the buffer \fBbuf\fR into the specified
\&\fBssl\fR connection.
.SH "NOTES"
.IX Header "NOTES"
If necessary, \fBSSL_write()\fR will negotiate a \s-1TLS/SSL\s0 session, if
not already explicitly performed by \fBSSL_connect\fR\|(3) or
\&\fBSSL_accept\fR\|(3). If the
peer requests a re-negotiation, it will be performed transparently during
the \fBSSL_write()\fR operation. The behaviour of \fBSSL_write()\fR depends on the
underlying \s-1BIO.\s0
.PP
For the transparent negotiation to succeed, the \fBssl\fR must have been
initialized to client or server mode. This is being done by calling
\&\fBSSL_set_connect_state\fR\|(3) or \fBSSL_set_accept_state()\fR
before the first call to an \fBSSL_read\fR\|(3) or \fBSSL_write()\fR function.
.PP
If the underlying \s-1BIO\s0 is \fBblocking\fR, \fBSSL_write()\fR will only return, once the
write operation has been finished or an error occurred, except when a
renegotiation take place, in which case a \s-1SSL_ERROR_WANT_READ\s0 may occur. 
This behaviour can be controlled with the \s-1SSL_MODE_AUTO_RETRY\s0 flag of the
\&\fBSSL_CTX_set_mode\fR\|(3) call.
.PP
If the underlying \s-1BIO\s0 is \fBnon-blocking\fR, \fBSSL_write()\fR will also return,
when the underlying \s-1BIO\s0 could not satisfy the needs of \fBSSL_write()\fR
to continue the operation. In this case a call to
\&\fBSSL_get_error\fR\|(3) with the
return value of \fBSSL_write()\fR will yield \fB\s-1SSL_ERROR_WANT_READ\s0\fR or
\&\fB\s-1SSL_ERROR_WANT_WRITE\s0\fR. As at any time a re-negotiation is possible, a
call to \fBSSL_write()\fR can also cause read operations! The calling process
then must repeat the call after taking appropriate action to satisfy the
needs of \fBSSL_write()\fR. The action depends on the underlying \s-1BIO.\s0 When using a
non-blocking socket, nothing is to be done, but \fBselect()\fR can be used to check
for the required condition. When using a buffering \s-1BIO,\s0 like a \s-1BIO\s0 pair, data
must be written into or retrieved out of the \s-1BIO\s0 before being able to continue.
.PP
\&\fBSSL_write()\fR will only return with success, when the complete contents
of \fBbuf\fR of length \fBnum\fR has been written. This default behaviour
can be changed with the \s-1SSL_MODE_ENABLE_PARTIAL_WRITE\s0 option of
\&\fBSSL_CTX_set_mode\fR\|(3). When this flag is set,
\&\fBSSL_write()\fR will also return with success, when a partial write has been
successfully completed. In this case the \fBSSL_write()\fR operation is considered
completed. The bytes are sent and a new \fBSSL_write()\fR operation with a new
buffer (with the already sent bytes removed) must be started.
A partial write is performed with the size of a message block, which is
16kB for SSLv3/TLSv1.
.SH "WARNING"
.IX Header "WARNING"
When an \fBSSL_write()\fR operation has to be repeated because of
\&\fB\s-1SSL_ERROR_WANT_READ\s0\fR or \fB\s-1SSL_ERROR_WANT_WRITE\s0\fR, it must be repeated
with the same arguments.
.PP
When calling \fBSSL_write()\fR with num=0 bytes to be sent the behaviour is
undefined.
.SH "RETURN VALUES"
.IX Header "RETURN VALUES"
The following return values can occur:
.IP "> 0" 4
.IX Item "> 0"
The write operation was successful, the return value is the number of
bytes actually written to the \s-1TLS/SSL\s0 connection.
.IP "<= 0" 4
.IX Item "<= 0"
The write operation was not successful, because either the connection was
closed, an error occurred or action must be taken by the calling process.
Call \fBSSL_get_error()\fR with the return value \fBret\fR to find out the reason.
.Sp
SSLv2 (deprecated) does not support a shutdown alert protocol, so it can
only be detected, whether the underlying connection was closed. It cannot
be checked, why the closure happened.
.Sp
Old documentation indicated a difference between 0 and \-1, and that \-1 was
retryable.
You should instead call \fBSSL_get_error()\fR to find out if it's retryable.
.SH "SEE ALSO"
.IX Header "SEE ALSO"
\&\fBSSL_get_error\fR\|(3), \fBSSL_read\fR\|(3),
\&\fBSSL_CTX_set_mode\fR\|(3), \fBSSL_CTX_new\fR\|(3),
\&\fBSSL_connect\fR\|(3), \fBSSL_accept\fR\|(3)
\&\fBSSL_set_connect_state\fR\|(3),
\&\fBssl\fR\|(3), \fBbio\fR\|(3)

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✏️ SSL_write.3ssl