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.\" ========================================================================
.\"
.IX Title "SSL_SET_FD 3ossl"
.TH SSL_SET_FD 3ossl "2026-01-16" "3.5.1" "OpenSSL"
.\" For nroff, turn off justification.  Always turn off hyphenation; it makes
.\" way too many mistakes in technical documents.
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.nh
.SH "NAME"
SSL_set_fd, SSL_set_rfd, SSL_set_wfd \- connect the SSL object with a file descriptor
.SH "SYNOPSIS"
.IX Header "SYNOPSIS"
.Vb 1
\& #include <openssl/ssl.h>
\&
\& int SSL_set_fd(SSL *ssl, int fd);
\& int SSL_set_rfd(SSL *ssl, int fd);
\& int SSL_set_wfd(SSL *ssl, int fd);
.Ve
.SH "DESCRIPTION"
.IX Header "DESCRIPTION"
\&\fBSSL_set_fd()\fR sets the file descriptor \fBfd\fR as the input/output facility
for the \s-1TLS/SSL\s0 (encrypted) side of \fBssl\fR. \fBfd\fR will typically be the
socket file descriptor of a network connection.
.PP
When performing the operation, a \fBsocket \s-1BIO\s0\fR is automatically created to
interface between the \fBssl\fR and \fBfd\fR. The \s-1BIO\s0 and hence the \s-1SSL\s0 engine
inherit the behaviour of \fBfd\fR. If \fBfd\fR is nonblocking, the \fBssl\fR will
also have nonblocking behaviour.
.PP
When used on a \s-1QUIC\s0 connection \s-1SSL\s0 object, a \fBdatagram \s-1BIO\s0\fR is automatically
created instead of a \fBsocket \s-1BIO\s0\fR. These functions fail if called
on a \s-1QUIC\s0 stream \s-1SSL\s0 object.
.PP
If there was already a \s-1BIO\s0 connected to \fBssl\fR, \fBBIO_free()\fR will be called
(for both the reading and writing side, if different).
.PP
\&\fBSSL_set_rfd()\fR and \fBSSL_set_wfd()\fR perform the respective action, but only
for the read channel or the write channel, which can be set independently.
.SH "RETURN VALUES"
.IX Header "RETURN VALUES"
The following return values can occur:
.IP "0" 4
The operation failed. Check the error stack to find out why.
.IP "1" 4
.IX Item "1"
The operation succeeded.
.SH "NOTES"
.IX Header "NOTES"
On Windows, a socket handle is a 64\-bit data type (\s-1UINT_PTR\s0), which leads to a
compiler warning (conversion from '\s-1SOCKET\s0' to 'int', possible loss of data) when
passing the socket handle to SSL_set_*\fBfd()\fR. For the time being, this warning can
safely be ignored, because although the Microsoft documentation claims that the
upper limit is \s-1INVALID_SOCKET\-1\s0 (2^64 \- 2), in practice the current \fBsocket()\fR
implementation returns an index into the kernel handle table, the size of which
is limited to 2^24.
.SH "SEE ALSO"
.IX Header "SEE ALSO"
\&\fBSSL_get_fd\fR\|(3), \fBSSL_set_bio\fR\|(3),
\&\fBSSL_connect\fR\|(3), \fBSSL_accept\fR\|(3),
\&\fBSSL_shutdown\fR\|(3), \fBssl\fR\|(7) , \fBbio\fR\|(7)
.SH "COPYRIGHT"
.IX Header "COPYRIGHT"
Copyright 2000\-2023 The OpenSSL Project Authors. All Rights Reserved.
.PP
Licensed under the Apache License 2.0 (the \*(L"License\*(R").  You may not use
this file except in compliance with the License.  You can obtain a copy
in the file \s-1LICENSE\s0 in the source distribution or at
<https://www.openssl.org/source/license.html>.
