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SSL_GET_FD(3) Library Functions Manual SSL_GET_FD(3)

SSL_get_fd, SSL_get_rfd, SSL_get_wfd
get file descriptor linked to an SSL object

#include <openssl/ssl.h>
SSL_get_fd(const SSL *ssl);
SSL_get_rfd(const SSL *ssl);
SSL_get_wfd(const SSL *ssl);

SSL_get_fd() returns the file descriptor which is linked to ssl. SSL_get_rfd() and SSL_get_wfd() return the file descriptors for the read or the write channel, which can be different. If the read and the write channel are different, SSL_get_fd() will return the file descriptor of the read channel.

The following return values can occur:
The operation failed, because the underlying BIO is not of the correct type (suitable for file descriptors).
The file descriptor linked to ssl.

BIO_new(3), ssl(3), SSL_set_fd(3)

SSL_get_fd() appeared in SSLeay 0.4 or earlier and has been available since OpenBSD 2.4.
SSL_get_rfd() and SSL_get_wfd() first appeared in OpenSSL 0.9.6c and have been available since OpenBSD 3.2.
March 27, 2018 OpenBSD-current