SSL_CONNECT(3) | Library Functions Manual | SSL_CONNECT(3)

# NAME

`SSL_connect` — initiate the TLS/SSL handshake with a TLS/SSL server

# SYNOPSIS

library “libssl” libcrypto
`#include <openssl/ssl.h>`

int
`SSL_connect`(SSL *ssl);

# DESCRIPTION

`SSL_connect`() initiates the TLS/SSL handshake with a server. The communication channel must already have been set and assigned to the ssl by setting an underlying BIO.

The behaviour of `SSL_connect`() depends on the underlying BIO.

If the underlying BIO is blocking, `SSL_connect`() will only return once the handshake has been finished or an error occurred.

If the underlying BIO is non-blocking, `SSL_connect`() will also return when the underlying BIO could not satisfy the needs of `SSL_connect`() to continue the handshake, indicating the problem with the return value −1. In this case a call to SSL_get_error(3) with the return value of `SSL_connect`() will yield `SSL_ERROR_WANT_READ` or `SSL_ERROR_WANT_WRITE`. The calling process then must repeat the call after taking appropriate action to satisfy the needs of `SSL_connect`(). The action depends on the underlying BIO. When using a non-blocking socket, nothing is to be done, but select(2) can be used to check for the required condition. When using a buffering BIO, like a BIO pair, data must be written into or retrieved out of the BIO before being able to continue.

# RETURN VALUES

The following return values can occur:

0
  The TLS/SSL handshake was not successful but was shut down controlled and by the specifications of the TLS/SSL protocol. Call SSL_get_error(3) with the return value ret to find out the reason.
1
  The TLS/SSL handshake was successfully completed, and a TLS/SSL connection has been established.
<0
  The TLS/SSL handshake was not successful, because either a fatal error occurred at the protocol level or a connection failure occurred. The shutdown was not clean. It can also occur if action is needed to continue the operation for non-blocking BIOs. Call SSL_get_error(3) with the return value ret to find out the reason.

# SEE ALSO

BIO_new(3), ssl(3), SSL_accept(3), SSL_CTX_new(3), SSL_do_handshake(3), SSL_get_error(3), SSL_set_connect_state(3), SSL_shutdown(3)

# HISTORY

`SSL_connect`() appeared in SSLeay 0.4 or earlier and has been available since OpenBSD 2.4.

June 8, 2025 | Debian