SSL_DO_HANDSHAKE(3) | Library Functions Manual | SSL_DO_HANDSHAKE(3)

# NAME

`SSL_do_handshake` — perform a TLS/SSL handshake

# SYNOPSIS

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

int
`SSL_do_handshake`(SSL *ssl);

# DESCRIPTION

`SSL_do_handshake`() will wait for a SSL/TLS handshake to take place. If the connection is in client mode, the handshake will be started. The handshake routines may have to be explicitly set in advance using either SSL_set_connect_state(3) or SSL_set_accept_state(3).

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

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

If the underlying BIO is non-blocking, `SSL_do_handshake`() will also return when the underlying BIO could not satisfy the needs of `SSL_do_handshake`() to continue the handshake. In this case a call to SSL_get_error(3) with the return value of `SSL_do_handshake`() 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_do_handshake`(). 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_connect(3), SSL_get_error(3), SSL_set_connect_state(3)

# HISTORY

`SSL_do_handshake`() first appeared in SSLeay 0.8.0 and has been available since OpenBSD 2.4.

June 8, 2025 | Debian