SSL_read(3)

NAME
SSL_read - read bytes from a TLS/SSL connection.

SYNOPSIS

 #include <openssl/ssl.h>
 int SSL_read(SSL *ssl, void *buf, int num);

DESCRIPTION
SSL_read() tries to read num bytes from the specified ssl into the buffer buf.

NOTES
If necessary, SSL_read() will negotiate a TLS/SSL session, if not already explicitly performed by SSL_connect(3) or SSL_accept(3). If the peer requests a re-negotiation, it will be performed transparently during the SSL_read() operation. The behaviour of SSL_read() depends on the underlying BIO.

For the transparent negotiation to succeed, the ssl must have been initialized to client or server mode. This is not the case if a generic method is being used (see SSL_CTX_new(3), so that SSL_set_connect_state(3) or SSL_set_accept_state() must be used before the first call to an SSL_read() or SSL_write(3) function.

If the underlying BIO is blocking, SSL_read() will only return, once the read operation has been finished or an error occurred, except when a renegotiation take place, in which case a SSL_ERROR_WANT_READ may occur. This behaviour can be controlled with the SSL_MODE_AUTO_RETRY flag of the SSL_CTX_set_mode(3) call.

If the underlying BIO is non-blocking, SSL_read() will also return when the underlying BIO could not satisfy the needs of SSL_read() to continue the operation. In this case a call to SSL_get_error(3) with the return value of SSL_read() will yield SSL_ERROR_WANT_READ or SSL_ERROR_WANT_WRITE. As at any time a re-negotiation is possible, a call to SSL_read() can also cause write operations! The calling process then must repeat the call after taking appropriate action to satisfy the needs of SSL_read(). The action depends on the underlying BIO. When using a non-blocking socket, nothing is to be done, but select() 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.

WARNING
When an SSL_read() operation has to be repeated because of SSL_ERROR_WANT_READ or SSL_ERROR_WANT_WRITE, it must be repeated with the same arguments.

RETURN VALUES
The following return values can occur: The read operation was successful; the return value is the number of bytes actually read from the TLS/SSL connection. The read operation was not successful, probably because no data was available. Call SSL_get_error() with the return value ret to find out, whether an error occurred. The read operation was not successful, because either an error occurred or action must be taken by the calling process. Call SSL_get_error() with the return value ret to find out the reason.

SEE ALSO
SSL_get_error(3), SSL_write(3), SSL_CTX_set_mode(3), SSL_CTX_new(3), SSL_connect(3), SSL_accept(3) SSL_set_connect_state(3), ssl(3), bio(3)