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SSL_WRITE(3) Library Functions Manual SSL_WRITE(3)

SSL_write
write bytes to a TLS/SSL connection

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

SSL_write() writes num bytes from the buffer buf into the specified ssl connection.
If necessary, SSL_write() 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_write() operation. The behaviour of SSL_write() depends on the underlying BIO.
For the transparent negotiation to succeed, the ssl must have been initialized to client or server mode. This is being done by calling SSL_set_connect_state(3) or SSL_set_accept_state(3) before the first call to an SSL_read(3) or SSL_write() function.
If the underlying BIO is blocking, SSL_write() will only return once the write 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_write() will also return when the underlying BIO could not satisfy the needs of SSL_write() to continue the operation. In this case a call to SSL_get_error(3) with the return value of SSL_write() will yield SSL_ERROR_WANT_READ or SSL_ERROR_WANT_WRITE. As at any time a re-negotiation is possible, a call to SSL_write() can also cause read operations! The calling process then must repeat the call after taking appropriate action to satisfy the needs of SSL_write(). 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.
SSL_write() will only return with success when the complete contents of buf of length num have been written. This default behaviour can be changed with the SSL_MODE_ENABLE_PARTIAL_WRITE option of SSL_CTX_set_mode(3). When this flag is set, SSL_write() will also return with success when a partial write has been successfully completed. In this case the SSL_write() operation is considered completed. The bytes are sent and a new SSL_write() operation with a new buffer (with the already sent bytes removed) must be started. A partial write is performed with the size of a message block, which is 16kB.
When an SSL_write() operation has to be repeated because SSL_get_error(3) returned SSL_ERROR_WANT_READ or SSL_ERROR_WANT_WRITE, it must be repeated with the same arguments.
When calling SSL_write() with num=0 bytes to be sent, the behaviour is undefined.

The following return values can occur:
 
 
>0
The write operation was successful. The return value is the number of bytes actually written to the TLS/SSL connection.
 
 
0
The write operation was not successful. Probably the underlying connection was closed. Call SSL_get_error(3) with the return value to find out whether an error occurred or the connection was shut down cleanly (SSL_ERROR_ZERO_RETURN).
 
 
<0
The write operation was not successful, because either an error occurred or action must be taken by the calling process. Call SSL_get_error(3) with the return value to find out the reason.

BIO_new(3), ssl(3), SSL_accept(3), SSL_connect(3), SSL_CTX_new(3), SSL_CTX_set_mode(3), SSL_get_error(3), SSL_read(3), SSL_set_connect_state(3)

SSL_write() appeared in SSLeay 0.4 or earlier and has been available since OpenBSD 2.4.
March 27, 2018 OpenBSD-current