$NetBSD: SSL_write.3,v 1.7 2013/02/12 19:52:14 christos Exp $

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Title "SSL_write 3" For nroff, turn off justification. Always turn off hyphenation; it makes
way too many mistakes in technical documents.
SSL_write - write bytes to a TLS/SSL connection. libcrypto, -lcrypto Header "SYNOPSIS" .Vb 1 #include <openssl/ssl.h> \& int SSL_write(SSL *ssl, const void *buf, int num); .Ve Header "DESCRIPTION" \fISSL_write() writes num bytes from the buffer buf into the specified \fBssl connection. Header "NOTES" If necessary, SSL_write() will negotiate a \s-1TLS/SSL\s0 session, if not already explicitly performed by SSL_connect\|(3) or \fISSL_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 \s-1BIO\s0.

For the transparent negotiation to succeed, the ssl must have been initialized to client or server mode. This is being done by calling \fISSL_set_connect_state\|(3) or SSL_set_accept_state() before the first call to an SSL_read\|(3) or SSL_write() function.

If the underlying \s-1BIO\s0 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 \s-1SSL_ERROR_WANT_READ\s0 may occur. This behaviour can be controlled with the \s-1SSL_MODE_AUTO_RETRY\s0 flag of the \fISSL_CTX_set_mode\|(3) call.

If the underlying \s-1BIO\s0 is non-blocking, SSL_write() will also return, when the underlying \s-1BIO\s0 could not satisfy the needs of SSL_write() to continue the operation. In this case a call to \fISSL_get_error\|(3) with the return value of SSL_write() will yield \s-1SSL_ERROR_WANT_READ\s0 or \fB\s-1SSL_ERROR_WANT_WRITE\s0. 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 \s-1BIO\s0. 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 \s-1BIO\s0, like a \s-1BIO\s0 pair, data must be written into or retrieved out of the \s-1BIO\s0 before being able to continue.

\fISSL_write() will only return with success, when the complete contents of buf of length num has been written. This default behaviour can be changed with the \s-1SSL_MODE_ENABLE_PARTIAL_WRITE\s0 option of \fISSL_CTX_set_mode\|(3). When this flag is set, \fISSL_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 for SSLv3/TLSv1.

Header "WARNING" When an SSL_write() operation has to be repeated because of \fB\s-1SSL_ERROR_WANT_READ\s0 or \s-1SSL_ERROR_WANT_WRITE\s0, it must be repeated with the same arguments.

When calling SSL_write() with num=0 bytes to be sent the behaviour is undefined.

Header "RETURN VALUES" The following return values can occur: Item ">0" The write operation was successful, the return value is the number of bytes actually written to the \s-1TLS/SSL\s0 connection. The write operation was not successful. Probably the underlying connection was closed. Call SSL_get_error() with the return value ret to find out, whether an error occurred or the connection was shut down cleanly (\s-1SSL_ERROR_ZERO_RETURN\s0). .Sp SSLv2 (deprecated) does not support a shutdown alert protocol, so it can only be detected, whether the underlying connection was closed. It cannot be checked, why the closure happened. Item "<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() with the return value ret to find out the reason. Header "SEE ALSO" \fISSL_get_error\|(3), SSL_read\|(3), \fISSL_CTX_set_mode\|(3), SSL_CTX_new\|(3), \fISSL_connect\|(3), SSL_accept\|(3) \fISSL_set_connect_state\|(3), \fIssl\|(3), openssl_bio\|(3)