SSL_WRITE(3) | Library Functions Manual | SSL_WRITE(3)

# NAME

`SSL_write_ex`, `SSL_write` — write bytes to a TLS connection

# SYNOPSIS

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

int
`SSL_write_ex`(SSL *ssl, const void *buf, size_t num, size_t *written);

int
`SSL_write`(SSL *ssl, const void *buf, int num);

# DESCRIPTION

`SSL_write_ex`() and `SSL_write`() write num bytes from the buffer buf into the specified ssl connection. On success `SSL_write_ex`() stores the number of bytes written in *written.

In the following, `SSL_write_ex`() and `SSL_write`() are called “write functions”.

If necessary, a write function negotiates a TLS 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 write function operation. The behaviour of the write functions depends on the underlying BIO.

For the transparent negotiation to succeed, the ssl must have been initialized to client or server mode. This is done by calling SSL_set_connect_state(3) or SSL_set_accept_state(3) before the first call to a write function.

If the underlying BIO is blocking, the write function will only return once the write operation has been finished or an error occurred, except when a renegotiation takes 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, the write function will also return when the underlying BIO could not satisfy the needs of the function to continue the operation. In this case a call to SSL_get_error(3) with the return value of the write function will yield `SSL_ERROR_WANT_READ` or `SSL_ERROR_WANT_WRITE`. As at any time a re-negotiation is possible, a call to a write function can also cause read operations. The calling process then must repeat the call after taking appropriate action to satisfy the needs of the write function. 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.

The write functions 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, the write functions will also return with success when a partial write has been successfully completed. In this case the write function operation is considered completed. The bytes are sent and a new write call 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 a write function call 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. `SSL_write_ex`() can be called with num=0, but will not send application data to the peer.

# RETURN VALUES

`SSL_write_ex`() returns 1 for success or 0 for failure. Success means that all requested application data bytes have been written to the TLS connection or, if `SSL_MODE_ENABLE_PARTIAL_WRITE` is in use, at least one application data byte has been written to the TLS connection. Failure means that not all the requested bytes have been written yet (if `SSL_MODE_ENABLE_PARTIAL_WRITE` is not in use) or no bytes could be written to the TLS connection (if `SSL_MODE_ENABLE_PARTIAL_WRITE` is in use). Failures can be retryable (e.g. the network write buffer has temporarily filled up) or non-retryable (e.g. a fatal network error). In the event of a failure, call SSL_get_error(3) to find out the reason which indicates whether the call is retryable or not.

For `SSL_write`(), 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 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.

# SEE ALSO

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)

# HISTORY

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

`SSL_write_ex`() first appeared in OpenSSL 1.1.1 and has been available since OpenBSD 7.1.

June 8, 2025 | Debian