SSL_WRITE(3ossl)

📦
version 3.5.9 · 8 versions
, in fedora 43, fedora 44

🌸 SSL_WRITE — the 3ossl manual, but make it cute 🌸

okay bestie, sit down, hydrate, because we're about to learn how to send bytes over a TLS/SSL connection and honestly? it's giving main character energy. this is the SSL_WRITE(3ossl) page, and yes, we're doing the whole thing, no skipping. 🫶

✨ NAME ✨

SSL_write_ex2, SSL_write_ex, SSL_write, SSL_sendfile, SSL_WRITE_FLAG_CONCLUDE — literally all of them write bytes to a TLS/SSL connection. that's the whole vibe. 💅

📜 SYNOPSIS 📜

#include <openssl/ssl.h>

#define SSL_WRITE_FLAG_CONCLUDE

ossl_ssize_t SSL_sendfile(SSL *s, int fd, off_t offset, size_t size, int flags);
int SSL_write_ex2(SSL *s, const void *buf, size_t num,
                  uint64_t flags,
                  size_t *written);
int SSL_write_ex(SSL *s, const void *buf, size_t num, size_t *written);
int SSL_write(SSL *ssl, const void *buf, int num);

look at all those function signatures, slay. 🎀

💖 DESCRIPTION 💖

so here's the tea. SSL_write_ex() and SSL_write() write num bytes from the buffer buf into the specified ssl connection. and when SSL_write_ex() wins? it stashes the number of bytes actually written into *written. cute, right? ✨

SSL_write_ex2() is basically SSL_write_ex()'s cooler sibling — it does the same thing BUT it also takes optional flags that change how it behaves. and get this: calling SSL_write_ex2() with a flags argument of 0 is exactly equivalent to calling SSL_write_ex(). no cap, literally identical. 💯

SSL_sendfile() writes size bytes starting from offset in the file descriptor fd into the specified SSL connection s. this one is iconic because it gives you efficient zero-copy semantics. ✨ but plot twist: SSL_sendfile() is only available when Kernel TLS is enabled — you can check that by calling BIO_get_ktls_send(). it's provided here so users can maintain the same interface. the meaning of flags is platform dependent, and currently, under Linux, it is ignored. (Linux said "flags? don't know her." 🙈)

🚩 the flags argument 🚩

the flags argument to SSL_write_ex2() can accept zero or more of the following flags. note: which flags are supported depends on the kind of SSL object and the underlying protocol being used. so do your research before you commit, bestie. 👑

  • SSL_WRITE_FLAG_CONCLUDE — this flag is only supported on QUIC stream SSL objects (or QUIC connection SSL objects with a default stream attached).

if this flag is set, AND the call to SSL_write_ex2() succeeds, AND all of the data passed to the call is written (meaning *written == num), then the relevant QUIC stream's send part is concluded automatically — as though SSL_stream_conclude(3) was called (which causes transmission of a FIN for the stream). 🏁

while using this flag is semantically equivalent to calling SSL_stream_conclude(3) after a successful call to this function, using this flag enables greater efficiency than making these two API calls separately — because it lets the written stream data and the FIN flag indicating the end of the stream be scheduled as part of the same QUIC STREAM frame and QUIC packet. efficiency? we love her. 💅

setting this flag does not cause a stream's send part to be concluded if not all of the data passed to the call was consumed. periodt.

⚠️ a call to SSL_write_ex2() fails if a flag is passed which is not supported or understood by the given SSL object. an application should determine if a flag is supported (for example, for SSL_WRITE_FLAG_CONCLUDE, that a QUIC stream SSL object is being used) before attempting to use it. pls don't just yolo it. 😭

📝 NOTES 📝

in the paragraphs below, a "write function" is defined as one of either SSL_write_ex(), or SSL_write(). (yes, just those two. SSL_write_ex2() and SSL_sendfile() are doing their own thing here. 💀)

if necessary, a write function will negotiate a TLS/SSL session, if that hasn't already been 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 gets done by calling SSL_set_connect_state(3) or SSL_set_accept_state() before the first call to a write function. don't forget, or it's over. 🙈

  • if the underlying BIO is blocking, the write functions will only return once the write operation has been finished or an error occurred.
  • if the underlying BIO is nonblocking, the write functions will also return when the underlying BIO could not satisfy the needs of the function to continue the operation.

in that nonblocking 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. and because 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 nonblocking 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.

the write functions 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 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 used with a QUIC SSL object, calling an I/O function such as SSL_write() allows internal network event processing to be performed. it is important that this processing is performed regularly. if an application is not using thread assisted mode, an application should ensure that an I/O function such as SSL_write() is called regularly, or alternatively ensure that SSL_handle_events() is called regularly. see openssl-quic(7) and SSL_handle_events(3) for more information. 🌈

🚨 WARNINGS 🚨

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. the data that was passed might have been partially processed. when SSL_MODE_ACCEPT_MOVING_WRITE_BUFFER was set using SSL_CTX_set_mode(3), the pointer can be different, but the data and length should still be the same. 🫠
you should not call SSL_write() with num=0 — it will return an error. SSL_write_ex() can be called with num=0, but will not send application data to the peer. 💀

🔙 RETURN VALUES 🔙

SSL_write_ex() and SSL_write_ex2() return 1 for success or 0 for failure.

success means that all requested application data bytes have been written to the SSL connection or, if SSL_MODE_ENABLE_PARTIAL_WRITE is in use, at least 1 application data byte has been written to the SSL 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 SSL 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/SSL connection. 🎉
  • <= 0 — the write operation was not successful, because either the connection was closed, 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.
old documentation indicated a difference between 0 and -1, and that -1 was retryable. you should instead call SSL_get_error() to find out if it's retryable. that lore is outdated, bestie, ignore it. 📜❌

for SSL_sendfile(), the following return values can occur:

  • >= 0 — the write operation was successful, the return value is the number of bytes of the file written to the TLS/SSL connection. the return value can be less than size for a partial write. 📬
  • < 0 — the write operation was not successful, because either the connection was closed, an error occurred or action must be taken by the calling process. call SSL_get_error() with the return value to find out the reason.

👀 SEE ALSO 👀

(the original list is a lil chaotic with its commas, but we're keeping every single one, fr fr. 🫶)

🕰️ HISTORY 🕰️

  • the SSL_write_ex() function was added in OpenSSL 1.1.1.
  • the SSL_sendfile() function was added in OpenSSL 3.0.
  • the SSL_write_ex2() function was added in OpenSSL 3.3.

glow-up timeline, honestly. 👑

Copyright 2000-2024 The OpenSSL Project Authors. All Rights Reserved.

Licensed under the Apache License 2.0 (the "License"). You may not use this file except in compliance with the License. You can obtain a copy in the file LICENSE in the source distribution or at <https://www.openssl.org/source/license.html>.

and that's the whole page, bestie!! 💖 this was OpenSSL 3.5.9, dated 2026-09-29, and it lives in the SSL_WRITE(3ossl) manual. now go forth and write those bytes responsibly. mwah. 💅✨🎀🦋🌸

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