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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_ex, SSL_write, SSL_sendfile - write bytes to a TLS/SSL connection Header "SYNOPSIS" .Vb 1 #include <openssl/ssl.h> \& ossl_ssize_t SSL_sendfile(SSL *s, int fd, off_t offset, size_t size, int flags); 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); .Ve Header "DESCRIPTION" \fBSSL_write_ex() and SSL_write() write num bytes from the buffer buf into the specified ssl connection. On success SSL_write_ex() will store the number of bytes written in *written.

\fBSSL_sendfile() writes size bytes from offset offset in the file descriptor fd to the specified \s-1SSL\s0 connection s. This function provides efficient zero-copy semantics. SSL_sendfile() is available only when Kernel \s-1TLS\s0 is enabled, which can be checked by calling BIO_get_ktls_send(). It is provided here to allow users to maintain the same interface. The meaning of flags is platform dependent. Currently, under Linux it is ignored.

Header "NOTES" In the paragraphs below a \*(L"write function\*(R" is defined as one of either \fBSSL_write_ex(), or SSL_write().

If necessary, a write function will negotiate a \s-1TLS/SSL\s0 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 \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 \fBSSL_set_connect_state\|(3) or SSL_set_accept_state() before the first call to a write function.

If the underlying \s-1BIO\s0 is blocking, the write functions will only return, once the write operation has been finished or an error occurred.

If the underlying \s-1BIO\s0 is nonblocking the write functions will also return when the underlying \s-1BIO\s0 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 \s-1SSL_ERROR_WANT_READ\s0 or \s-1SSL_ERROR_WANT_WRITE\s0. 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 \s-1BIO.\s0 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 \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.

The write functions will only return with success when the complete contents of \fBbuf of length num has been written. This default behaviour can be changed with the \s-1SSL_MODE_ENABLE_PARTIAL_WRITE\s0 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.

Header "WARNINGS" When a write function call has to be repeated because SSL_get_error\|(3) returned \s-1SSL_ERROR_WANT_READ\s0 or \s-1SSL_ERROR_WANT_WRITE\s0, it must be repeated with the same arguments. The data that was passed might have been partially processed. When \s-1SSL_MODE_ACCEPT_MOVING_WRITE_BUFFER\s0 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. \fBSSL_write_ex() can be called with num=0, but will not send application data to the peer.

Header "RETURN VALUES" \fBSSL_write_ex() will return 1 for success or 0 for failure. Success means that all requested application data bytes have been written to the \s-1SSL\s0 connection or, if \s-1SSL_MODE_ENABLE_PARTIAL_WRITE\s0 is in use, at least 1 application data byte has been written to the \s-1SSL\s0 connection. Failure means that not all the requested bytes have been written yet (if \s-1SSL_MODE_ENABLE_PARTIAL_WRITE\s0 is not in use) or no bytes could be written to the \s-1SSL\s0 connection (if \s-1SSL_MODE_ENABLE_PARTIAL_WRITE\s0 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:

Item "> 0" The write operation was successful, the return value is the number of bytes actually written to the \s-1TLS/SSL\s0 connection. Item "<= 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. .Sp 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.

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

Item ">= 0" The write operation was successful, the return value is the number of bytes of the file written to the \s-1TLS/SSL\s0 connection. The return value can be less than size for a partial write. Item "< 0" The write operation was not successful, because either the connection was closed, an error occured or action must be taken by the calling process. Call SSL_get_error() with the return value to find out the reason. Header "SEE ALSO" \fBSSL_get_error\|(3), SSL_read_ex\|(3), SSL_read\|(3) \fBSSL_CTX_set_mode\|(3), SSL_CTX_new\|(3), \fBSSL_connect\|(3), SSL_accept\|(3) \fBSSL_set_connect_state\|(3), BIO_ctrl\|(3), \fBssl\|(7), bio\|(7) Header "HISTORY" The SSL_write_ex() function was added in OpenSSL 1.1.1. The SSL_sendfile() function was added in OpenSSL 3.0.0. Header "COPYRIGHT" Copyright 2000-2020 The OpenSSL Project Authors. All Rights Reserved.

Licensed under the OpenSSL license (the \*(L"License\*(R"). You may not use this file except in compliance with the License. You can obtain a copy in the file \s-1LICENSE\s0 in the source distribution or at <https://www.openssl.org/source/license.html>.