---
name: "SSL_CTX_set_read_ahead"
section: "3ssl"
platform: "linux"
package: "openssl"
version: "3.6.5-1"
release: "Arch Linux 2026-11-01"
platforms: ["linux"]
url: "https://new.carta.tech/SSL_CTX_set_read_ahead"
---

# SSL\_CTX\_set\_read\_ahead(3ssl)

manage whether to read as many input bytes as possible

## NAME

SSL\_CTX\_set\_read\_ahead, SSL\_CTX\_get\_read\_ahead, SSL\_set\_read\_ahead, SSL\_get\_read\_ahead, SSL\_CTX\_get\_default\_read\_ahead - manage whether to read as many input bytes as possible

## SYNOPSIS

```
 #include <openssl/ssl.h>
 void SSL_set_read_ahead(SSL *s, int yes);
 int SSL_get_read_ahead(const SSL *s);
 SSL_CTX_set_read_ahead(SSL_CTX *ctx, int yes);
 long SSL_CTX_get_read_ahead(SSL_CTX *ctx);
 long SSL_CTX_get_default_read_ahead(SSL_CTX *ctx);
```

## DESCRIPTION

**SSL\_CTX\_set\_read\_ahead()** and **SSL\_set\_read\_ahead()** set whether we should read as many input bytes as possible (for nonblocking reads) or not. For example if **x** bytes are currently required by OpenSSL, but **y** bytes are available from the underlying BIO (where **y** \> **x**), then OpenSSL will read all **y** bytes into its buffer (providing that the buffer is large enough) if reading ahead is on, or **x** bytes otherwise. Setting the parameter **yes** to 0 turns reading ahead is off, other values turn it on. **SSL\_CTX\_set\_default\_read\_ahead()** is identical to **SSL\_CTX\_set\_read\_ahead()**.

**SSL\_CTX\_get\_read\_ahead()** and **SSL\_get\_read\_ahead()** indicate whether reading ahead has been set or not. **SSL\_CTX\_get\_default\_read\_ahead()** is identical to **SSL\_CTX\_get\_read\_ahead()**.

These functions cannot be used with QUIC SSL objects. **SSL\_set\_read\_ahead()** has no effect if called on a QUIC SSL object.

## NOTES

These functions have no impact when used with DTLS. The return values for **SSL\_CTX\_get\_read\_head()** and **SSL\_get\_read\_ahead()** are undefined for DTLS. Setting **read\_ahead** can impact the behaviour of the **SSL\_pending()** function (see **SSL\_pending**(3)).

Since **SSL\_read()** can return **SSL\_ERROR\_WANT\_READ** for non-application data records, and **SSL\_has\_pending()** can't tell the difference between processed and unprocessed data, it's recommended that if read ahead is turned on that **SSL\_MODE\_AUTO\_RETRY** is not turned off using **SSL\_CTX\_clear\_mode()**. That will prevent getting **SSL\_ERROR\_WANT\_READ** when there is still a complete record available that hasn't been processed.

If the application wants to continue to use the underlying transport (e.g. TCP connection) after the SSL connection is finished using **SSL\_shutdown()** reading ahead should be turned off. Otherwise the SSL structure might read data that it shouldn't.

## RETURN VALUES

**SSL\_get\_read\_ahead()** and **SSL\_CTX\_get\_read\_ahead()** return 0 if reading ahead is off, and non zero otherwise.

## SEE ALSO

[**ssl**(7)](/ssl/7.md), **SSL\_pending**(3)

## COPYRIGHT

Copyright 2015-2023 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\>.