SSL_CTX_SET_MODE(3) | Library Functions Manual | SSL_CTX_SET_MODE(3)

# NAME

`SSL_CTX_set_mode`, `SSL_set_mode`, `SSL_CTX_clear_mode`, `SSL_clear_mode`, `SSL_CTX_get_mode`, `SSL_get_mode` — manipulate SSL engine mode

# SYNOPSIS

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

long
`SSL_CTX_set_mode`(SSL_CTX *ctx, long mode);

long
`SSL_set_mode`(SSL *ssl, long mode);

long
`SSL_CTX_clear_mode`(SSL_CTX *ctx, long mode);

long
`SSL_clear_mode`(SSL *ssl, long mode);

long
`SSL_CTX_get_mode`(SSL_CTX *ctx);

long
`SSL_get_mode`(SSL *ssl);

# DESCRIPTION

`SSL_CTX_set_mode`() and `SSL_set_mode`() enable the options contained in the bitmask mode for the ctx or ssl object, respectively. Options that were already enabled before the call are not disabled.

`SSL_CTX_clear_mode`() and `SSL_clear_mode`() disable the options contained in the bitmask mode for the ctx or ssl object.

`SSL_CTX_get_mode`() and `SSL_get_mode`() return a bitmask representing the options that are currently enabled for the ctx or ssl object.

The following options are available:

`SSL_MODE_ENABLE_PARTIAL_WRITE`
  Allow `SSL_write`(..., n) to return r with 0<r<n (i.e., report success when just a single record has been written). When not set (the default), SSL_write(3) will only report success once the complete chunk was written. Once SSL_write(3) returns with r, r bytes have been successfully written and the next call to SSL_write(3) must only send the n − r bytes left, imitating the behaviour of write(2).
`SSL_MODE_ACCEPT_MOVING_WRITE_BUFFER`
  Make it possible to retry SSL_write(3) with changed buffer location (the buffer contents must stay the same). This is not the default to avoid the misconception that non-blocking SSL_write(3) behaves like non-blocking write(2).
`SSL_MODE_AUTO_RETRY`
  Never bother the application with retries if the transport is blocking. If a renegotiation takes place during normal operation, a SSL_read(3) or SSL_write(3) would return with −1 and indicate the need to retry with `SSL_ERROR_WANT_READ`. In a non-blocking environment applications must be prepared to handle incomplete read/write operations. In a blocking environment, applications are not always prepared to deal with read/write operations returning without success report. The flag `SSL_MODE_AUTO_RETRY` will cause read/write operations to only return after the handshake and successful completion.
`SSL_MODE_RELEASE_BUFFERS`
  When we no longer need a read buffer or a write buffer for a given SSL, then release the memory we were using to hold it. Using this flag can save around 34k per idle SSL connection. This flag has no effect on SSL v2 connections, or on DTLS connections.

# RETURN VALUES

`SSL_CTX_set_mode`(), `SSL_set_mode`(), `SSL_CTX_clear_mode`(), and `SSL_clear_mode`() return the new mode bitmask after adding or clearing mode.

`SSL_CTX_get_mode`() and `SSL_get_mode`() return the current bitmask.

# SEE ALSO

ssl(3), SSL_CTX_ctrl(3), SSL_read(3), SSL_write(3)

# HISTORY

`SSL_CTX_set_mode`(), `SSL_set_mode`(), `SSL_CTX_get_mode`(), and `SSL_get_mode`() first appeared in OpenSSL 0.9.4 and have been available since OpenBSD 2.6.

`SSL_CTX_clear_mode`() and `SSL_clear_mode`() first appeared in OpenSSL 0.9.8m and have been available since OpenBSD 4.9.

`SSL_MODE_AUTO_RETRY` was added in OpenSSL 0.9.6.

June 8, 2025 | Debian