Cryptography and SSL/TLS Toolkit



BIO_ctrl, BIO_callback_ctrl, BIO_ptr_ctrl, BIO_int_ctrl, BIO_reset, BIO_seek, BIO_tell, BIO_flush, BIO_eof, BIO_set_close, BIO_get_close, BIO_pending, BIO_wpending, BIO_ctrl_pending, BIO_ctrl_wpending, BIO_get_info_callback, BIO_set_info_callback, BIO_info_cb, BIO_get_ktls_send, BIO_get_ktls_recv, BIO_set_conn_mode, BIO_get_conn_mode, BIO_set_tfo - BIO control operations


#include <openssl/bio.h>

typedef int BIO_info_cb(BIO *b, int state, int res);

long BIO_ctrl(BIO *bp, int cmd, long larg, void *parg);
long BIO_callback_ctrl(BIO *b, int cmd, BIO_info_cb *cb);
void *BIO_ptr_ctrl(BIO *bp, int cmd, long larg);
long BIO_int_ctrl(BIO *bp, int cmd, long larg, int iarg);

int BIO_reset(BIO *b);
int BIO_seek(BIO *b, int ofs);
int BIO_tell(BIO *b);
int BIO_flush(BIO *b);
int BIO_eof(BIO *b);
int BIO_set_close(BIO *b, long flag);
int BIO_get_close(BIO *b);
int BIO_pending(BIO *b);
int BIO_wpending(BIO *b);
size_t BIO_ctrl_pending(BIO *b);
size_t BIO_ctrl_wpending(BIO *b);

int BIO_get_info_callback(BIO *b, BIO_info_cb **cbp);
int BIO_set_info_callback(BIO *b, BIO_info_cb *cb);

int BIO_get_ktls_send(BIO *b);
int BIO_get_ktls_recv(BIO *b);

int BIO_set_conn_mode(BIO *b, int mode);
int BIO_get_conn_mode(BIO *b);

int BIO_set_tfo(BIO *b, int onoff);


BIO_ctrl(), BIO_callback_ctrl(), BIO_ptr_ctrl() and BIO_int_ctrl() are BIO "control" operations taking arguments of various types. These functions are not normally called directly, various macros are used instead. The standard macros are described below, macros specific to a particular type of BIO are described in the specific BIOs manual page as well as any special features of the standard calls.

BIO_reset() typically resets a BIO to some initial state, in the case of file related BIOs for example it rewinds the file pointer to the start of the file.

BIO_seek() resets a file related BIO's (that is file descriptor and FILE BIOs) file position pointer to ofs bytes from start of file.

BIO_tell() returns the current file position of a file related BIO.

BIO_flush() normally writes out any internally buffered data, in some cases it is used to signal EOF and that no more data will be written.

BIO_eof() returns 1 if the BIO has read EOF, the precise meaning of "EOF" varies according to the BIO type.

BIO_set_close() sets the BIO b close flag to flag. flag can take the value BIO_CLOSE or BIO_NOCLOSE. Typically BIO_CLOSE is used in a source/sink BIO to indicate that the underlying I/O stream should be closed when the BIO is freed.

BIO_get_close() returns the BIOs close flag.

BIO_pending(), BIO_ctrl_pending(), BIO_wpending() and BIO_ctrl_wpending() return the number of pending characters in the BIOs read and write buffers. Not all BIOs support these calls. BIO_ctrl_pending() and BIO_ctrl_wpending() return a size_t type and are functions, BIO_pending() and BIO_wpending() are macros which call BIO_ctrl().

BIO_get_ktls_send() returns 1 if the BIO is using the Kernel TLS data-path for sending. Otherwise, it returns zero. BIO_get_ktls_recv() returns 1 if the BIO is using the Kernel TLS data-path for receiving. Otherwise, it returns zero.

BIO_get_conn_mode() returns the BIO connection mode. BIO_set_conn_mode() sets the BIO connection mode.

BIO_set_tfo() disables TCP Fast Open when onoff is 0, and enables TCP Fast Open when onoff is nonzero. Setting the value to 1 is equivalent to setting BIO_SOCK_TFO in BIO_set_conn_mode().


BIO_reset() normally returns 1 for success and <=0 for failure. File BIOs are an exception, they return 0 for success and -1 for failure.

BIO_seek() and BIO_tell() both return the current file position on success and -1 for failure, except file BIOs which for BIO_seek() always return 0 for success and -1 for failure.

BIO_flush() returns 1 for success and <=0 for failure.

BIO_eof() returns 1 if EOF has been reached, 0 if not, or negative values for failure.

BIO_set_close() returns 1 on success or <=0 for failure.

BIO_get_close() returns the close flag value: BIO_CLOSE or BIO_NOCLOSE. It also returns other negative values if an error occurs.

BIO_pending(), BIO_ctrl_pending(), BIO_wpending() and BIO_ctrl_wpending() return the amount of pending data. BIO_pending() and BIO_wpending() return negative value or 0 on error. BIO_ctrl_pending() and BIO_ctrl_wpending() return 0 on error.

BIO_get_ktls_send() returns 1 if the BIO is using the Kernel TLS data-path for sending. Otherwise, it returns zero. BIO_get_ktls_recv() returns 1 if the BIO is using the Kernel TLS data-path for receiving. Otherwise, it returns zero.

BIO_set_conn_mode() returns 1 for success and 0 for failure. BIO_get_conn_mode() returns the current connection mode. Which may contain the bitwise-or of the following flags:


BIO_set_tfo() returns 1 for success, and 0 for failure.


BIO_flush(), because it can write data may return 0 or -1 indicating that the call should be retried later in a similar manner to BIO_write_ex(). The BIO_should_retry() call should be used and appropriate action taken is the call fails.

The return values of BIO_pending() and BIO_wpending() may not reliably determine the amount of pending data in all cases. For example in the case of a file BIO some data may be available in the FILE structures internal buffers but it is not possible to determine this in a portably way. For other types of BIO they may not be supported.

Filter BIOs if they do not internally handle a particular BIO_ctrl() operation usually pass the operation to the next BIO in the chain. This often means there is no need to locate the required BIO for a particular operation, it can be called on a chain and it will be automatically passed to the relevant BIO. However, this can cause unexpected results: for example no current filter BIOs implement BIO_seek(), but this may still succeed if the chain ends in a FILE or file descriptor BIO.

Source/sink BIOs return an 0 if they do not recognize the BIO_ctrl() operation.


Some of the return values are ambiguous and care should be taken. In particular a return value of 0 can be returned if an operation is not supported, if an error occurred, if EOF has not been reached and in the case of BIO_seek() on a file BIO for a successful operation.

In older versions of OpenSSL the BIO_ctrl_pending() and BIO_ctrl_wpending() could return values greater than INT_MAX on error.


The BIO_get_ktls_send() and BIO_get_ktls_recv() macros were added in OpenSSL 3.0. They were modified to never return -1 in OpenSSL 3.0.4.

The BIO_get_conn_mode(), BIO_set_conn_mode() and BIO_set_tfo() functions were added in OpenSSL 3.2.

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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