BIO_CTRL(3) | Library Functions Manual | BIO_CTRL(3)

# NAME

`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_info_cb` — BIO control operations

# SYNOPSIS

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

long
`BIO_ctrl`(BIO *b, int cmd, long larg, void *parg);

long
`BIO_callback_ctrl`(BIO *b, int cmd, BIO_info_cb *cb);

char *
`BIO_ptr_ctrl`(BIO *b, int cmd, long larg);

long
`BIO_int_ctrl`(BIO *b, 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);

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

typedef int
`bio_info_cb`(BIO *b, int state, int res);

# DESCRIPTION

`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 BIO's manual page as well as any special features of the standard calls.

Depending on the cmd and on the type of b, `BIO_ctrl`() may have a read-only effect on b or change data in b or in its sub-structures. It may also have a side effect of changing the memory pointed to by parg.

`BIO_callback_ctrl`() does not call `BIO_ctrl`() but instead requires that the BIO type of b provides a dedicated callback_ctrl function pointer, which is built into the library for some standard BIO types and can be provided with BIO_meth_set_callback_ctrl(3) for application-defined BIO types. The only cmd supported by `BIO_callback_ctrl`() is `BIO_CTRL_SET_CALLBACK`.

`BIO_ptr_ctrl`() calls `BIO_ctrl`() with parg pointing to the location of a temporary pointer variable initialized to `NULL`.

`BIO_int_ctrl`() calls `BIO_ctrl`() with parg pointing to the location of a temporary int variable initialized to iarg. If `BIO_ctrl`() changes the value stored at *parg, the new value is ignored.

`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 BIO's close flag.

`BIO_pending`(), `BIO_ctrl_pending`(), `BIO_wpending`(), and `BIO_ctrl_wpending`() return the number of pending characters in the BIO's 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_set_info_callback`() installs the function pointer cb as an info callback in b by calling `BIO_callback_ctrl`() with a command of `BIO_CTRL_SET_CALLBACK`. Among the BIO types built into the library, only BIO_s_connect(3) and BIO_f_ssl(3) support this functionality. Some filter BIO types forward this control call to the next BIO in the chain instead of processing it themselves.

`BIO_get_info_callback`() places the function pointer to the info callback into *cbp if any was installed using `BIO_set_info_callback`() or `BIO_callback_ctrl`(). If the type of b supports setting an info callback but none was installed, it stores a `NULL` pointer in *cbp.

The function type name bio_info_cb is a deprecated synonym for BIO_info_cb provided for backward compatibility with some existing application software.

The following cmd constants correspond to macros:

cmd constant | corresponding macro
`BIO_C_DESTROY_BIO_PAIR` | BIO_destroy_bio_pair(3)
`BIO_C_DO_STATE_MACHINE` | BIO_do_handshake(3)
`BIO_C_FILE_SEEK` | `BIO_seek`()
`BIO_C_FILE_TELL` | `BIO_tell`()
`BIO_C_GET_ACCEPT` | BIO_get_accept_port(3)
`BIO_C_GET_BIND_MODE` | BIO_get_bind_mode(3)
`BIO_C_GET_BUF_MEM_PTR` | BIO_get_mem_ptr(3)
`BIO_C_GET_BUFF_NUM_LINES` | BIO_get_buffer_num_lines(3)
`BIO_C_GET_CIPHER_CTX` | BIO_get_cipher_ctx(3)
`BIO_C_GET_CIPHER_STATUS` | BIO_get_cipher_status(3)
`BIO_C_GET_FD` | BIO_get_fd(3)
`BIO_C_GET_FILE_PTR` | BIO_get_fp(3)
`BIO_C_GET_MD` | BIO_get_md(3)
`BIO_C_GET_MD_CTX` | BIO_get_md_ctx(3)
`BIO_C_GET_READ_REQUEST` | BIO_get_read_request(3)
`BIO_C_GET_SSL` | BIO_get_ssl(3)
`BIO_C_GET_SSL_NUM_RENEGOTIATES` | BIO_get_num_renegotiates(3)
`BIO_C_GET_WRITE_BUF_SIZE` | BIO_get_write_buf_size(3)
`BIO_C_GET_WRITE_GUARANTEE` | BIO_get_write_guarantee(3)
`BIO_C_MAKE_BIO_PAIR` | BIO_make_bio_pair(3)
`BIO_C_RESET_READ_REQUEST` | BIO_ctrl_reset_read_request(3)
`BIO_C_SET_BIND_MODE` | BIO_set_bind_mode(3)
`BIO_C_SET_BUF_MEM` | BIO_set_mem_buf(3)
`BIO_C_SET_BUF_MEM_EOF_RETURN` | BIO_set_mem_eof_return(3)
`BIO_C_SET_BUFF_READ_DATA` | BIO_set_buffer_read_data(3)
`BIO_C_SET_FD` | BIO_set_fd(3)
`BIO_C_SET_FILE_PTR` | BIO_set_fp(3)
`BIO_C_SET_MD` | BIO_set_md(3)
`BIO_C_SET_MD_CTX` | BIO_set_md_ctx(3)
`BIO_C_SET_NBIO` | BIO_set_nbio(3)
`BIO_C_SET_SSL` | BIO_set_ssl(3)
`BIO_C_SET_SSL_RENEGOTIATE_BYTES` | BIO_set_ssl_renegotiate_bytes(3)
`BIO_C_SET_SSL_RENEGOTIATE_TIMEOUT` | BIO_set_ssl_renegotiate_timeout(3)
`BIO_C_SET_WRITE_BUF_SIZE` | BIO_set_write_buf_size(3)
`BIO_C_SHUTDOWN_WR` | BIO_shutdown_wr(3)
`BIO_C_SSL_MODE` | BIO_set_ssl_mode(3)
`BIO_CTRL_DGRAM_CONNECT` | BIO_ctrl_dgram_connect(3)
`BIO_CTRL_DGRAM_GET_PEER` | BIO_dgram_get_peer(3)
`BIO_CTRL_DGRAM_GET_RECV_TIMER_EXP` | BIO_dgram_recv_timedout(3)
`BIO_CTRL_DGRAM_GET_SEND_TIMER_EXP` | BIO_dgram_send_timedout(3)
`BIO_CTRL_DGRAM_SET_CONNECTED` | BIO_ctrl_set_connected(3)
`BIO_CTRL_DGRAM_SET_PEER` | BIO_dgram_set_peer(3)
`BIO_CTRL_DUP` | BIO_dup_state(3)
`BIO_CTRL_EOF` | `BIO_eof`()
`BIO_CTRL_FLUSH` | `BIO_flush`()
`BIO_CTRL_GET_CALLBACK` | `BIO_get_info_callback`()
`BIO_CTRL_GET_CLOSE` | `BIO_get_close`()
`BIO_CTRL_INFO` | BIO_get_mem_data(3)
`BIO_CTRL_PENDING` | `BIO_pending`()
`BIO_CTRL_RESET` | `BIO_reset`()
`BIO_CTRL_SET_CALLBACK` | `BIO_set_info_callback`()
`BIO_CTRL_SET_CLOSE` | `BIO_set_close`()
`BIO_CTRL_WPENDING` | `BIO_wpending`()

A few cmd constants serve more than one macro each and are documented in the following manual pages:

cmd constant | manual page
`BIO_C_GET_CONNECT` | BIO_s_connect(3)
`BIO_C_SET_ACCEPT` | BIO_s_accept(3)
`BIO_C_SET_BUFF_SIZE` | BIO_f_buffer(3)
`BIO_C_SET_CONNECT` | BIO_s_connect(3)
`BIO_C_SET_FILENAME` | BIO_s_file(3)

Some cmd constants are not associated with any macros. They are documented in the following manual pages:

cmd constant | manual page
`BIO_CTRL_DGRAM_GET_FALLBACK_MTU` | BIO_dgram_set_peer(3)
`BIO_CTRL_DGRAM_GET_MTU` | BIO_dgram_set_peer(3)
`BIO_CTRL_DGRAM_GET_RECV_TIMEOUT` | BIO_dgram_recv_timedout(3)
`BIO_CTRL_DGRAM_GET_SEND_TIMEOUT` | BIO_dgram_send_timedout(3)
`BIO_CTRL_DGRAM_SET_MTU` | BIO_dgram_set_peer(3)
`BIO_CTRL_DGRAM_SET_NEXT_TIMEOUT` | BIO_dgram_recv_timedout(3)
`BIO_CTRL_DGRAM_SET_RECV_TIMEOUT` | BIO_dgram_recv_timedout(3)
`BIO_CTRL_DGRAM_SET_SEND_TIMEOUT` | BIO_dgram_send_timedout(3)
`BIO_CTRL_DGRAM_MTU_EXCEEDED` | BIO_s_datagram(3)
`BIO_CTRL_POP` | BIO_pop(3)
`BIO_CTRL_PUSH` | BIO_push(3)

# RETURN VALUES

The meaning of the return values of `BIO_ctrl`(), `BIO_callback_ctrl`(), and `BIO_int_ctrl`() depends on both the type of b and on the cmd. If b is a `NULL` pointer, no action occurs and 0 is returned. The return value -2 usually indicates a fatal error. In particular, it is returned if the cmd is unsupported by the type of b.

`BIO_callback_ctrl`() and `BIO_set_info_callback`() return 1 on success, 0 if b is `NULL` or to indicate failure of a valid cmd, or -2 if the cmd is not supported by b.

`BIO_ptr_ctrl`() returns `NULL` if the `BIO_ctrl`() call returns a negative value or does not change *parg, or the pointer it puts into *parg otherwise.

`BIO_int_ctrl`() returns the return value of `BIO_ctrl`().

`BIO_reset`() normally returns 1 for success and 0 or -1 for failure. File BIOs are an exception, returning 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 or -1 for failure.

`BIO_eof`() returns 1 if EOF has been reached or 0 otherwise.

`BIO_set_close`() always returns 1.

`BIO_get_close`() returns the close flag value `BIO_CLOSE` or `BIO_NOCLOSE`.

`BIO_pending`(), `BIO_ctrl_pending`(), `BIO_wpending`(), and `BIO_ctrl_wpending`() return the amount of pending data.

`BIO_get_info_callback`() returns 1 on success, including when the type of b supports an info callback but none is installed, 0 if b is `NULL` or -2 if the type of b does not support an info callback.

If a callback was installed in b using BIO_set_callback_ex(3) or BIO_set_callback(3), it can modify the return values of all these functions.

# NOTES

Because it can write data, `BIO_flush`() may return 0 or -1 indicating that the call should be retried later in a similar manner to BIO_write(3). The BIO_should_retry(3) call should be used and appropriate action taken if 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 structure's internal buffers but it is not possible to determine this in a portable way. For other types of BIO they may not be supported.

If they do not internally handle a particular `BIO_ctrl`() operation, filter BIOs 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 a 0 if they do not recognize the `BIO_ctrl`() operation.

# SEE ALSO

BIO_meth_new(3), BIO_new(3)

# HISTORY

`BIO_ctrl`(), `BIO_reset`(), `BIO_flush`(), `BIO_eof`(), `BIO_set_close`(), `BIO_get_close`(), and `BIO_pending`() first appeared in SSLeay 0.6.0. `BIO_wpending`() first appeared in SSLeay 0.8.1. `BIO_ptr_ctrl`(), `BIO_int_ctrl`(), `BIO_get_info_callback`() and `BIO_set_info_callback`() first appeared in SSLeay 0.9.0. All these functions have been available since OpenBSD 2.4.

`BIO_seek`() and `BIO_tell`() first appeared in SSLeay 0.9.1. `BIO_ctrl_pending`() and `BIO_ctrl_wpending`() first appeared in OpenSSL 0.9.4. These functions have been available since OpenBSD 2.6.

`BIO_callback_ctrl`() first appeared in OpenSSL 0.9.5 and has been available since OpenBSD 2.7.

`bio_info_cb`() first appeared with a more complicated prototype in OpenSSL 0.9.6 and has been available since OpenBSD 2.9.

`BIO_info_cb`() first appeared in OpenSSL 1.1.0h and has been available since OpenBSD 6.3.

# BUGS

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.

June 8, 2025 | Debian