- NAME
- SYNOPSIS
- DESCRIPTION
- RETURN VALUES
- BUGS
- SEE ALSO
- HISTORY
- COPYRIGHT

# NAME

BIO_get_new_index, BIO_meth_new, BIO_meth_free, BIO_meth_get_read_ex, BIO_meth_set_read_ex, BIO_meth_get_write_ex, BIO_meth_set_write_ex, BIO_meth_get_write, BIO_meth_set_write, BIO_meth_get_read, BIO_meth_set_read, BIO_meth_get_puts, BIO_meth_set_puts, BIO_meth_get_gets, BIO_meth_set_gets, BIO_meth_get_ctrl, BIO_meth_set_ctrl, BIO_meth_get_create, BIO_meth_set_create, BIO_meth_get_destroy, BIO_meth_set_destroy, BIO_meth_get_callback_ctrl, BIO_meth_set_callback_ctrl, BIO_meth_set_sendmmsg, BIO_meth_get_sendmmsg, BIO_meth_set_recvmmsg, BIO_meth_get_recvmmsg - Routines to build up BIO methods

# SYNOPSIS

    #include <openssl/bio.h>

    int BIO_get_new_index(void);

    BIO_METHOD *BIO_meth_new(int type, const char *name);

    void BIO_meth_free(BIO_METHOD *biom);

    int BIO_meth_set_write_ex(BIO_METHOD *biom,
                              int (*bwrite)(BIO *, const char *, size_t, size_t *));
    int BIO_meth_set_write(BIO_METHOD *biom,
                           int (*write)(BIO *, const char *, int));

    int BIO_meth_set_read_ex(BIO_METHOD *biom,
                             int (*bread)(BIO *, char *, size_t, size_t *));
    int BIO_meth_set_read(BIO_METHOD *biom, int (*read)(BIO *, char *, int));

    int BIO_meth_set_puts(BIO_METHOD *biom, int (*puts)(BIO *, const char *));
    int BIO_meth_set_gets(BIO_METHOD *biom,
                          int (*gets)(BIO *, char *, int));

    int BIO_meth_set_ctrl(BIO_METHOD *biom,
                          long (*ctrl)(BIO *, int, long, void *));

    int BIO_meth_set_create(BIO_METHOD *biom, int (*create)(BIO *));
    int BIO_meth_set_destroy(BIO_METHOD *biom, int (*destroy)(BIO *));

    int BIO_meth_set_callback_ctrl(BIO_METHOD *biom,
                                   long (*callback_ctrl)(BIO *, int, BIO_info_cb *));

    int BIO_meth_set_sendmmsg(BIO_METHOD *biom,
                              ossl_ssize_t (*f) (BIO *, BIO_MSG *, size_t,
                                                 size_t, uint64_t));
    int BIO_meth_set_recvmmsg(BIO_METHOD *biom,
                              ossl_ssize_t (*f) (BIO *, BIO_MSG *, size_t,
                                                 size_t, uint64_t));

The following functions have been deprecated since OpenSSL 3.5:

    int (*BIO_meth_get_write_ex(const BIO_METHOD *biom))(BIO *, const char *, size_t,
                                                   size_t *);
    int (*BIO_meth_get_write(const BIO_METHOD *biom))(BIO *, const char *, int);

    int (*BIO_meth_get_read_ex(const BIO_METHOD *biom))(BIO *, char *, size_t, size_t *);
    int (*BIO_meth_get_read(const BIO_METHOD *biom))(BIO *, char *, int);

    int (*BIO_meth_get_puts(const BIO_METHOD *biom))(BIO *, const char *);
    int (*BIO_meth_get_gets(const BIO_METHOD *biom))(BIO *, char *, int);

    long (*BIO_meth_get_ctrl(const BIO_METHOD *biom))(BIO *, int, long, void *);

    int (*BIO_meth_get_create(const BIO_METHOD *bion))(BIO *);
    int (*BIO_meth_get_destroy(const BIO_METHOD *biom))(BIO *);

    long (*BIO_meth_get_callback_ctrl(const BIO_METHOD *biom))(BIO *, int, BIO_info_cb *);

    ossl_ssize_t (*BIO_meth_get_sendmmsg(const BIO_METHOD *biom))(BIO *,
                                                                  BIO_MSG *,
                                                                  size_t,
                                                                  size_t,
                                                                  uint64_t);
    ossl_ssize_t (*BIO_meth_get_recvmmsg(const BIO_METHOD *biom))(BIO *,
                                                                  BIO_MSG *,
                                                                  size_t,
                                                                  size_t,
                                                                  uint64_t);

# DESCRIPTION

The BIO_METHOD type is a structure used for the implementation of new BIO types. It provides a set of functions used by OpenSSL for the implementation of the various BIO capabilities. See the bio(7) page for more information.

BIO_meth_new() creates a new BIO_METHOD structure that contains a type identifier type and a string that represents its name. type can be set to either BIO_TYPE_NONE or via BIO_get_new_index() if a unique type is required for searching (See BIO_find_type(3))

Note that BIO_get_new_index() can only be used 127 times before it returns an error.

The set of standard OpenSSL provided BIO types is provided in <openssl/bio.h>. Some examples include BIO_TYPE_BUFFER and BIO_TYPE_CIPHER. Filter BIOs should have a type which have the "filter" bit set (BIO_TYPE_FILTER). Source/sink BIOs should have the "source/sink" bit set (BIO_TYPE_SOURCE_SINK). File descriptor based BIOs (e.g. socket, fd, connect, accept etc) should additionally have the "descriptor" bit set (BIO_TYPE_DESCRIPTOR). See the BIO_find_type(3) page for more information.

BIO_meth_free() destroys a BIO_METHOD structure and frees up any memory associated with it. If the argument is NULL, nothing is done.

BIO_meth_get_write_ex() and BIO_meth_set_write_ex() get and set the function used for writing arbitrary length data to the BIO respectively. This function will be called in response to the application calling BIO_write_ex() or BIO_write(). The parameters for the function have the same meaning as for BIO_write_ex() and it must return values as described for BIO_write_ex().

Older code may call BIO_meth_get_write() and BIO_meth_set_write() instead to set an old-style write function. The parameters for the function have the same meaning as for BIO_write() and it must return values as described for BIO_write().

Functions set by BIO_meth_set_write_ex() and BIO_meth_set_write() must call BIO_set_flags() to set the BIO_FLAGS_SHOULD_RETRY flag in relevant situations.

Applications should not call both BIO_meth_set_write_ex() and BIO_meth_set_write() or call BIO_meth_get_write() when the function was set with BIO_meth_set_write_ex().

BIO_meth_get_read_ex() and BIO_meth_set_read_ex() get and set the function used for reading arbitrary length data from the BIO respectively. This function will be called in response to the application calling BIO_read_ex() or BIO_read(). The parameters for the function have the same meaning as for BIO_read_ex() and it must return values as described for BIO_read_ex(). The function must handle the end-of-file condition (if applicable) and return 0 in this case.

Older code may call BIO_meth_get_read() and BIO_meth_set_read() instead to set an old-style read function. The parameters for the function have the same meaning as for BIO_read() and it must return values as described for BIO_read().

Functions set by BIO_meth_set_read_ex() and BIO_meth_set_read() must call BIO_set_flags() to set the BIO_FLAGS_SHOULD_RETRY flag in relevant situations.

Applications should not call both BIO_meth_set_read_ex() and BIO_meth_set_read() or call BIO_meth_get_read() when the function was set with BIO_meth_set_read_ex().

BIO_meth_get_puts() and BIO_meth_set_puts() get and set the function used for writing a NULL terminated string to the BIO respectively. This function will be called in response to the application calling BIO_puts(). The parameters for the function have the same meaning as for BIO_puts().

BIO_meth_get_gets() and BIO_meth_set_gets() get and set the function typically used for reading a line of data from the BIO respectively (see the BIO_gets(3) page for more information). This function will be called in response to the application calling BIO_gets(). The parameters for the function have the same meaning as for BIO_gets().

BIO_meth_get_ctrl() and BIO_meth_set_ctrl() get and set the function used for processing ctrl messages in the BIO respectively. See the BIO_ctrl(3) page for more information. This function will be called in response to the application calling BIO_ctrl(). The parameters for the function have the same meaning as for BIO_ctrl(). If the concept of end-of-file is meaningful for a BIO and the read method is set using BIO_meth_set_read_ex(), the ctrl function must handle the BIO_CTRL_EOF command and return an appropriate value (1 if EOF has been reached, 0 if not, or a negative value on failure), at least immediately after a read operation. If the read method is set using BIO_meth_set_read(), handling of the BIO_CTRL_EOF command is not mandatory; however, if such handling is implemented, it must return 1 if the read function returned 0 when attempting to read a nonzero number of bytes.

BIO_meth_get_create() and BIO_meth_set_create() get and set the function used for creating a new instance of the BIO respectively. This function will be called in response to the application calling BIO_new() and passing in a pointer to the current BIO_METHOD. The BIO_new() function will allocate the memory for the new BIO, and a pointer to this newly allocated structure will be passed as a parameter to the function. If a create function is set, BIO_new() will not mark the BIO as initialised on allocation. BIO_set_init(3) must then be called either by the create function, or later, by a BIO ctrl function, once BIO initialisation is complete.

BIO_meth_get_destroy() and BIO_meth_set_destroy() get and set the function used for destroying an instance of a BIO respectively. This function will be called in response to the application calling BIO_free(). A pointer to the BIO to be destroyed is passed as a parameter. The destroy function should be used for BIO specific clean up. The memory for the BIO itself should not be freed by this function.

BIO_meth_get_callback_ctrl() and BIO_meth_set_callback_ctrl() get and set the function used for processing callback ctrl messages in the BIO respectively. See the BIO_callback_ctrl(3) page for more information. This function will be called in response to the application calling BIO_callback_ctrl(). The parameters for the function have the same meaning as for BIO_callback_ctrl().

BIO_meth_get_sendmmsg(), BIO_meth_set_sendmmsg(), BIO_meth_get_recvmmsg() and BIO_meth_set_recvmmsg() get and set the functions used for handling BIO_sendmmsg() and BIO_recvmmsg() calls respectively. See BIO_sendmmsg(3) for more information.

# RETURN VALUES

BIO_get_new_index() returns the new BIO type value or -1 if an error occurred.

BIO_meth_new(int type, const char *name) returns a valid BIO_METHOD or NULL if an error occurred.

The BIO_meth_set functions return 1 on success or 0 on error.

The BIO_meth_get functions return the corresponding function pointers.

# BUGS

It is not safe to use `BIO_meth_get_` functions to reuse the BIO implementation of BIOs implemented by OpenSSL itself with application-implemented BIOs. Instead either the applications ought to implement these functions themselves or they should implement a filter BIO.

For more details please see https://github.com/openssl/openssl/issues/26047.

# SEE ALSO

bio(7), BIO_find_type(3), BIO_ctrl(3), BIO_read_ex(3), BIO_new(3)

# HISTORY

The functions BIO_meth_get_sendmmsg(), BIO_meth_set_sendmmsg(), BIO_meth_get_recvmmsg() and BIO_meth_set_recvmmsg() were added in OpenSSL 3.2.

All the other functions described here were added in OpenSSL 1.1.0.

The functions BIO_meth_get_read_ex(), BIO_meth_get_write_ex(), BIO_meth_get_write(), BIO_meth_get_read(), BIO_meth_get_puts(), BIO_meth_get_gets(), BIO_meth_get_ctrl(), BIO_meth_get_create(), BIO_meth_get_destroy(), BIO_meth_get_callback_ctrl(), BIO_meth_get_sendmmsg() and BIO_meth_get_recvmmsg() are deprecated since OpenSSL 3.5.

# COPYRIGHT

Copyright 2016-2026 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.