Cryptography and SSL/TLS Toolkit



BIO_s_fd, BIO_set_fd, BIO_get_fd, BIO_new_fd - file descriptor BIO


 #include <openssl/bio.h>

 const BIO_METHOD *BIO_s_fd(void);

 int BIO_set_fd(BIO *b, int fd, int c);
 int BIO_get_fd(BIO *b, int *c);

 BIO *BIO_new_fd(int fd, int close_flag);


BIO_s_fd() returns the file descriptor BIO method. This is a wrapper round the platforms file descriptor routines such as read() and write().

BIO_read_ex() and BIO_write_ex() read or write the underlying descriptor. BIO_puts() is supported but BIO_gets() is not.

If the close flag is set then close() is called on the underlying file descriptor when the BIO is freed.

BIO_reset() attempts to change the file pointer to the start of file such as by using lseek(fd, 0, 0).

BIO_seek() sets the file pointer to position ofs from start of file such as by using lseek(fd, ofs, 0).

BIO_tell() returns the current file position such as by calling lseek(fd, 0, 1).

BIO_set_fd() sets the file descriptor of BIO b to fd and the close flag to c.

BIO_get_fd() places the file descriptor in c if it is not NULL, it also returns the file descriptor.

BIO_new_fd() returns a file descriptor BIO using fd and close_flag.


The behaviour of BIO_read_ex() and BIO_write_ex() depends on the behavior of the platforms read() and write() calls on the descriptor. If the underlying file descriptor is in a non blocking mode then the BIO will behave in the manner described in the BIO_read_ex(3) and BIO_should_retry(3) manual pages.

File descriptor BIOs should not be used for socket I/O. Use socket BIOs instead.

BIO_set_fd() and BIO_get_fd() are implemented as macros.


BIO_s_fd() returns the file descriptor BIO method.

BIO_set_fd() always returns 1.

BIO_get_fd() returns the file descriptor or -1 if the BIO has not been initialized.

BIO_new_fd() returns the newly allocated BIO or NULL is an error occurred.


This is a file descriptor BIO version of "Hello World":

 BIO *out;

 out = BIO_new_fd(fileno(stdout), BIO_NOCLOSE);
 BIO_printf(out, "Hello World\n");


BIO_seek(3), BIO_tell(3), BIO_reset(3), BIO_read_ex(3), BIO_write_ex(3), BIO_puts(3), BIO_gets(3), BIO_printf(3), BIO_set_close(3), BIO_get_close(3)

Copyright 2000-2016 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