BIO Routines
This documentation is rather sparse, you are probably best
off looking at the code for specific details.
The BIO library is a IO abstraction that was originally
inspired by the need to have callbacks to perform IO to FILE
pointers when using Windows 3.1 DLLs. There are two types
of BIO; a source/sink type and a filter type.
The source/sink methods are as follows:
- BIO_s_mem() memory buffer - a read/write byte array that
grows until memory runs out :-).
- BIO_s_file() FILE pointer - A wrapper around the normal
'FILE *' commands, good for use with stdin/stdout.
- BIO_s_fd() File descriptor - A wrapper around file
descriptors, often used with pipes.
- BIO_s_socket() Socket - Used around sockets. It is
mostly in the Microsoft world that sockets are different
from file descriptors and there are all those ugly winsock
commands.
- BIO_s_null() Null - read nothing and write nothing.; a
useful endpoint for filter type BIO's specifically things
like the message digest BIO.
The filter types are
- BIO_f_buffer() IO buffering - does output buffering into
larger chunks and performs input buffering to allow gets()
type functions.
- BIO_f_md() Message digest - a transparent filter that can
be asked to return a message digest for the data that has
passed through it.
- BIO_f_cipher() Encrypt or decrypt all data passing
through the filter.
- BIO_f_base64() Base64 decode on read and encode on write.
- BIO_f_ssl() A filter that performs SSL encryption on the
data sent through it.
Base BIO functions.
The BIO library has a set of base functions that are
implemented for each particular type. Filter BIOs will
normally call the equivalent function on the source/sink BIO
that they are layered on top of after they have performed
some modification to the data stream. Multiple filter BIOs
can be 'push' into a stack of modifers, so to read from a
file, unbase64 it, then decrypt it, a BIO_f_cipher,
BIO_f_base64 and a BIO_s_file would probably be used. If a
sha-1 and md5 message digest needed to be generated, a stack
two BIO_f_md() BIOs and a BIO_s_null() BIO could be used.
The base functions are
- BIO *BIO_new(BIO_METHOD *type); Create a new BIO of type 'type'.
- int BIO_free(BIO *a); Free a BIO structure. Depending on
the configuration, this will free the underlying data
object for a source/sink BIO.
- int BIO_read(BIO *b, char *data, int len); Read upto 'len'
bytes into 'data'.
- int BIO_gets(BIO *bp,char *buf, int size); Depending on
the BIO, this can either be a 'get special' or a get one
line of data, as per fgets();
- int BIO_write(BIO *b, char *data, int len); Write 'len'
bytes from 'data' to the 'b' BIO.
- int BIO_puts(BIO *bp,char *buf); Either a 'put special' or
a write null terminated string as per fputs().
- long BIO_ctrl(BIO *bp,int cmd,long larg,char *parg); A
control function which is used to manipulate the BIO
structure and modify it's state and or report on it. This
function is just about never used directly, rather it
should be used in conjunction with BIO_METHOD specific
macros.
- BIO *BIO_push(BIO *new_top, BIO *old); new_top is apped to the
top of the 'old' BIO list. new_top should be a filter BIO.
All writes will go through 'new_top' first and last on read.
'old' is returned.
- BIO *BIO_pop(BIO *bio); the new topmost BIO is returned, NULL if
there are no more.
If a particular low level BIO method is not supported
(normally BIO_gets()), -2 will be returned if that method is
called. Otherwise the IO methods (read, write, gets, puts)
will return the number of bytes read or written, and 0 or -1
for error (or end of input). For the -1 case,
BIO_should_retry(bio) can be called to determine if it was a
genuine error or a temporary problem. -2 will also be
returned if the BIO has not been initalised yet, in all
cases, the correct error codes are set (accessible via the
ERR library).
The following functions are convenience functions:
- int BIO_printf(BIO *bio, char * format, ..); printf but
to a BIO handle.
- long BIO_ctrl_int(BIO *bp,int cmd,long larg,int iarg); a
convenience function to allow a different argument types
to be passed to BIO_ctrl().
- int BIO_dump(BIO *b,char *bytes,int len); output 'len'
bytes from 'bytes' in a hex dump debug format.
- long BIO_debug_callback(BIO *bio, int cmd, char *argp, int
argi, long argl, long ret) - a default debug BIO callback,
this is mentioned below. To use this one normally has to
use the BIO_set_callback_arg() function to assign an
output BIO for the callback to use.
- BIO *BIO_find_type(BIO *bio,int type); when there is a 'stack'
of BIOs, this function scan the list and returns the first
that is of type 'type', as listed in buffer.h under BIO_TYPE_XXX.
- void BIO_free_all(BIO *bio); Free the bio and all other BIOs
in the list. It walks the bio->next_bio list.
Extra commands are normally implemented as macros calling BIO_ctrl().
- BIO_number_read(BIO *bio) - the number of bytes processed
by BIO_read(bio,.).
- BIO_number_written(BIO *bio) - the number of bytes written
by BIO_write(bio,.).
- BIO_reset(BIO *bio) - 'reset' the BIO.
- BIO_eof(BIO *bio) - non zero if we are at the current end
of input.
- BIO_set_close(BIO *bio, int close_flag) - set the close flag.
- BIO_get_close(BIO *bio) - return the close flag.
BIO_pending(BIO *bio) - return the number of bytes waiting
to be read (normally buffered internally).
- BIO_flush(BIO *bio) - output any data waiting to be output.
- BIO_should_retry(BIO *io) - after a BIO_read/BIO_write
operation returns 0 or -1, a call to this function will
return non zero if you should retry the call later (this
is for non-blocking IO).
- BIO_should_read(BIO *io) - we should retry when data can
be read.
- BIO_should_write(BIO *io) - we should retry when data can
be written.
- BIO_method_name(BIO *io) - return a string for the method name.
- BIO_method_type(BIO *io) - return the unique ID of the BIO method.
- BIO_set_callback(BIO *io, long (*callback)(BIO *io, int
cmd, char *argp, int argi, long argl, long ret); - sets
the debug callback.
- BIO_get_callback(BIO *io) - return the assigned function
as mentioned above.
- BIO_set_callback_arg(BIO *io, char *arg) - assign some
data against the BIO. This is normally used by the debug
callback but could in reality be used for anything. To
get an idea of how all this works, have a look at the code
in the default debug callback mentioned above. The
callback can modify the return values.
Details of the BIO_METHOD structure.
typedef struct bio_method_st
{
int type;
char *name;
int (*bwrite)();
int (*bread)();
int (*bputs)();
int (*bgets)();
long (*ctrl)();
int (*create)();
int (*destroy)();
} BIO_METHOD;
The 'type' is the numeric type of the BIO, these are listed in buffer.h;
'Name' is a textual representation of the BIO 'type'.
The 7 function pointers point to the respective function
methods, some of which can be NULL if not implemented.
The BIO structure
typedef struct bio_st
{
BIO_METHOD *method;
long (*callback)(BIO * bio, int mode, char *argp, int
argi, long argl, long ret);
char *cb_arg; /* first argument for the callback */
int init;
int shutdown;
int flags; /* extra storage */
int num;
char *ptr;
struct bio_st *next_bio; /* used by filter BIOs */
int references;
unsigned long num_read;
unsigned long num_write;
} BIO;
- 'Method' is the BIO method.
- 'callback', when configured, is called before and after
each BIO method is called for that particular BIO. This
is intended primarily for debugging and of informational feedback.
- 'init' is 0 when the BIO can be used for operation.
Often, after a BIO is created, a number of operations may
need to be performed before it is available for use. An
example is for BIO_s_sock(). A socket needs to be
assigned to the BIO before it can be used.
- 'shutdown', this flag indicates if the underlying
comunication primative being used should be closed/freed
when the BIO is closed.
- 'flags' is used to hold extra state. It is primarily used
to hold information about why a non-blocking operation
failed and to record startup protocol information for the
SSL BIO.
- 'num' and 'ptr' are used to hold instance specific state
like file descriptors or local data structures.
- 'next_bio' is used by filter BIOs to hold the pointer of the
next BIO in the chain. written data is sent to this BIO and
data read is taken from it.
- 'references' is used to indicate the number of pointers to
this structure. This needs to be '1' before a call to
BIO_free() is made if the BIO_free() function is to
actually free() the structure, otherwise the reference
count is just decreased. The actual BIO subsystem does
not really use this functionality but it is useful when
used in more advanced applicaion.
- num_read and num_write are the total number of bytes
read/written via the 'read()' and 'write()' methods.
BIO_ctrl operations.
The following is the list of standard commands passed as the
second parameter to BIO_ctrl() and should be supported by
all BIO as best as possible. Some are optional, some are
manditory, in any case, where is makes sense, a filter BIO
should pass such requests to underlying BIO's.
- BIO_CTRL_RESET - Reset the BIO back to an initial state.
- BIO_CTRL_EOF - return 0 if we are not at the end of input,
non 0 if we are.
- BIO_CTRL_INFO - BIO specific special command, normal
information return.
- BIO_CTRL_SET - set IO specific parameter.
- BIO_CTRL_GET - get IO specific parameter.
- BIO_CTRL_GET_CLOSE - Get the close on BIO_free() flag, one
of BIO_CLOSE or BIO_NOCLOSE.
- BIO_CTRL_SET_CLOSE - Set the close on BIO_free() flag.
- BIO_CTRL_PENDING - Return the number of bytes available
for instant reading
- BIO_CTRL_FLUSH - Output pending data, return number of bytes output.
- BIO_CTRL_SHOULD_RETRY - After an IO error (-1 returned)
should we 'retry' when IO is possible on the underlying IO object.
- BIO_CTRL_RETRY_TYPE - What kind of IO are we waiting on.
The following command is a special BIO_s_file() specific option.
- BIO_CTRL_SET_FILENAME - specify a file to open for IO.
The BIO_CTRL_RETRY_TYPE needs a little more explanation.
When performing non-blocking IO, or say reading on a memory
BIO, when no data is present (or cannot be written),
BIO_read() and/or BIO_write() will return -1.
BIO_should_retry(bio) will return true if this is due to an
IO condition rather than an actual error. In the case of
BIO_s_mem(), a read when there is no data will return -1 and
a should retry when there is more 'read' data.
The retry type is deduced from 2 macros
BIO_should_read(bio) and BIO_should_write(bio).
Now while it may appear obvious that a BIO_read() failure
should indicate that a retry should be performed when more
read data is available, this is often not true when using
things like an SSL BIO. During the SSL protocol startup
multiple reads and writes are performed, triggered by any
SSL_read or SSL_write.
So to write code that will transparently handle either a
socket or SSL BIO,
i=BIO_read(bio,..)
if (I == -1)
{
if (BIO_should_retry(bio))
{
if (BIO_should_read(bio))
{
/* call us again when BIO can be read */
}
if (BIO_should_write(bio))
{
/* call us again when BIO can be written */
}
}
}
At this point in time only read and write conditions can be
used but in the future I can see the situation for other
conditions, specifically with SSL there could be a condition
of a X509 certificate lookup taking place and so the non-
blocking BIO_read would require a retry when the certificate
lookup subsystem has finished it's lookup. This is all
makes more sense and is easy to use in a event loop type
setup.
When using the SSL BIO, either SSL_read() or SSL_write()s
can be called during the protocol startup and things will
still work correctly.
The nice aspect of the use of the BIO_should_retry() macro
is that all the errno codes that indicate a non-fatal error
are encapsulated in one place. The Windows specific error
codes and WSAGetLastError() calls are also hidden from the
application.
Notes on each BIO method.
Normally buffer.h is just required but depending on the
BIO_METHOD, ssl.h or envelope.h will also be required.
BIO_METHOD *BIO_s_mem(void);
- BIO_set_mem_buf(BIO *bio, BUF_MEM *bm, int close_flag) -
set the underlying BUF_MEM structure for the BIO to use.
- BIO_get_mem_ptr(BIO *bio, char **pp) - if pp is not NULL,
set it to point to the memory array and return the number
of bytes available.
A read/write BIO. Any data written is appended to the
memory array and any read is read from the front. This BIO
can be used for read/write at the same time. BIO_gets() is
supported in the fgets() sense.
BIO_CTRL_INFO can be used to retrieve pointers to the memory
buffer and it's length.
BIO_METHOD *BIO_s_file(void);
- BIO_set_fp(BIO *bio, FILE *fp, int close_flag) - set 'FILE *' to use.
- BIO_get_fp(BIO *bio, FILE **fp) - get the 'FILE *' in use.
- BIO_read_filename(BIO *bio, char *name) - read from file.
- BIO_write_filename(BIO *bio, char *name) - write to file.
- BIO_append_filename(BIO *bio, char *name) - append to file.
This BIO sits over the normal system fread()/fgets() type
functions. Gets() is supported. This BIO in theory could be
used for read and write but it is best to think of each BIO
of this type as either a read or a write BIO, not both.
BIO_METHOD *BIO_s_socket(void);
BIO_METHOD *BIO_s_fd(void);
- BIO_sock_should_retry(int i) - the underlying function
used to determine if a call should be retried; the
argument is the '0' or '-1' returned by the previous BIO
operation.
- BIO_fd_should_retry(int i) - same as the
- BIO_sock_should_retry() except that it is different internally.
- BIO_set_fd(BIO *bio, int fd, int close_flag) - set the
file descriptor to use
- BIO_get_fd(BIO *bio, int *fd) - get the file descriptor.
These two methods are very similar. Gets() is not
supported, if you want this functionality, put a
BIO_f_buffer() onto it. This BIO is bi-directional if the
underlying file descriptor is. This is normally the case
for sockets but not the case for stdio descriptors.
BIO_METHOD *BIO_s_null(void);
Read and write as much data as you like, it all disappears
into this BIO.
BIO_METHOD *BIO_f_buffer(void);
- BIO_get_buffer_num_lines(BIO *bio) - return the number of
complete lines in the buffer.
- BIO_set_buffer_size(BIO *bio, long size) - set the size of
the buffers.
This type performs input and output buffering. It performs
both at the same time. The size of the buffer can be set
via the set buffer size option. Data buffered for output is
only written when the buffer fills.
BIO_METHOD *BIO_f_ssl(void);
- BIO_set_ssl(BIO *bio, SSL *ssl, int close_flag) - the SSL
structure to use.
- BIO_get_ssl(BIO *bio, SSL **ssl) - get the SSL structure
in use.
The SSL bio is a little different from normal BIOs because
the underlying SSL structure is a little different. A SSL
structure performs IO via a read and write BIO. These can
be different and are normally set via the
SSL_set_rbio()/SSL_set_wbio() calls. The SSL_set_fd() calls
are just wrappers that create socket BIOs and then call
SSL_set_bio() where the read and write BIOs are the same.
The BIO_push() operation makes the SSLs IO BIOs the same, so
make sure the BIO pushed is capable of two directional
traffic. If it is not, you will have to install the BIOs
via the more conventional SSL_set_bio() call. BIO_pop() will retrieve
the 'SSL read' BIO.
BIO_METHOD *BIO_f_md(void);
- BIO_set_md(BIO *bio, EVP_MD *md) - set the message digest
to use.
- BIO_get_md(BIO *bio, EVP_MD **mdp) - return the digest
method in use in mdp, return 0 if not set yet.
- BIO_reset() reinitializes the digest (EVP_DigestInit())
and passes the reset to the underlying BIOs.
All data read or written via BIO_read() or BIO_write() to
this BIO will be added to the calculated digest. This
implies that this BIO is only one directional. If read and
write operations are performed, two separate BIO_f_md() BIOs
are reuqired to generate digests on both the input and the
output. BIO_gets(BIO *bio, char *md, int size) will place the
generated digest into 'md' and return the number of bytes.
The EVP_MAX_MD_SIZE should probably be used to size the 'md'
array. Reading the digest will also reset it.
BIO_METHOD *BIO_f_cipher(void);
- BIO_reset() reinitializes the cipher.
- BIO_flush() should be called when the last bytes have been
output to flush the final block of block ciphers.
- BIO_get_cipher_status(BIO *b), when called after the last
read from a cipher BIO, returns non-zero if the data
decrypted correctly, otherwise, 0.
- BIO_set_cipher(BIO *b, EVP_CIPHER *c, unsigned char *key,
unsigned char *iv, int encrypt) This function is used to
setup a cipher BIO. The length of key and iv are
specified by the choice of EVP_CIPHER. Encrypt is 1 to
encrypt and 0 to decrypt.
BIO_METHOD *BIO_f_base64(void);
- BIO_flush() should be called when the last bytes have been output.
This BIO base64 encodes when writing and base64 decodes when
reading. It will scan the input until a suitable begin line
is found. After reading data, BIO_reset() will reset the
BIO to start scanning again. Do not mix reading and writing
on the same base64 BIO. It is meant as a single stream BIO.
Directions type
both BIO_s_mem()
one/both BIO_s_file()
both BIO_s_fd()
both BIO_s_socket()
both BIO_s_null()
both BIO_f_buffer()
one BIO_f_md()
one BIO_f_cipher()
one BIO_f_base64()
both BIO_f_ssl()
It is easy to mix one and two directional BIOs, all one has
to do is to keep two separate BIO pointers for reading and
writing and be careful about usage of underlying BIOs. The
SSL bio by it's very nature has to be two directional but
the BIO_push() command will push the one BIO into the SSL
BIO for both reading and writing.
The best example program to look at is apps/enc.c and/or perhaps apps/dgst.c.