BIO_SET_CALLBACK(3)

NAME
BIO_set_callback, BIO_get_callback, BIO_set_callback_arg, BIO_get_callback_arg, BIO_debug_callback - BIO callback functions

SYNOPSIS
#include <openssl/bio.h>
void
BIO_set_callback(BIO *b, BIO_callback_fn cb);
BIO_callback_fn
BIO_get_callback(BIO *b);
void
BIO_set_callback_arg(BIO *b, char *arg);
char *
BIO_get_callback_arg(const BIO *b);
long
BIO_debug_callback(BIO *bio, int oper, const char *argp, int argi, long argl, long ret);
typedef long
(*BIO_callback_fn)(BIO *b, int oper, const char *argp, int argi, long argl, long ret);

DESCRIPTION
BIO_set_callback() and BIO_get_callback() set and retrieve the BIO callback. The callback is called during most high level BIO operations. It can be used for debugging purposes to trace operations on a BIO or to modify its operation.

BIO_set_callback_arg() and BIO_get_callback_arg() set and retrieve an argument for use in the callback.

BIO_debug_callback() is a standard debugging callback which prints out information relating to each BIO operation. If the callback argument is set, it is interpreted as a BIO to send the information to, otherwise stderr is used.

BIO_callback_fn() is the type of the callback function. The meaning of each argument is described below.

The BIO the callback is attached to is passed in b.

oper is set to the operation being performed. For some operations the callback is called twice, once before and once after the actual operation. The latter case has oper or'ed with BIO_CB_RETURN.

The meaning of the arguments argp, argi and argl depends on the value of oper (i.e. the operation being performed).

ret is the return value that would be returned to the application if no callback were present. The actual value returned is the return value of the callback itself. In the case of callbacks called before the actual BIO operation, 1 is placed in ret. If the return value is not positive, it will be immediately returned to the application and the BIO operation will not be performed.

The callback should normally simply return ret when it has finished processing, unless it specifically wishes to modify the value returned to the application.

  Callback operations

BIO_free(b)
    callback(b, BIO_CB_FREE, NULL, 0L, 0L, 1L) is called before the free operation.

BIO_read(b, out, outl)
    callback(b, BIO_CB_READ, out, outl, 0L, 1L) is called before the read and callback(b, BIO_CB_READ|BIO_CB_RETURN, out, outl, 0L, ret) after.

BIO_write(b, in, inl)
    callback(b, BIO_CB_WRITE, in, inl, 0L, 1L) is called before the write and callback(b, BIO_CB_WRITE|BIO_CB_RETURN, in, inl, 0L, ret) after.

BIO_gets(b, out, outl)
    callback(b, BIO_CB_GETS, out, outl, 0L, 1L) is called before the operation and callback(b, BIO_CB_GETS|BIO_CB_RETURN, out, outl, 0L, ret) after.

BIO_puts(b, in)
    callback(b, BIO_CB_WRITE, in, 0, 0L, 1L) is called before the operation and callback(b, BIO_CB_WRITE|BIO_CB_RETURN, in, 0, 0L, ret) after.

BIO_ctrl(b, oper, larg, parg)
    callback(b, BIO_CB_CTRL, parg, oper, larg, 1L) is called before the call and callback(b, BIO_CB_CTRL|BIO_CB_RETURN, parg, oper, larg, ret) after.

EXAMPLES
The BIO_debug_callback() function is a good example. Its source is in the file crypto/bio/bio_cb.c.

SEE ALSO
BIO_new(3)