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BIO_SET_CALLBACK(3) Library Functions Manual BIO_SET_CALLBACK(3)

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

#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);

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).

When oper does not include BIO_CB_RETURN, i.e. when the callback is invoked before an operation, the value passed into the callback via ret is always 1. In this case, if the callback returns a negative value, the library aborts the requested operation and instead returns the negative return value from the callback to the application. If the callback returns a non-negative value, that return value is ignored by the library, and the operation is performed normally.

When oper includes BIO_CB_RETURN, i.e. when the callback is invoked after an operation, the value passed into the callback via ret is the return value that the operation would return to the application if no callback were present. When a callback is present, the operation only passes this value to the callback and instead of it returns the return value of the callback to the application.

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

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_PUTS, in, 0, 0L, 1L) is called before the operation and callback(b, BIO_CB_PUTS|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.

BIO_get_callback() returns a pointer to the function cb previously installed with BIO_set_callback(), or NULL if no callback was installed.

BIO_get_callback_arg() returns a pointer to the arg previously set with BIO_set_callback_arg(), or NULL if no such argument was set.

BIO_debug_callback() returns ret if the bit BIO_CB_RETURN is set in cmd, or 1 otherwise.

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

BIO_new(3)

BIO_set_callback(), BIO_get_callback(), BIO_set_callback_arg(), and BIO_debug_callback() first appeared in SSLeay 0.6.0. BIO_get_callback_arg() first appeared in SSLeay 0.8.0. All these functions have been available since OpenBSD 2.4.
March 29, 2018 OpenBSD-current