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.\" ========================================================================
.\"
.IX Title "BIO_set_callback 3"
.TH BIO_set_callback 3 "2014-07-11" "LibreSSL" "LibreSSL"
.\" For nroff, turn off justification.  Always turn off hyphenation; it makes
.\" way too many mistakes in technical documents.
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.nh
.SH "NAME"
BIO_set_callback, BIO_get_callback, BIO_set_callback_arg, BIO_get_callback_arg,
BIO_debug_callback \- BIO callback functions
.SH "SYNOPSIS"
.IX Header "SYNOPSIS"
.Vb 1
\& #include <openssl/bio.h>
\&
\& #define BIO_set_callback(b,cb)         ((b)\->callback=(cb))
\& #define BIO_get_callback(b)            ((b)\->callback)
\& #define BIO_set_callback_arg(b,arg)    ((b)\->cb_arg=(char *)(arg))
\& #define BIO_get_callback_arg(b)                ((b)\->cb_arg)
\&
\& long BIO_debug_callback(BIO *bio,int cmd,const char *argp,int argi,
\&        long argl,long ret);
\&
\& typedef long (*callback)(BIO *b, int oper, const char *argp,
\&                        int argi, long argl, long retvalue);
.Ve
.SH "DESCRIPTION"
.IX Header "DESCRIPTION"
\&\fIBIO_set_callback()\fR and \fIBIO_get_callback()\fR set and retrieve the \s-1BIO\s0 callback,
they are both macros. The callback is called during most high level \s-1BIO\s0
operations. It can be used for debugging purposes to trace operations on
a \s-1BIO\s0 or to modify its operation.
.PP
\&\fIBIO_set_callback_arg()\fR and \fIBIO_get_callback_arg()\fR are macros which can be
used to set and retrieve an argument for use in the callback.
.PP
\&\fIBIO_debug_callback()\fR is a standard debugging callback which prints
out information relating to each \s-1BIO\s0 operation. If the callback
argument is set if is interpreted as a \s-1BIO\s0 to send the information
to, otherwise stderr is used.
.PP
\&\fIcallback()\fR is the callback function itself. The meaning of each
argument is described below.
.PP
The \s-1BIO\s0 the callback is attached to is passed in \fBb\fR.
.PP
\&\fBoper\fR 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 \fBoper\fR or'ed with \s-1BIO_CB_RETURN.\s0
.PP
The meaning of the arguments \fBargp\fR, \fBargi\fR and \fBargl\fR depends on
the value of \fBoper\fR, that is the operation being performed.
.PP
\&\fBretvalue\fR 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 \s-1BIO\s0 operation 1 is placed in retvalue, if
the return value is not positive it will be immediately returned to
the application and the \s-1BIO\s0 operation will not be performed.
.PP
The callback should normally simply return \fBretvalue\fR when it has
finished processing, unless if specifically wishes to modify the
value returned to the application.
.SH "CALLBACK OPERATIONS"
.IX Header "CALLBACK OPERATIONS"
.IP "\fBBIO_free(b)\fR" 4
.IX Item "BIO_free(b)"
callback(b, \s-1BIO_CB_FREE, NULL, 0L, 0L, 1L\s0) is called before the
free operation.
.IP "\fBBIO_read(b, out, outl)\fR" 4
.IX Item "BIO_read(b, out, outl)"
callback(b, \s-1BIO_CB_READ,\s0 out, outl, 0L, 1L) is called before
the read and callback(b, BIO_CB_READ|BIO_CB_RETURN, out, outl, 0L, retvalue)
after.
.IP "\fBBIO_write(b, in, inl)\fR" 4
.IX Item "BIO_write(b, in, inl)"
callback(b, \s-1BIO_CB_WRITE,\s0 in, inl, 0L, 1L) is called before
the write and callback(b, BIO_CB_WRITE|BIO_CB_RETURN, in, inl, 0L, retvalue)
after.
.IP "\fBBIO_gets(b, out, outl)\fR" 4
.IX Item "BIO_gets(b, out, outl)"
callback(b, \s-1BIO_CB_GETS,\s0 out, outl, 0L, 1L) is called before the operation and
callback(b, BIO_CB_GETS|BIO_CB_RETURN, out, outl, 0L, retvalue) after.
.IP "\fBBIO_puts(b, in)\fR" 4
.IX Item "BIO_puts(b, in)"
callback(b, \s-1BIO_CB_WRITE,\s0 in, 0, 0L, 1L) is called before
the operation and callback(b, BIO_CB_WRITE|BIO_CB_RETURN, in, 0, 0L, retvalue)
after.
.IP "\fBBIO_ctrl(\s-1BIO\s0 *b, int cmd, long larg, void *parg)\fR" 4
.IX Item "BIO_ctrl(BIO *b, int cmd, long larg, void *parg)"
callback(b,BIO_CB_CTRL,parg,cmd,larg,1L) is called before the call and
callback(b,BIO_CB_CTRL|BIO_CB_RETURN,parg,cmd, larg,ret) after.
.SH "EXAMPLE"
.IX Header "EXAMPLE"
The \fIBIO_debug_callback()\fR function is a good example, its source is
in crypto/bio/bio_cb.c