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BIO_set_callback_arg.3
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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\s0, \s-1NULL\s0, 0L, 0L, 1L) 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 .SH "SEE ALSO" .IX Header "SEE ALSO" \&\s-1TBA\s0