[#BIO_callback_fn]
= BIO_callback_fn
:mrdocs:

Legacy BIO callback invoked immediately before and after I/O operations (deprecated).

== Synopsis

Declared in `<openssl/bio.h>`

[source,cpp,subs="verbatim,replacements,macros,-callouts"]
----
typedef long(* BIO_callback_fn)(xref:BIO.adoc[BIO]*, int, char const*, int, long, long);
----

== Return Value

Post‐call return value (usually `ret` unchanged).

== Parameters

[cols="1,4"]
|===
| Name| Description
| *b*
| BIO performing the operation.
| *oper*
| BIO_CB_* operation code OR'd with BIO_CB_RETURN after the call.
| *argp*
| Pointer argument (buffer or ctrl pointer), or NULL.
| *argi*
| Integer argument for ctrl‐style operations.
| *argl*
| Long argument for ctrl‐style operations.
| *ret*
| Return value from the underlying operation (post‐call only).
|===


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