[#BIO_set_callback_arg]
= BIO&lowbar;set&lowbar;callback&lowbar;arg
:mrdocs:

Store an opaque pointer passed to BIO callbacks as the callback argument&period;

== Synopsis

Declared in `&lt;openssl&sol;bio&period;h&gt;`

[source,cpp,subs="verbatim,replacements,macros,-callouts"]
----
void
BIO&lowbar;set&lowbar;callback&lowbar;arg(
    xref:BIO.adoc[BIO]* b,
    char* arg);
----

== Parameters

[cols="1,4"]
|===
| Name| Description
| *b*
| BIO whose callback argument is set&period;
| *arg*
| Opaque pointer (often a BIO&ast; used for debug logging), or NULL&period;
|===


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