[#OSSL_trace_cb]
= OSSL&lowbar;trace&lowbar;cb
:mrdocs:

Application callback that receives OpenSSL trace output for a category&period;

== Synopsis

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

[source,cpp,subs="verbatim,replacements,macros,-callouts"]
----
typedef size&lowbar;t(* OSSL&lowbar;trace&lowbar;cb)(char const*, size&lowbar;t, int, int, void*);
----

== Return Value

Number of bytes written (must be non&hyphen;zero on success), or 0 on error&period;

== Parameters

[cols="1,4"]
|===
| Name| Description
| *buffer*
| Trace text (may span multiple lines)&semi; never empty on WRITE&period;
| *count*
| Number of bytes at `buffer&period;`
| *category*
| OSSL&lowbar;TRACE&lowbar;CATEGORY&lowbar;&ast; for this channel&period;
| *cmd*
| One of OSSL&lowbar;TRACE&lowbar;CTRL&lowbar;BEGIN, OSSL&lowbar;TRACE&lowbar;CTRL&lowbar;WRITE, or OSSL&lowbar;TRACE&lowbar;CTRL&lowbar;END&period;
| *data*
| User pointer supplied to OSSL&lowbar;trace&lowbar;set&lowbar;callback()&period;
|===


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