[#OSSL_trace_cb]
= OSSL_trace_cb
:mrdocs:

Application callback that receives OpenSSL trace output for a category.

== Synopsis

Declared in `<openssl/trace.h>`

[source,cpp,subs="verbatim,replacements,macros,-callouts"]
----
typedef size_t(* OSSL_trace_cb)(char const*, size_t, int, int, void*);
----

== Return Value

Number of bytes written (must be non‐zero on success), or 0 on error.

== Parameters

[cols="1,4"]
|===
| Name| Description
| *buffer*
| Trace text (may span multiple lines); never empty on WRITE.
| *count*
| Number of bytes at `buffer.`
| *category*
| OSSL_TRACE_CATEGORY_* for this channel.
| *cmd*
| One of OSSL_TRACE_CTRL_BEGIN, OSSL_TRACE_CTRL_WRITE, or OSSL_TRACE_CTRL_END.
| *data*
| User pointer supplied to OSSL_trace_set_callback().
|===


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