% PT_QRY_ALLOC_DECODER(3)

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# NAME

pt_qry_alloc_decoder, pt_qry_free_decoder - allocate/free an Intel(R) Processor
Trace query decoder


# SYNOPSIS

| **\#include `<intel-pt.h>`**
|
| **struct pt_query_decoder \***
| **pt_qry_alloc_decoder(const struct pt_config \**config*);**
|
| **void pt_qry_free_decoder(struct pt_query_decoder \**decoder*);**

Link with *-lipt*.


# DESCRIPTION

A query decoder decodes raw Intel Processor Trace (Intel PT) and provides
functions for querying the decoder about:

  - whether the next conditional branch was taken (or not taken)
  - the destination of the next indirect branch

This information can be used to reconstruct the execution flow of the traced
code.  As long as the flow is clear, follow the flow.  If the flow cannot be
determined by examining the current instruction, ask the query decoder.

In addition, the query decoder indicates asynchronous events via the return
value of its query functions and provides an additional function to query for
such asynchronous events.  See **pt_qry_cond_branch**(3),
**pt_qry_indirect_branch**(3), and **pt_qry_event**(3).

**pt_qry_alloc_decoder**() allocates a new query decoder and returns a pointer
to it.  The *config* argument points to a *pt_config* object.  See
**pt_config**(3).  The *config* argument will not be referenced by the returned
decoder but the trace buffer defined by the *config* argument's *begin* and
*end* fields will.

The returned query decoder needs to be synchronized onto the trace stream
before it can be used.  To synchronize the query decoder, use
**pt_qry_sync_forward**(3), **pt_qry_sync_backward**(3), or
**pt_qry_sync_set**(3).

**pt_qry_free_decoder**() frees the Intel PT query decoder pointed to by
*decoder*.  The *decoder* argument must be NULL or point to a decoder that has
been allocated by a call to **pt_qry_alloc_decoder**().


# RETURN VALUE

**pt_qry_alloc_decoder**() returns a pointer to a *pt_query_decoder* object on
success or NULL in case of an error.


# EXAMPLE

~~~{.c}
int foo(const struct pt_config *config) {
	struct pt_query_decoder *decoder;
	errcode;

	decoder = pt_qry_alloc_decoder(config);
	if (!decoder)
		return pte_nomem;

	errcode = bar(decoder);

	pt_qry_free_decoder(decoder);
	return errcode;
}
~~~


# SEE ALSO

**pt_config**(3), **pt_qry_sync_forward**(3), **pt_qry_sync_backward**(3),
**pt_qry_sync_set**(3), **pt_qry_get_offset**(3), **pt_qry_get_sync_offset**(3),
**pt_qry_get_config**(3), **pt_qry_cond_branch**(3),
**pt_qry_indirect_branch**(3), **pt_qry_event**(3), **pt_qry_time**(3),
**pt_qry_core_bus_ratio**(3)