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LIBTRACEEVENT(3) libtraceevent Manual LIBTRACEEVENT(3)

NAME

tep_register_print_function, tep_unregister_print_function - Registers / Unregisters a helper function.

SYNOPSIS

#include <event-parse.h>
enum tep_func_arg_type {

TEP_FUNC_ARG_VOID,
TEP_FUNC_ARG_INT,
TEP_FUNC_ARG_LONG,
TEP_FUNC_ARG_STRING,
TEP_FUNC_ARG_PTR,
TEP_FUNC_ARG_MAX_TYPES }; typedef unsigned long long (*tep_func_handler)(struct trace_seq *s, unsigned long long *args); int tep_register_print_function(struct tep_handle *tep, tep_func_handler func, enum tep_func_arg_type ret_type, char *name, ...); int tep_unregister_print_function(struct tep_handle *tep, tep_func_handler func, char *name);

DESCRIPTION

Some events may have helper functions in the print format arguments. This allows a plugin to dynamically create a way to process one of these functions.

The tep_register_print_function() registers such helper function. The tep argument is the trace event parser context. The func argument is a pointer to the helper function. The ret_type argument is the return type of the helper function, value from the tep_func_arg_type enum. The name is the name of the helper function, as seen in the print format arguments. The ... is a variable list of tep_func_arg_type enums, the func function arguments. This list must end with TEP_FUNC_ARG_VOID. See EXAMPLE section.

The tep_unregister_print_function() unregisters a helper function, previously registered with tep_register_print_function(). The tep argument is the trace event parser context. The func and name arguments are the same, used when the helper function was registered.

The tep_func_handler is the type of the helper function. The s argument is the trace sequence, it can be used to create a custom string. The args is a list of arguments, defined when the helper function was registered.

RETURN VALUE

The tep_register_print_function() function returns 0 in case of success. In case of an error, TEP_ERRNO_... code is returned.

The tep_unregister_print_function() returns 0 in case of success, or -1 in case of an error.

EXAMPLE

Some events have internal functions calls, that appear in the print format output. For example "tracefs/events/i915/g4x_wm/format" has:

print fmt: "pipe %c, frame=%u, scanline=%u, wm %d/%d/%d, sr %s/%d/%d/%d, hpll %s/%d/%d/%d, fbc %s",

((REC->pipe) + 'A'), REC->frame, REC->scanline, REC->primary,
REC->sprite, REC->cursor, yesno(REC->cxsr), REC->sr_plane,
REC->sr_cursor, REC->sr_fbc, yesno(REC->hpll), REC->hpll_plane,
REC->hpll_cursor, REC->hpll_fbc, yesno(REC->fbc)

Notice the call to function yesno() in the print arguments. In the kernel context, this function has the following implementation:

static const char *yesno(int x)
{

static const char *yes = "yes";
static const char *no = "no";
return x ? yes : no; }

The user space event parser has no idea how to handle this yesno() function. The tep_register_print_function() API can be used to register a user space helper function, mapped to the kernel’s yesno():

#include <event-parse.h>
#include <trace-seq.h>
...
struct tep_handle *tep = tep_alloc();
...
static const char *yes_no_helper(int x)
{

return x ? "yes" : "no"; } ...
if ( tep_register_print_function(tep,
yes_no_helper,
TEP_FUNC_ARG_STRING,
"yesno",
TEP_FUNC_ARG_INT,
TEP_FUNC_ARG_VOID) != 0) {
/* Failed to register yes_no_helper function */
} /*
Now, when the event parser encounters this yesno() function, it will know
how to handle it. */ ...
if (tep_unregister_print_function(tep, yes_no_helper, "yesno") != 0) {
/* Failed to unregister yes_no_helper function */
}

FILES

event-parse.h

Header file to include in order to have access to the library APIs. trace-seq.h
Header file to include in order to have access to trace sequences
related APIs. Trace sequences are used to allow a function to call
several other functions to create a string of data to use. -ltraceevent
Linker switch to add when building a program that uses the library.

SEE ALSO

libtraceevent(3), trace-cmd(1)

AUTHOR

Steven Rostedt <rostedt@goodmis.org[1]>, author of libtraceevent.
Tzvetomir Stoyanov <tz.stoyanov@gmail.com[2]>, author of this man page.

REPORTING BUGS

Report bugs to <linux-trace-devel@vger.kernel.org[3]>

LICENSE

libtraceevent is Free Software licensed under the GNU LGPL 2.1

RESOURCES

https://git.kernel.org/pub/scm/libs/libtrace/libtraceevent.git/

NOTES

1.
rostedt@goodmis.org
mailto:rostedt@goodmis.org
2.
tz.stoyanov@gmail.com
mailto:tz.stoyanov@gmail.com
3.
linux-trace-devel@vger.kernel.org
mailto:linux-trace-devel@vger.kernel.org
09/24/2022 libtraceevent 1.6.3