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NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | EXAMPLE | FILES | SEE ALSO | AUTHOR | REPORTING BUGS | LICENSE | RESOURCES | NOTES | COLOPHON |
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LIBTRACEEVENT(3) libtraceevent Manual LIBTRACEEVENT(3)
tep_register_print_function, tep_unregister_print_function -
Registers / Unregisters a helper function.
#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);
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.
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.
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 */
}
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.
libtraceevent(3), trace-cmd(1)
Steven Rostedt <rostedt@goodmis.org[1]>, author of libtraceevent.
Tzvetomir Stoyanov <tz.stoyanov@gmail.com[2]>, author of this man page.
Report bugs to <linux-trace-devel@vger.kernel.org[3]>
libtraceevent is Free Software licensed under the GNU LGPL 2.1
https://git.kernel.org/pub/scm/libs/libtrace/libtraceevent.git/
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
This page is part of the libtraceevent (Linux kernel trace event
library) project. Information about the project can be found at
⟨https://www.trace-cmd.org/⟩. If you have a bug report for this
manual page, see ⟨https://www.trace-cmd.org/⟩. This page was
obtained from the project's upstream Git repository
⟨https://git.kernel.org/pub/scm/libs/libtrace/libtraceevent.git⟩
on 2025-08-11. (At that time, the date of the most recent commit
that was found in the repository was 2025-05-30.) If you discover
any rendering problems in this HTML version of the page, or you
believe there is a better or more up-to-date source for the page,
or you have corrections or improvements to the information in this
COLOPHON (which is not part of the original manual page), send a
mail to man-pages@man7.org
libtraceevent 1.7.3 09/24/2023 LIBTRACEEVENT(3)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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