tep_register_print_function - Man Page

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

Referenced By

The man page tep_unregister_print_function(3) is an alias of tep_register_print_function(3).

02/08/2021 libtraceevent 1.1.1 libtraceevent Manual