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