# libtraceevent(3) Manual Page

## 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:

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

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():

## 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>, author of libtraceevent.
    Tzvetomir Stoyanov <tz.stoyanov@gmail.com>, author of this man page.

## REPORTING BUGS

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

## LICENSE

libtraceevent is Free Software licensed under the GNU LGPL 2.1

## RESOURCES

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