---
name: "tep_unregister_print_function"
section: "3"
platform: "linux"
package: "libtraceevent-doc"
version: "1:1.8.4-2"
release: "Debian 13 (Trixie)"
platforms: ["linux"]
url: "https://new.carta.tech/tep_register_print_function"
---

# tep\_unregister\_print\_function(3)

Registers / Unregisters a helper function.

Part of [tep\_register\_print\_function(3)](/tep_register_print_function/3.md).

## 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)](/libtraceevent/3.md), [**trace-cmd**(1)](/trace-cmd/1.md)

## 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