libtraceevent(3)

# libtraceevent(3) Manual Page

## NAME

kbuffer_alloc, kbuffer_dup, kbuffer_free, kbuffer_load_subbuffer, kbuffer_subbuffer, kbuffer_refresh, kbuffer_subbuffer_size, kbuffer_start_of_data - Creating of kbuffer element to parse the Linux kernel tracing ring buffer

## SYNOPSIS

    #include <kbuffer.h>

    enum kbuffer_endian {
            KBUFFER_ENDIAN_BIG,
            KBUFFER_ENDIAN_LITTLE,
            KBUFFER_ENDIAN_SAME_AS_HOST,
    };

    enum kbuffer_long_size {
            KBUFFER_LSIZE_4,
            KBUFFER_LSIZE_8,
            KBUFFER_LSIZE_SAME_AS_HOST,
    };

    struct kbuffer;
    struct tep_handle;

    struct kbuffer *kbuffer_alloc(enum kbuffer_long_size size, enum kbuffer_endian endian);
    struct kbuffer *kbuffer_dup(struct kbuffer *kbuf);
    void kbuffer_free(struct kbuffer *kbuf);
    int kbuffer_load_subbuffer(struct kbuffer *kbuf, void *subbuffer);
    int kbuffer_subbuffer_size(struct kbuffer *kbuf);
    int kbuffer_refresh(struct kbuffer *kbuf);
    int kbuffer_start_of_data(struct kbuffer *kbuf);
    void *kbuffer_subbuffer(struct kbuffer *_kbuf);

## DESCRIPTION

These functions create a kbuffer handle that can be used to parse the raw sub buffers of the Linux kernel tracing ring buffer. The ring buffer is found in the tracefs directory, and can be retrieved by tracefs_instance_get_file(3) at per_cpu/cpuX/trace_pipe_raw where X is replaced by the per CPU number of the specified ring buffer. The ring buffer inside the kernel is split up per CPU, such that the raw ring buffer must be retrieved per CPU as well.

The kbuffer_alloc() will create a descriptor that can be used to manage a sub buffer read by the ring buffer. The size parameter denotes what the word size is for the given buffer (note, this works from reading raw data from machines other than the machine that is calling this function). The endian denotes the endian for the machine.

If endian is set to KBUFFER_ENDIAN_SAME_AS_HOST the endian will be set to the same as the host endianess, which is useful when the application is reading the ring buffer data directly from the same machine it is running on.

If size is set to KBUFFER_LSIZE_SAME_AS_HOST, if the word size is 8, it will set the kbuffer descriptor to long size of 8. But if the size is 4, then it will then perform a uname(2) call, and if the machine field has the string "64" in it, it will be set to 8 byte long size and not 4 byte. This is because the ring buffer long size is dependent on the kernel and not user space.

The kbuffer_dup() function will duplicate an existing kbuffer structure with an allocated new one. It will have all the properties of the passed in kbuf, including pointing to the same subbuffer that was loaded in the kbuf. It must be freed with kbuffer_free().

The kbuffer_free() function will free the resources created by kbuffer_alloc().

The kbuffer_load_subbuffer() will take a subbuffer which is a raw data blob from the tracefs trace_pipe_raw file. The Linux tracing ring buffer is broken up into sub buffers. Each sub buffer is as stand alone data segment that has all the information to split out the individual events and time stamps. This sub buffer is what kbuffer uses to walk the events.

The kbuffer_subbuffer_size() returns the location of the end of the last event on the sub-buffer. It does not return the size of the sub-buffer itself.

The kbuffer_refresh() is to be used if more writes were done on the loaded kbuffer where the size of the kbuffer needs to be refreshed to be able to read the new events that were written since the last kbuffer_load_subbuffer() was called on it.

Note, no memory barriers are implemented with this function and any synchronization with the writer is the responsibility of the application.

The kbuffer_start_of_data() function returns the offset of where the actual data load of the sub-buffer begins.

The kbuffer_subbuffer() function returns the pointer to the currently loaded subbuffer. That is, the last subbuffer that was loaded by kbuffer_load_subbuffer(). If no subbuffer was loaded NULL is returned.

## RETURN VALUE

kbuffer_alloc() returns an allocated kbuffer descriptor or NULL on error. The returned descriptor must be freed with kbuffer_free()

kbuffer_load_subbuffer() returns 0 on success and -1 on error.

kbuffer_subbuffer_size() returns the index on the subbuffer where the end of the last event is located.

kbuffer_start_of_data() returns the offset of where the data begins on the sub-buffer loaded in kbuf.

kbuffer_subbuffer() returns the last loaded subbuffer to kbuf that was loaded by kbuffer_load_subbuffer() or NULL if none was loaded.

kbuffer_refresh() returns 0 on success and -1 if kbuf is NULL or it does not have a subbuffer loaded via kbuffer_load_subbuffer().

## EXAMPLE

## FILES

    event-parse.h
            Header file to include in order to have access to the library APIs.
    -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.

## 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/