# libtracefs(3) Manual Page

## NAME

tracefs_cpu_read_buf, tracefs_cpu_buffered_read_buf, tracefs_cpu_flush_buf - Reading trace_pipe_raw data returning a kbuffer

## SYNOPSIS

    #include <tracefs.h>

    struct kbuffer *tracefs_cpu_read_buf(struct tracefs_cpu *tcpu, bool nonblock);
    struct kbuffer *tracefs_cpu_buffered_read_buf(struct tracefs_cpu *tcpu, bool nonblock);
    struct kbuffer *tracefs_cpu_flush_buf(struct tracefs_cpu *tcpu);

## DESCRIPTION

This set of APIs can be used to read the raw data from the trace_pipe_raw files in the tracefs file system and return a kbuffer structure to read it with.

The tracefs_cpu_read_buf() reads the trace_pipe_raw files associated to tcpu and returns a kbuffer structure that can be used to iterate the events. If nonblock is set, and there’s no data available, it will return immediately. Otherwise depending on how tcpu was opened, it will block. If tcpu was opened with nonblock set, then this nonblock will make no difference.

The tracefs_cpu_buffered_read_buf() is basically the same as tracefs_cpu_read_buf() except that it uses a pipe through splice to buffer reads. This will batch reads keeping the reading from the ring buffer less intrusive to the system, as just reading all the time can cause quite a disturbance. Note, one difference between this and tracefs_cpu_read() is that it will read only in sub buffer pages. If the ring buffer has not filled a page, then it will not return anything, even with nonblock set. Calls to tracefs_cpu_flush_buf() or tracefs_cpu_flush() should be done to read the rest of the file at the end of the trace.

The tracefs_cpu_flush_buf() reads the trace_pipe_raw file associated by the tcpu and puts it into buffer, which must be the size of the sub buffer which is retrieved. This should be called at the end of tracing to get the rest of the data. This call will convert the file descriptor of trace_pipe_raw into non-blocking mode.

## RETURN VALUE

The functions tracefs_cpu_read_buf(), tracefs_cpu_buffered_read_buf()* and tracefs_cpu_flush() returns a kbuffer descriptor that can be iterated over to find the events. Note, this descriptor is part of the tracefs_cpu structure and should not be freed. It will be freed. It returns NULL on error or if nonblock is set and there are no events available. In the case of no events, errno will be set with EAGAIN.

## EXAMPLE

    #include <stdlib.h>
    #include <ctype.h>
    #include <tracefs.h>

    static void read_page(struct tep_handle *tep, struct kbuffer *kbuf)
    {
            static struct trace_seq seq;
            struct tep_record record;

            if (seq.buffer)
                    trace_seq_reset(&seq);
            else
                    trace_seq_init(&seq);

            while ((record.data = kbuffer_read_event(kbuf, &record.ts))) {
                    record.size = kbuffer_event_size(kbuf);
                    kbuffer_next_event(kbuf, NULL);
                    tep_print_event(tep, &seq, &record,
                                    "%s-%d %9d\t%s: %s\n",
                                    TEP_PRINT_COMM,
                                    TEP_PRINT_PID,
                                    TEP_PRINT_TIME,
                                    TEP_PRINT_NAME,
                                    TEP_PRINT_INFO);
                    trace_seq_do_printf(&seq);
                    trace_seq_reset(&seq);
            }
    }

    int main (int argc, char **argv)
    {
            struct tracefs_cpu *tcpu;
            struct tep_handle *tep;
            struct kbuffer *kbuf;
            int cpu;

            if (argc < 2 || !isdigit(argv[1][0])) {
                    printf("usage: %s cpu\n\n", argv[0]);
                    exit(-1);
            }

            cpu = atoi(argv[1]);

            tep = tracefs_local_events(NULL);
            if (!tep) {
                    perror("Reading trace event formats");
                    exit(-1);
            }

            tcpu = tracefs_cpu_open(NULL, cpu, 0);
            if (!tcpu) {
                    perror("Open CPU 0 file");
                    exit(-1);
            }

            while ((kbuf = tracefs_cpu_buffered_read_buf(tcpu, true))) {
                    read_page(tep, kbuf);
            }

            kbuf = tracefs_cpu_flush_buf(tcpu);
            if (kbuf)
                    read_page(tep, kbuf);

            tracefs_cpu_close(tcpu);
            tep_free(tep);

            return 0;
    }

## FILES

    tracefs.h
            Header file to include in order to have access to the library APIs.
    -ltracefs
            Linker switch to add when building a program that uses the library.

## SEE ALSO

tracefs_cpu_open(3) tracefs_cpu_close(3) tracefs_cpu_read(3) tracefs_cpu_buffered_read(3) tracefs_cpu_flush(3) libtracefs(3), libtraceevent(3), trace-cmd(1)

## AUTHOR

    Steven Rostedt <rostedt@goodmis.org>

## REPORTING BUGS

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

## LICENSE

libtracefs is Free Software licensed under the GNU LGPL 2.1

## RESOURCES

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

## COPYING

Copyright (C) 2022 Google, Inc. Free use of this software is granted under the terms of the GNU Public License (GPL).