# libtracefs(3) Manual Page

## NAME

tracefs_trace_pipe_stream, tracefs_trace_pipe_print, tracefs_trace_pipe_stop - redirect the stream of trace data to an output or stdout.

## SYNOPSIS

    #include <tracefs.h>

    ssize_t tracefs_trace_pipe_stream(int fd, struct tracefs_instance *instance, int flags);
    ssize_t tracefs_trace_pipe_print(struct tracefs_instance *instance, int flags);
    void tracefs_trace_pipe_stop(struct tracefs_instance *instance);

## DESCRIPTION

If NULL is passed as instance, the top trace instance is used.

The reading of the trace_pipe file can be stopped by calling tracefs_trace_pipe_stop() which could be placed in a signal handler in case the application wants to stop the reading, for example, with the user pressing Ctrl-C.

The tracefs_trace_pipe_stream() function redirects the stream of trace data to an output file. The "splice" system call is used to moves the data without copying between kernel address space and user address space. The fd is the file descriptor of the output file and flags is a bit mask of flags to be passed to the open system call of the trace_pipe file (see open(2)). If flags contain O_NONBLOCK, then that is also passed to the splice calls that may read the file to the output stream file descriptor. Note, O_RDONLY is or’d to the flags and only O_NONBLOCK is useful for this parameter.

The tracefs_trace_pipe_print() function is similar to tracefs_trace_pipe_stream(), but the stream of trace data is redirected to stdout.

## RETURN VALUE

The tracefs_trace_pipe_stream(), and tracefs_trace_pipe_print() functions return the number of bytes transfered if the operation is successful, or -1 in case of an error.

## EXAMPLE

    #include <stdio.h>
    #include <stdlib.h>
    #include <unistd.h>
    #include <signal.h>
    #include <fcntl.h>

    #include <tracefs.h>

    void stop(int sig)
    {
            tracefs_trace_pipe_stop(NULL);
    }

    int main(int argc, char **argv)
    {
            mode_t mode = S_IRUSR | S_IWUSR | S_IRGRP | S_IROTH;
            const char *filename;
            int fd;
            int ret;

            if (argc < 2) {
                    fprintf(stderr, "usage: %s output_file\n", argv[0]);
                    exit(-1);
            }
            filename = argv[1];
            fd = creat(filename, mode);
            if (fd < 0) {
                    perror(filename);
                    exit(-1);
            }
            signal(SIGINT, stop);
            ret = tracefs_trace_pipe_stream(fd, NULL, SPLICE_F_NONBLOCK);
            close(fd);

            return ret;
    }

## 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

libtracefs(3), libtraceevent(3), trace-cmd(1), Documentation/trace/ftrace.rst from the Linux kernel tree

## AUTHOR

    Steven Rostedt <rostedt@goodmis.org>
    Tzvetomir Stoyanov <tz.stoyanov@gmail.com>

## 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) 2021 VMware, Inc. Free use of this software is granted under the terms of the GNU Public License (GPL).