libtracefs(3)

# libtracefs(3) Manual Page

## NAME

tracefs_cpu_open, tracefs_cpu_close, tracefs_cpu_alloc_fd, tracefs_cpu_free_fd, tracefs_cpu_snapshot_open - Opening trace_pipe_raw data for reading

## SYNOPSIS

    #include <tracefs.h>

    struct tracefs_cpu *tracefs_cpu_open(struct tracefs_instance *instance,
                                         int cpu, bool nonblock);
    void tracefs_cpu_close(struct tracefs_cpu *tcpu);

    struct tracefs_cpu *tracefs_cpu_alloc_fd(int fd, int subbuf_size, bool nonblock);
    void tracefs_cpu_free_fd(struct tracefs_cpu *tcpu);

    struct tracefs_cpu *tracefs_cpu_snapshot_open(struct tracefs_instance *instance,
                                                 int cpu, bool nonblock);

## DESCRIPTION

This set of APIs can be used to open the raw data from the trace_pipe_raw files in the tracefs file system in oder to read them with the tracefs_cpu_read(3) functions.

The tracefs_cpu_open() creates a descriptor that can read the tracefs trace_pipe_raw file for a given cpu in a given instance. If instance is NULL than the toplevel trace_pipe_raw file is used.

The tracefs_cpu_close() closes all the file descriptors associated to the trace_pipe_raw opened by tracefs_cpu_open().

The tracefs_cpu_alloc_fd() will create a tracefs_cpu descriptor from an existing file descriptor fd. This is useful to use when connecting to a socket or pipe where the other end is feeding raw tracing data in the same format as the trace_pipe_raw file would (like in guest to host tracing). The caller is responsible for determining the subbuf_size that will be used to break up the sub-buffers being read by the file descriptor. The nonblock is treated the same as the same parameter in tracefs_cpu_open().

The tracefs_cpu_free_fd() is used to free the descriptor returned by tracefs_cpu_alloc_fd(). It does all the clean up that tracefs_cpu_close() performs, and that could also be used to free up the descriptor created by tracefs_cpu_alloc_fd() but will also close the file descriptor passed in. Note that tracefs_cpu_free_fd() should not be used on the descriptor returned by tracefs_cpu_open() as it will not close the file descriptor created by it.

The tracefs_cpu_snapshot_open() is similar to tracefs_cpu_open() except that it opens the snapshot buffer (see tracefs_snapshot_snap(3)). The snapshot buffer does not have a writer to it, it is only created by a snapshot action that swaps the current ring buffer with the snapshot buffer. The nonblock, when false, acts a little differently here too. Reads are not affected by the "buffer_percent" file. If the snapshot buffer is empty, it will block until a new snapshot happens.

## RETURN VALUE

The tracefs_cpu_open() and *tracefs_cpu_snapshot_open() both return a struct tracefs_cpu descriptor that can be used by the other functions or NULL on error.

The tracefs_cpu_alloc_fd() returns a struct tracefs_cpu descriptor that can be used by the tracefs_cpu_read(3) related functions, where the descriptor will be reading the passed in fd file descriptor.

## EXAMPLE

See tracefs_cpu_read(3) for an example.

## 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)

## 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).