---
name: "tracefs_cpu_flush_buf"
section: "3"
platform: "linux"
package: "libtracefs-doc"
version: "1.8.1-1"
release: "Debian 13 (Trixie)"
platforms: ["linux"]
url: "https://new.carta.tech/tracefs_cpu_read_buf"
---

# tracefs\_cpu\_flush\_buf(3)

Reading trace\_pipe\_raw data returning a kbuffer

Part of [tracefs\_cpu\_read\_buf(3)](/tracefs_cpu_read_buf/3.md).

## 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_open/3.md) [**tracefs\_cpu\_close**(3)](/tracefs_cpu_close/3.md) [**tracefs\_cpu\_read**(3)](/tracefs_cpu_read/3.md) [**tracefs\_cpu\_buffered\_read**(3)](/tracefs_cpu_buffered_read/3.md) [**tracefs\_cpu\_flush**(3)](/tracefs_cpu_flush/3.md) [**libtracefs**(3)](/libtracefs/3.md), [**libtraceevent**(3)](/libtraceevent/3.md), [**trace-cmd**(1)](/trace-cmd/1.md)

## AUTHOR

```
Steven Rostedt <rostedt@goodmis.org[1]>
```

## REPORTING BUGS

Report bugs to \<**linux-trace-devel@vger.kernel.org**\[2\]\>

## 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).

## NOTES

- 1\.

  rostedt@goodmis.org

  mailto:rostedt@goodmis.org

- 2\.

  linux-trace-devel@vger.kernel.org

  mailto:linux-trace-devel@vger.kernel.org