---
name: "tracefs_instance_get_subbuf_size"
section: "3"
platform: "linux"
package: "libtracefs"
version: "1.8.4-1"
release: "Arch Linux 2026-11-01"
platforms: ["linux"]
url: "https://new.carta.tech/tracefs_instance_get_subbuf_size"
---

# tracefs\_instance\_get\_subbuf\_size(3)

Helper functions for working with ring buffer sub buffers.

## NAME

tracefs\_instance\_get\_subbuf\_size, tracefs\_instance\_set\_subbuf\_size - Helper functions for working with ring buffer sub buffers.

## SYNOPSIS

```
#include <tracefs.h>
size_t tracefs_instance_get_subbuf_size(struct tracefs_instance *instance);
int tracefs_instance_set_subbuf_size(struct tracefs_instance *instance, size_t size);
```

## DESCRIPTION

Helper functions for working with the sub-buffers of the tracing ring buffer. The tracing ring buffer is broken up into **sub-buffers**. An event can not be bigger than the data section of the sub-buffer (see [**tep\_get\_sub\_buffer\_data\_size**(3)](/tep_get_sub_buffer_data_size/3.md)). By default, the ring buffer uses the architectures **page\_size** as the default size of the sub-buffer, but this can be limiting if there is a need for large events, for example, the application wants to write large strings into the trace\_marker file.

The **tracefs\_instance\_get\_subbuf\_size()** returns the current size in kilobytes fo the ring buffer sub-buffers.

The **tracefs\_instance\_set\_subbuf\_size()** will write the size in kilobytes of what the new sub-buffer size should be. Note, that this is only a hint to what the minimum sub-buffer size should be. It also does not take into account the meta-data that is used by the sub-buffer, so the size written should be no less than 16 bytes more than the maximum event size that will be used. The kernel will likely make the sub-buffer size larger than specified, as it may need to align the size for implementation purposes.

## RETURN VALUE

The **tracefs\_instance\_get\_subbuf\_size()** returns the size of the current sub-buffer for the given *instance* ring buffer or -1 on error.

The **tracefs\_instance\_set\_subbuf\_size()** will return 0 if it successfully set the *instance* ring buffer sub-buffer size in kilobytes, or -1 on error.

## EXAMPLE

```
#include <stdlib.h>
#include <tracefs.h>
#include <errno.h>
int main(int argc, char **argv)
{
        struct tep_handle *tep;
        ssize_t save_subsize;
        ssize_t subsize;
        char *trace;
        char buf[3000];
        int meta_size;
        int ret;
        int i;
        tep = tep_alloc();
        ret = tracefs_load_headers(NULL, tep);
        tep_free(tep);
        if (ret < 0) {
                perror("reading headers");
                exit(-1);
        }
        meta_size = tep_get_sub_buffer_size(tep) - tep_get_sub_buffer_data_size(tep);
        save_subsize = tracefs_instance_get_subbuf_size(NULL);
        if (save_subsize < 0) {
                printf("Changing sub-buffer size not available\n");
                exit(-1);
        }
        subsize = save_subsize * 1024;
        /* Have at least 4 writes fit on a sub-buffer */
        if (subsize - meta_size < sizeof(buf) *4 ) {
                subsize = ((sizeof(buf) * 4 + meta_size) + 1023) / 1024;
                tracefs_instance_set_subbuf_size(NULL, subsize);
        }
        for (i = 0; i < sizeof(buf) - 1; i++) {
                buf[i] = '0' + i % 10;
        }
        buf[i] = '\0';
        tracefs_instance_clear(NULL);
        for (i = 0; i < 4; i++) {
                ret = tracefs_printf(NULL, "%s\n", buf);
                if (ret < 0)
                        perror("write");
        }
        trace = tracefs_instance_file_read(NULL, "trace", NULL);
        printf("%s\n", trace);
        free(trace);
        printf("Buffer size was: %zd * 1024\n",
               tracefs_instance_get_subbuf_size(NULL));
        tracefs_instance_set_subbuf_size(NULL, save_subsize);
}
```

## 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)](/libtracefs/3.md), [**libtraceevent**(3)](/libtraceevent/3.md), [**trace-cmd**(1)](/trace-cmd/1.md)

## AUTHOR

```
Steven Rostedt <rostedt@goodmis.org[1]>
Tzvetomir Stoyanov <tz.stoyanov@gmail.com[2]>
```

## REPORTING BUGS

Report bugs to \<**linux-trace-devel@vger.kernel.org**\[3\]\>

## 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) 2020 VMware, 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\.

  tz.stoyanov@gmail.com

  mailto:tz.stoyanov@gmail.com

- 3\.

  linux-trace-devel@vger.kernel.org

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