Man page - tracefs_instance_get_subbuf_size(3)

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LIBTRACEFS

NAME
SYNOPSIS
DESCRIPTION
RETURN VALUE
EXAMPLE
FILES
SEE ALSO
AUTHOR
REPORTING BUGS
LICENSE
RESOURCES
COPYING
NOTES

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)). 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.

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

NOTES

rostedt@goodmis.org

mailto:rostedt@goodmis.org

tz.stoyanov@gmail.com

mailto:tz.stoyanov@gmail.com

linux-trace-devel@vger.kernel.org

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