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NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | EXAMPLE | FILES | SEE ALSO | AUTHOR | REPORTING BUGS | LICENSE | RESOURCES | COPYING | NOTES | COLOPHON |
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LIBTRACEFS(3) libtracefs Manual LIBTRACEFS(3)
tracefs_instance_get_subbuf_size, tracefs_instance_set_subbuf_size
- Helper functions for working with ring buffer sub buffers.
#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);
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.
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.
#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);
}
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.
libtracefs(3), libtraceevent(3), trace-cmd(1)
Steven Rostedt <rostedt@goodmis.org[1]>
Tzvetomir Stoyanov <tz.stoyanov@gmail.com[2]>
Report bugs to <linux-trace-devel@vger.kernel.org[3]>
libtracefs is Free Software licensed under the GNU LGPL 2.1
https://git.kernel.org/pub/scm/libs/libtrace/libtracefs.git/
Copyright (C) 2020 VMware, Inc. Free use of this software is
granted under the terms of the GNU Public License (GPL).
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
This page is part of the libtracefs (Linux kernel trace file
system library) project. Information about the project can be
found at ⟨https://www.trace-cmd.org/⟩. If you have a bug report
for this manual page, see ⟨https://www.trace-cmd.org/⟩. This page
was obtained from the project's upstream Git repository
⟨https://git.kernel.org/pub/scm/libs/libtrace/libtracefs.git⟩ on
2025-08-11. (At that time, the date of the most recent commit
that was found in the repository was 2025-06-02.) If you discover
any rendering problems in this HTML version of the page, or you
believe there is a better or more up-to-date source for the page,
or you have corrections or improvements to the information in this
COLOPHON (which is not part of the original manual page), send a
mail to man-pages@man7.org
libtracefs 1.8.1 01/02/2025 LIBTRACEFS(3)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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