tracefs_instance_get_subbuf_size - Helper functions for working with ring buffer sub buffers.

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);

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);
}

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Updated 2026-06-01 - jenkler.se | uex.se