NAME

tracefs_snapshot_snap, tracefs_snapshot_clear, tracefs_snapshot_free - API to create, clear and read snapshots

SYNOPSIS

#include <tracefs.h>
int tracefs_snapshot_snap(struct tracefs_instance *instance);
int tracefs_snapshot_clear(struct tracefs_instance *instance);
int tracefs_snapshot_free(struct tracefs_instance *instance);

DESCRIPTION

The Linux kernel tracing provides a "snapshot" feature. The kernel has two ring buffers. One that is written to by the tracing system and another one that is the "snapshot" buffer. When a snapshot happens, the two buffers are swapped, and the current snapshot buffer becomes the one being written to, and the buffer that was being written to becomes the saved snapshot.

Note, the snapshot buffer is allocated the first time it is taken, so it is best to take a snapshot at the start before one is needed so that it is allocated and a snapshot is ready, then the snapshot will happen immediately.

The tracefs_snapshot_snap() will allocate (if not already allocated) the snapshot buffer and then take a "snapshot" (swap the main buffer that’s being written to with the allocated snapshot buffer). It will do this to the given instance buffer or the top instance if instance is NULL.

The tracefs_snapshot_clear() will erase the content of the snapshot buffer for the given instance or the top level instance if instance is NULL.

The tracefs_snapshot_free() will free the allocated snapshot for the given instance or the top level instance if instance is NULL. That is, if another call to tracefs_snapshot_snap() is done after this, then it will need to allocate the snapshot buffer again before it can take a snapshot. This function should be used to free up the kernel memory used by hte snapshot buffer when no longer in use.

RETURN VALUE

The tracefs_snapshot_snap(), tracefs_snapshot_clear() and the tracefs_snapshot_free() all return 0 on success and -1 on failure.

EXAMPLE

#include <stdlib.h>
#include <stdio.h>
#include <tracefs.h>
static int callback(struct tep_event *event, struct tep_record *record, int cpu, void *data)
{
        static struct trace_seq seq;
        struct tep_handle *tep = event->tep;
        if (!seq.buffer)
                trace_seq_init(&seq);
        trace_seq_reset(&seq);
        tep_print_event(tep, &seq, record, "[%03d] %s-%d %6.1000d\t%s: %s\n",
                                TEP_PRINT_CPU,
                                TEP_PRINT_COMM,
                                TEP_PRINT_PID,
                                TEP_PRINT_TIME,
                                TEP_PRINT_NAME,
                                TEP_PRINT_INFO);
        trace_seq_do_printf(&seq);
        return 0;
}
int main (int argc, char **argv)
{
        struct tracefs_instance *instance;
        struct tep_handle *tep;
        char *line = NULL;
        size_t len = 0;
        int ret;
        instance = tracefs_instance_create("my_snapshots");
        if (!instance) {
                perror("creating instance");
                exit(-1);
        }
        tep = tracefs_local_events(NULL);
        if (!tep) {
                perror("reading event formats");
                goto out;
        }
        /* Make sure the snapshot buffer is allocated */
        ret = tracefs_snapshot_snap(instance);
        if (ret < 0)
                goto out;
        ret = tracefs_event_enable(instance, "sched", NULL);
        if (ret < 0) {
                perror("enabling event");
                goto out;
        }
        for (;;) {
                printf("Hit enter without text to take snapshot!\n");
                printf("Enter any text to display the snapshot\n");
                printf("Enter 'quit' to exit\n");
                getline(&line, &len, stdin);
                ret = tracefs_snapshot_snap(instance);
                if (ret < 0) {
                        perror("taking snapshot");
                        goto out;
                }
                if (!line)
                        break;
                if (strlen(line) < 2)
                        continue;
                if (strncmp(line, "quit", 4) == 0)
                        break;
                tracefs_iterate_snapshot_events(tep, instance, NULL, 0, callback, NULL);
        }
        free(line);
        tracefs_instance_clear(instance);
 out:
        tracefs_snapshot_free(instance);
        tracefs_event_disable(instance, "sched", NULL);
        tracefs_instance_destroy(instance);
        tracefs_instance_free(instance);
        exit(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.