'\" t
.\"     Title: libtracefs
.\"    Author: [see the "AUTHOR" section]
.\" Generator: DocBook XSL Stylesheets vsnapshot <http://docbook.sf.net/>
.\"      Date: 01/13/2024
.\"    Manual: libtracefs Manual
.\"    Source: libtracefs 1.8.0
.\"  Language: English
.\"
.TH "LIBTRACEFS" "3" "01/13/2024" "libtracefs 1\&.8\&.0" "libtracefs Manual"
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.SH "NAME"
tracefs_snapshot_snap, tracefs_snapshot_clear, tracefs_snapshot_free \- API to create, clear and read snapshots
.SH "SYNOPSIS"
.sp
.nf
\fB#include <tracefs\&.h>\fR

int \fBtracefs_snapshot_snap\fR(struct tracefs_instance *instance);
int \fBtracefs_snapshot_clear\fR(struct tracefs_instance *instance);
int \fBtracefs_snapshot_free\fR(struct tracefs_instance *instance);
.fi
.SH "DESCRIPTION"
.sp
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\&.
.sp
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\&.
.sp
The \fBtracefs_snapshot_snap()\fR will allocate (if not already allocated) the snapshot buffer and then take a "snapshot" (swap the main buffer that\(cqs being written to with the allocated snapshot buffer)\&. It will do this to the given \fIinstance\fR buffer or the top instance if \fIinstance\fR is NULL\&.
.sp
The \fBtracefs_snapshot_clear()\fR will erase the content of the snapshot buffer for the given \fIinstance\fR or the top level instance if \fIinstance\fR is NULL\&.
.sp
The \fBtracefs_snapshot_free()\fR will free the allocated snapshot for the given \fIinstance\fR or the top level instance if \fIinstance\fR is NULL\&. That is, if another call to \fBtracefs_snapshot_snap()\fR 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\&.
.SH "RETURN VALUE"
.sp
The \fBtracefs_snapshot_snap()\fR, \fBtracefs_snapshot_clear()\fR and the \fBtracefs_snapshot_free()\fR all return 0 on success and \-1 on failure\&.
.SH "EXAMPLE"
.sp
.if n \{\
.RS 4
.\}
.nf
#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\et%s: %s\en",
                                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!\en");
                printf("Enter any text to display the snapshot\en");
                printf("Enter \*(Aqquit\*(Aq to exit\en");
                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);
}
.fi
.if n \{\
.RE
.\}
.SH "FILES"
.sp
.if n \{\
.RS 4
.\}
.nf
\fBtracefs\&.h\fR
        Header file to include in order to have access to the library APIs\&.
\fB\-ltracefs\fR
        Linker switch to add when building a program that uses the library\&.
.fi
.if n \{\
.RE
.\}
.SH "SEE ALSO"
.sp
\fBtracefs_iterate_snapshot_events\fR(3) \fBlibtracefs\fR(3), \fBlibtraceevent\fR(3), \fBtrace\-cmd\fR(1)
.SH "AUTHOR"
.sp
.if n \{\
.RS 4
.\}
.nf
\fBSteven Rostedt\fR <\m[blue]\fBrostedt@goodmis\&.org\fR\m[]\&\s-2\u[1]\d\s+2>
.fi
.if n \{\
.RE
.\}
.SH "REPORTING BUGS"
.sp
Report bugs to <\m[blue]\fBlinux\-trace\-devel@vger\&.kernel\&.org\fR\m[]\&\s-2\u[2]\d\s+2>
.SH "LICENSE"
.sp
libtracefs is Free Software licensed under the GNU LGPL 2\&.1
.SH "RESOURCES"
.sp
\m[blue]\fBhttps://git\&.kernel\&.org/pub/scm/libs/libtrace/libtracefs\&.git/\fR\m[]
.SH "COPYING"
.sp
Copyright (C) 2023 Google, LLC\&. Free use of this software is granted under the terms of the GNU Public License (GPL)\&.
.SH "NOTES"
.IP " 1." 4
rostedt@goodmis.org
.RS 4
\%mailto:rostedt@goodmis.org
.RE
.IP " 2." 4
linux-trace-devel@vger.kernel.org
.RS 4
\%mailto:linux-trace-devel@vger.kernel.org
.RE