'\" t
.\" Title: libtracefs
.\" Author: [see the "AUTHOR" section]
.\" Generator: DocBook XSL Stylesheets vsnapshot
.\" Date: 07/25/2024
.\" Manual: libtracefs Manual
.\" Source: libtracefs 1.8.1
.\" Language: English
.\"
.TH "LIBTRACEFS" "3" "07/25/2024" "libtracefs 1\&.8\&.1" "libtracefs Manual"
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.nh
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.\" * MAIN CONTENT STARTS HERE *
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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 \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
#include
#include
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