libc/sys/sys_checkpoint.2

.\" Copyright (c) 2003,2004 The DragonFly Project.  All rights reserved.
.\"
.\" This code is derived from software contributed to The DragonFly Project
.\" by Matthew Dillon <dillon@backplane.com>
.\"
.\" Redistribution and use in source and binary forms, with or without
.\" modification, are permitted provided that the following conditions
.\" are met:
.\"
.\" 1. Redistributions of source code must retain the above copyright
.\"    notice, this list of conditions and the following disclaimer.
.\" 2. Redistributions in binary form must reproduce the above copyright
.\"    notice, this list of conditions and the following disclaimer in
.\"    the documentation and/or other materials provided with the
.\"    distribution.
.\" 3. Neither the name of The DragonFly Project nor the names of its
.\"    contributors may be used to endorse or promote products derived
.\"    from this software without specific, prior written permission.
.\"
.\" THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
.\" ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
.\" LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
.\" FOR A PARTICULAR PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL THE
.\" COPYRIGHT HOLDERS OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
.\" INCIDENTAL, SPECIAL, EXEMPLARY OR CONSEQUENTIAL DAMAGES (INCLUDING,
.\" BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
.\" LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED
.\" AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
.\" OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT
.\" OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
.\" SUCH DAMAGE.
.\"
.\" $DragonFly: src/lib/libc/sys/sys_checkpoint.2,v 1.2 2004/11/24 02:56:43 hmp Exp $
.\"
.Dd November 22, 2004
.Dt SYS_CHECKPOINT 2
.Os
.Sh NAME
.Nm sys_checkpoint
.Nd checkpoint or restore a process
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In sys/types.h
.In sys/checkpoint.h
.Ft int
.Fn sys_checkpoint "int type" "int fd" "pid_t pid" "int retval"
.Sh DESCRIPTION
The
.Fn sys_checkpoint
system call executes a checkpoint function as specified by
.Fa type .
Supported types are as follows:
.Pp
.Bl -tag -width CKPT_FREEZE -offset indent
.It Dv CKPT_FREEZE
Generate a checkpoint file.
Currently
.Fa pid
must be -1 or the pid of the current process.
The checkpoint file will be written out to
.Fa fd ,
and
.Fa retval
is unused but must be specified as -1.
As a special case, if
.Fa pid
and
.Fa fd
are both specified as -1, the system will generate a checkpoint file
using the system checkpoint template.
.Pp
This function returns 0 on success, -1 on error, and typically 1
on resume.  The value returned on resume is controlled by the
.Fa retval
argument passed to
.Fn sys_checkpoint
when resuming a checkpoint file.  A user program which installs its
own
.Dv SIGCKPT
signal handler and calls
.Fn sys_checkpoint
manually thus has control over both termination/continuance and
resumption.
.It Dv CKPT_THAW
Restore a checkpointed program.
The
.Fa pid
must be specified as -1, and
.Fa fd
represents the checkpoint file.
The
.Fa retval
specifies the value returned to the resumed program if
.Fn sys_checkpoint
was called directly.
.Pp
The checkpointed program will replace the current program, similar to
an exec call.
.El
.Sh RETURN VALUES
Upon successful completion, the value 0 is typically returned.  A checkpoint
being resumed typically returns a positive value; otherwise the value -1
is returned and the global variable
.Li errno
is set to indicate the error.
.Sh ERRORS
.Bl -tag -width Er
.It Bq Er EBADF
The given
.Fa fd
is not a valid regular file, socket descriptor, or pipe.  Note that not
all systems necessarily support checkpointing to sockets and pipes.
.It Bq Er EPERM
The caller does not have permission to issue the checkpoint command.
Checkpointing may be restricted or disabled using sysctls.
.It Bq Er EIO
An I/O error occurred while reading from the file system.
.It Bq Er EINVAL
An invalid parameter was specified.
.El
.Sh CHECKPOINT FEATURES
The system checkpointing code will save the process register state (including
floating point registers), signal state, file descriptors representing
regular files or directories (anything that can be converted into a file
handle for storage), and both shared and private memory mappings.
Private, writable mappings are copied to the checkpoint file while shared
mappings and stored by referencing the file handle and offset.
Note that the system checkpointing code does not retain references to
deleted files, so mappings and open descriptors of deleted files
cannot be restored.
Unpredictable operation will occur if a checkpoint-unaware program
is restored and some of the underlying files mapped by the program
have changed.
.Pp
The system checkpointing code is not able to retain the process pid, process
group, user/group creds, or descriptors 0, 1, and 2.  These will be inherited
from whomever restores the checkpoint.
.Pp
When a checkpointed program is restored modified private mappings will
be mapped from the checkpoint file itself, but major portions of the
original program binary will be mapped from the original program binary.
If the resumed program is checkpointed again the system will automatically
copy any mappings from the original checkpoint file to the new one, since
the original is likely being replaced.
The caller must not truncate the existing checkpoint file when creating
a new one or specify the existing file's file descriptor as the new
one as this will destroy the data that the checkpoint operation needs
to copy to the new file.
.Sh SECURITY
The sysctl
.Em kern.ckptgroup
controls which group can use system checkpointing.
By default, only users in the
.Ql wheel
group are allowed to checkpoint and restore processes.
To allow users in any group to have this capability (risky), set sysctl
.Em kern.ckptgroup
to -1.
.Sh SIGNALS
Two signals are associated with checkpointing.
.Dv SIGCKPT
is delivered via the tty ckpt character, usually control-E.  Its default
action is to checkpoint a program and continue running it.  The
.Dv SIGCKPTEXIT
signal can only be delivered by
.Xr kill 2 .
Its default action is to checkpoint a program and then exit.
.Dv SIGCKPTEXIT
might not be implemented by the system.  Both signals are defined to
be greater or equal to signal 32 and cannot be manipulated using legacy
masking functions.
.Pp
If a program overrides the default action for a checkpoint signal the
system will not undertake any action of its own.  The program may issue
the checkpoint command from the signal handler itself or simply set a
reminder for later action.  It is usually safest to set a reminder and
do the actual checkpointing from your main loop.
.Sh EXAMPLE
.Bd -literal -offset indent -compact

/*
 * Demonstrate checkpointing.  Use control-E to checkpoint
 * the program and 'checkpt -r x.ckpt' to resume it.
 */
#include <sys/types.h>
#include <sys/signal.h>
#include <sys/checkpoint.h>
#include <stdio.h>
#include <unistd.h>
#include <fcntl.h>
#include <errno.h>

void docheckpoint(void);

int wantckpt;

void
dockpt(int sig)
{
    wantckpt = 1;
}

int
main(int argc, char** argv)
{
     int i = 0;

     signal(SIGCKPT, dockpt);

     for (;;) {
	printf("iteration: %d\en", i);
	++i;
	sleep(1);
	if (wantckpt) {
		wantckpt = 0;
		printf("Checkpoint requested\en");
		docheckpoint();
	}
    }
    return(0);
}

void
docheckpoint(void)
{
    int ret;
    int fd;

    fd = open("x.ckpt", O_RDWR|O_CREAT|O_TRUNC, 0666);
    if (fd < 0) {
	printf("unable to create checkpoint file: %s\en",
		strerror(errno));
	return;
    }

    ret = sys_checkpoint(CKPT_FREEZE, fd, -1, -1);
    if (ret < 0) {
	printf("unable to checkpoint: %s\en",
		strerror(errno));
    } else if (ret == 0) {
	printf("checkpoint successful, continuing\en");
    } else if (ret == 1) {
	printf("resuming from checkpoint.\en");
    } else {
	printf("unknown return value %d from sys_checkpoint\en", ret);
	exit(1);
    }
    /* note that the file descriptor is still valid on a resume */
    close(fd);
}
.Ed
.Sh SEE ALSO
.Xr checkpt 1 ,
.Xr signal 3
.Sh HISTORY
The
.Fn sys_checkpoint
function call
appeared in
DragonFly 1.1 .