libc/sys/sys_checkpoint.2

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.Dd June 29, 2007
.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:
.Bl -tag -width ".Dv 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
.Xr exec 3
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
.Va errno
is set to indicate the error.
.Sh EXAMPLES
.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 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.
It is best to checkpoint to a new file and then rename-over the old, or to
.Xr remove 3
the old file before creating the new
one so it remains valid as long as the program continues to run.
.Pp
Threaded programs cannot currently be checkpointed.
The program must be
reduced to a single thread before it can be safely checkpointed.
.Pp
.Dv MAP_VPAGETABLE
mappings cannot currently be checkpointed.
A program must restore such mappings manually on resumption.
Only regular file and
anonymous memory mappings are checkpointed and restored.
Device and other special mappings are not.
Only regular file descriptors are checkpointed and restored.
Devices, pipes, sockets, and other special descriptors are not.
Memory wiring states are not checkpointed or restored.
.Xr madvise 2
states are not checkpointed or restored.
Basic mapping permissions are checkpointed and restored.
.Sh SECURITY
The sysctl
.Va 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
.Va 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 SEE ALSO
.Xr checkpt 1 ,
.Xr signal 3
.Sh HISTORY
The
.Fn sys_checkpoint
function call appeared in
.Dx 1.1 .