man/man9/fork1.9

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.\" NASA Ames Research Center.
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.Dd $Mdocdate: June 13 2012 $
.Dt FORK1 9
.Os
.Sh NAME
.Nm fork1
.Nd create a new process
.Sh SYNOPSIS
.Fd #include <sys/types.h>
.Fd #include <sys/proc.h>
.Ft int
.Fo "fork1"
.Fa "struct proc *p1"
.Fa "int exitsig"
.Fa "int flags"
.Fa "void *stack"
.Fa "pid_t *tidptr"
.Fa "void (*func)(void *)"
.Fa "void *arg"
.Fa "register_t *retval"
.Fa "struct proc **rnewprocp"
.Fc
.Sh DESCRIPTION
.Fn fork1
creates a new process out of
.Ar p1 ,
which should be the current process.
This function is used primarily to implement the
.Xr fork 2 ,
.Xr __tfork 2 ,
.Xr vfork 2
system calls, as well as the
.Xr kthread_create 9
function.
.Pp
The
.Ar flags
argument is used to control the behavior of the fork and is created by
a bitwise-OR of the following values:
.Bl -tag -width FORK_SHAREFILES
.It Dv FORK_FORK
The call is done by the
.Xr fork 2
system call.
Used only for statistics.
.It Dv FORK_VFORK
The call is done by the
.Xr vfork 2
system call.
Used only for statistics.
.It Dv FORK_TFORK
The call is done by the
.Xr __tfork 2
system call.
Used only for statistics.
.It Dv FORK_PPWAIT
Suspend the parent process until the child is terminated (by calling
.Xr _exit 2
or abnormally), or makes a call to
.Xr execve 2 .
.It Dv FORK_SHAREFILES
Let the child share the file descriptor table with the parent through
fdshare().
The default behavior is to copy the table through
fdcopy().
.It Dv FORK_CLEANFILES
The child starts with a clean file descriptor table created by
fdinit().
.It Dv FORK_NOZOMBIE
The child will be dissociated from the parent and will not leave a status
for the parent to collect.
See
.Xr wait 2 .
.It Dv FORK_SHAREVM
The child will share the parent's address space.
The default behavior is
that the child gets a copy-on-write copy of the address space.
.It Dv FORK_SIGHAND
The child will share the parent's signal actions, including the handler,
mask, and flags, with sigactsshare().
The default behavior is to copy the signal actions from the parent with
sigactsinit().
.Dv FORK_SHAREVM
must also be set.
.It Dv FORK_PTRACE
The child will start with tracing enabled, as if
ptrace(PT_TRACE_ME, 0, 0, 0) had been invoked in the child.
.It Dv FORK_THREAD
The child will instead be a kernel-level thread in the same process
as the parent.
.Dv FORK_NOZOMBIE ,
.Dv FORK_SHAREVM ,
and
.Dv FORK_SIGHAND
must also be set.
.El
.Pp
If
.Fa stack
is not
.Dv NULL ,
it will be used as the initial value of the child's stack pointer,
instead of using the child's copy of the parent's stack.
.Pp
If
.Fa tidptr
is not
.Dv NULL ,
the PID of the child process will be written there in the parent
on success.
This is guaranteed to be done before
the child process is started.
.Pp
If
.Fa retval
is not
.Dv NULL ,
it will hold the following values after successful completion
of the fork operation:
.Bl -tag -width retval[0]
.It Fa retval[0]
This will contain the PID of the child process.
.It Fa retval[1]
In the parent process, this will contain the value 0.
In the child process, this will contain 1.
.El
.Pp
The signal
.Fa exitsig
is sent to the parent
.Fa p1
on exit of the new process.
.Pp
If
.Fa func
is not
.Dv NULL ,
the new process will begin execution by calling this function.
It defaults to child_return, which returns to userland.
.Pp
If
.Fa arg
is not
.Dv NULL ,
it is the argument to the previous function.
It defaults to a pointer to the new process.
.Pp
The newly created process is returned through
.Fa *rnewprocp .
.Sh RETURN VALUES
Upon successful completion of the fork operation,
.Fn fork1
returns 0.
Otherwise, the following error values are returned:
.Bl -tag -width [EAGAIN]
.It Bq Er EAGAIN
The limit on the total number of system processes would be exceeded.
.It Bq Er EAGAIN
The limit
.Dv RLIMIT_NPROC
on the total number of processes under execution by this
user id would be exceeded.
.El
.Sh SEE ALSO
.Xr __tfork 2 ,
.Xr execve 2 ,
.Xr fork 2 ,
.Xr vfork 2 ,
.Xr kthread_create 9 ,
.Xr pfind 9 ,
.Xr psignal 9 ,
.Xr uvm_fork 9
.Sh CAVEATS
The
.Nm
function semantics are specific to
.Ox .
Other BSD systems have different semantics.
.Pp
The only use of a non-null
.Fa stack
is for
.Xr compat_linux 8
on i386, so that feature is mostly untested.