FORK1(9)

NAME
fork1 - create a new process

SYNOPSIS
#include <sys/types.h>
#include <sys/proc.h>
int
fork1(struct proc *p1, int exitsig, int flags, void *stack, pid_t *tidptr, void (*func)(void *), void *arg, register_t *retval, struct proc **rnewprocp);

DESCRIPTION
fork1() creates a new process out of p1, which should be the current process. This function is used primarily to implement the fork(2), __tfork(2), vfork(2) system calls, as well as the kthread_create(9) function.

The flags argument is used to control the behavior of the fork and is created by a bitwise-OR of the following values:

FORK_FORK
    The call is done by the fork(2) system call. Used only for statistics.

FORK_VFORK
    The call is done by the vfork(2) system call. Used only for statistics.

FORK_TFORK
    The call is done by the __tfork(2) system call. Used only for statistics.

FORK_PPWAIT
    Suspend the parent process until the child is terminated (by calling _exit(2) or abnormally), or makes a call to execve(2).

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().

FORK_CLEANFILES
    The child starts with a clean file descriptor table created by fdinit().

FORK_NOZOMBIE
    The child will be dissociated from the parent and will not leave a status for the parent to collect. See wait(2).

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.

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(). FORK_SHAREVM must also be set.

FORK_PTRACE
    The child will start with tracing enabled, as if ptrace(PT_TRACE_ME, 0, 0, 0) had been invoked in the child.

FORK_THREAD
    The child will instead be a kernel-level thread in the same process as the parent. FORK_NOZOMBIE, FORK_SHAREVM, and FORK_SIGHAND must also be set.

If stack is not 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.

If tidptr is not 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.

If retval is not NULL, it will hold the following values after successful completion of the fork operation:

retval[0]
    This will contain the PID of the child process.

retval[1]
    In the parent process, this will contain the value 0. In the child process, this will contain 1.

The signal exitsig is sent to the parent p1 on exit of the new process.

If func is not NULL, the new process will begin execution by calling this function. It defaults to child_return, which returns to userland.

If arg is not NULL, it is the argument to the previous function. It defaults to a pointer to the new process.

The newly created process is returned through *rnewprocp.

RETURN VALUES
Upon successful completion of the fork operation, fork1() returns 0. Otherwise, the following error values are returned:

[EAGAIN]
    The limit on the total number of system processes would be exceeded.

[EAGAIN]
    The limit RLIMIT_NPROC on the total number of processes under execution by this user id would be exceeded.

SEE ALSO
__tfork(2), execve(2), fork(2), vfork(2), kthread_create(9), pfind(9), psignal(9), uvm_fork(9)

CAVEATS
The fork1 function semantics are specific to OpenBSD. Other BSD systems have different semantics.

The only use of a non-null stack is for compat_linux(8) on i386, so that feature is mostly untested.