fork1() creates a new process out of the process behind
l1, which is assumed to be the current lwp. This function is used primarily to implement the
fork(2) and
vfork(2) system calls, but is versatile enough to be used as a backend for e.g. the
__clone(2) call.
The
flags argument controls the semantics of the fork operation, and is made up of the bitwise-OR of the following values:
FORK_PPWAIT
The parent process will sleep until the child process successfully calls
execve(2) or exits (either by a call to
_exit(2) or abnormally).
FORK_SHAREVM
The child process will share the parent's virtual address space. If this flag is not specified, the child will get a copy-on-write snapshot of the parent's address space.
FORK_SHARECWD
The child process will share the parent's current directory, root directory, and file creation mask.
FORK_SHAREFILES
The child process will share the parent's file descriptors.
FORK_SHARESIGS
The child process will share the parent's signal actions.
FORK_NOWAIT
The child process will at creation time be inherited by the init process.
FORK_CLEANFILES
The child process will not copy or share the parent's descriptors, but rather will start out with a clean set.
A
flags value of 0 indicates a standard fork operation.
The
exitsig argument controls the signal sent to the parent on child death. If normal operation desired, SIGCHLD should be supplied.
It is possible to specify the child userspace stack location and size by using the
stack and
stacksize arguments, respectively. Values
NULL and 0, respectively, will give the child the default values for the machine architecture in question.
The arguments
func and
arg can be used to specify a kernel function to be called when the child process returns instead of
child_return(). These are used for example in starting the init process and creating kernel threads.
The
retval argument is provided for the use of system call stubs. 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.
User level system call stubs typically subtract 1 from
retval[1] and bitwise-AND it with
retval[0], thus returning the pid to the parent process and 0 to the child.
If
rnewprocp is not NULL,
*rnewprocp will point to the newly created process upon successful completion of the fork operation.