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FORK1(9) Kernel Developer's Manual FORK1(9)

fork1
create a new process

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

fork1() creates a new process out of p1, which should be the current thread. This function is used primarily to implement the fork(2) and 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:
 
 
The call is done by the fork(2) system call. Used only for statistics.
 
 
The call is done by the vfork(2) system call. Used only for statistics.
 
 
Suspend the parent process until the child is terminated (by calling _exit(2) or abnormally), or makes a call to execve(2).
 
 
Let the child share the file descriptor table with the parent through fdshare(). The default behavior is to copy the table through fdcopy().
 
 
The new thread will be left in the SIDL state. The default behavior is to make it runnable and add it to the run queue.
 
 
The child will be dissociated from the parent and will not leave a status for the parent to collect. See wait(2).
 
 
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.
 
 
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.
 
 
The child will start with tracing enabled, as if ptrace(PT_TRACE_ME, 0, 0, 0) had been invoked in the child.
If func is not NULL, the new thread 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 thread.
If retval is not NULL, the PID of the child process will be stored in *retval on successful completion.
If rnewprocp is not NULL, the newly created thread is stored in *rnewprocp on successful completion.

Upon successful completion of the fork operation, fork1() returns 0. Otherwise, the following error values are returned:
 
 
[EAGAIN]
The system limits on the total number of threads or processes would be exceeded.
 
 
[EAGAIN]
The limit RLIMIT_NPROC on the total number of processes under execution by this user id would be exceeded.
 
 
[ENOMEM]
There is insufficient swap space for the new thread.

execve(2), fork(2), vfork(2), kthread_create(9), psignal(9), tfind(9)

The fork1 function semantics are specific to OpenBSD. Other BSD systems have different semantics.
June 11, 2017 OpenBSD-current