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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 *stack, pid_t *tidptr, void (*func)(void *), void *arg, register_t *retval, struct proc **rnewprocp);

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:
 
 
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.
 
 
The call is done by the __tfork(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 child 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.
 
 
The child will instead be a kernel-level thread in the same process as the parent. FORK_SHAREFILES, 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.
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.

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.

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

The fork1 function semantics are specific to OpenBSD. Other BSD systems have different semantics.
September 14, 2015 OpenBSD-5.9