Updated: 2021/Apr/14

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

     fork1 - create a new process

     #include <sys/types.h>
     #include <sys/proc.h>

     fork1(struct lwp *l1, int flags, int exitsig, void *stack,
         size_t stacksize, void (*func)(void *), void *arg,
         register_t *retval);

     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

     FORK_SHARESIGS   The child process will share the parent's signal

     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

     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.

     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; or the limit RLIMIT_NPROC on the total number of
               processes under execution by this user id would be exceeded.

     execve(2), fork(2), vfork(2)

NetBSD 9.99                     April 16, 2018                     NetBSD 9.99