_lwp_create(2)

NAME

_lwp_create − create a new light-weight process

SYNOPSIS

#include <sys/lwp.h>

int _lwp_create(ucontext_t ∗contextp, unsigned long flags, lwpid_t ∗new_lwp);

DESCRIPTION

The function _lwp_create() adds a lightweight process (LWP) to the current process. The context parameter specifies the initial signal mask, stack, and machine context (including the program counter and stack pointer) for the new LWP. The new LWP inherits the scheduling class and priority of the caller.

If _lwp_create() is successful, the ID of the new LWP is stored in the location pointed to by new_lwp.

flags specifies additional attributes for the new LWP. The value in flags is constructed by the bit-wise inclusive OR of the following values:

LWP_DETACHED The LWP is created detached.

LWP_SUSPENDED
The LWP is created suspended. If __LWP_ASLWP is specified, then the LWP created is the special, designated LWP which handles signals sent to a multi-threaded process — the aslwp. There can be only one aslwp in a multi-threaded process, so the creation of another aslwp will return an error code — EINVAL.

__LWP_ASLWP The LWP created is the aslwp (Asynchronous Signals LWP) (see signal(5)). The aslwp should always have all signals blocked — that is how it should be created. It should never exit via _lwp_exit(2) or exit(2). This flag should not be used by any user program. It is documented here purely for the sake of documentation and not for use by an application.

If LWP_DETACHED is specified, then the LWP is created in the detached state. Otherwise the LWP is created in the undetached state. The ID (and system resources) associated with a detached LWP can be automatically reclaimed when the LWP exits. The ID of an undetached LWP cannot be reclaimed until it exits and another LWP has reported its termination via _lwp_wait(2). This allows the waiting LWP to determine that the waited for LWP has terminated and to reclaim any process resources that it was using.

If LWP_SUSPENDED is specified, then the LWP is created in a suspended state. This allows the creator to change the LWP’s inherited attributes before it starts to execute. The suspended LWP can only be resumed via _lwp_continue(2). If LWP_SUSPENDED is not specified the LWP can begin to run immediately after it has been created.

RETURN VALUES

Zero is returned when successful. A non-zero value indicates an error.

ERRORS

If any of the following conditions are detected, _lwp_create() fails and returns the corresponding value:

EFAULT Either the context parameter or the new_lwp parameter point to invalid addresses.

EAGAIN A system limit is exceeded, e.g., too many LWP were created for this real user ID.

EINVAL The __LWP_ASLWP flag was used to create more than one aslwp in the process. There can be only one aslwp within a process.

EXAMPLES

This example shows how a stack is allocated to a new LWP. _lwp_makecontext() is used to set up the context parameter so that the new LWP begins executing a function.

contextp = (ucontext_t ∗)malloc(sizeof(ucontext_t));
stackbase = malloc(stacksize);
sigprocmask(SIGSETMASK, NULL, &contextp->uc_sigmask);
_lwp_makecontext(contextp, func, arg, private, stackbase, stacksize);
error = _lwp_create(contextp, NULL, &new_lwp);

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