Platform-Specific Implementation

This design note lists platform-specific implementation details, such as the default demultiplexing mechanism, the number of threads created internally, and when threads are created.

Linux Kernel 2.4

Demultiplexing mechanism:

Uses select for demultiplexing. This means that the number of file descriptors in the process cannot be permitted to exceed FD_SETSIZE.

Threads:

Demultiplexing using select is performed in one of the threads that calls io_service::run(), io_service::run_one(), io_service::poll() or io_service::poll_one().
An additional thread per io_service is used to emulate asynchronous host resolution. This thread is created on the first call to either ip::tcp::resolver::async_resolve() or ip::udp::resolver::async_resolve().

Linux Kernel 2.6

Demultiplexing mechanism:

Uses epoll for demultiplexing.

Threads:

Demultiplexing using epoll is performed in one of the threads that calls io_service::run(), io_service::run_one(), io_service::poll() or io_service::poll_one().
An additional thread per io_service is used to emulate asynchronous host resolution. This thread is created on the first call to either ip::tcp::resolver::async_resolve() or ip::udp::resolver::async_resolve().

Solaris

Demultiplexing mechanism:

Uses /dev/poll for demultiplexing.

Threads:

Demultiplexing using /dev/poll is performed in one of the threads that calls io_service::run(), io_service::run_one(), io_service::poll() or io_service::poll_one().
An additional thread per io_service is used to emulate asynchronous host resolution. This thread is created on the first call to either ip::tcp::resolver::async_resolve() or ip::udp::resolver::async_resolve().

QNX Neutrino

Demultiplexing mechanism:

Uses select for demultiplexing. This means that the number of file descriptors in the process cannot be permitted to exceed FD_SETSIZE.

Threads:

Demultiplexing using select is performed in one of the threads that calls io_service::run(), io_service::run_one(), io_service::poll() or io_service::poll_one().
An additional thread per io_service is used to emulate asynchronous host resolution. This thread is created on the first call to either ip::tcp::resolver::async_resolve() or ip::udp::resolver::async_resolve().

Mac OS X

Demultiplexing mechanism:

Uses kqueue for demultiplexing.

Threads:

Demultiplexing using kqueue is performed in one of the threads that calls io_service::run(), io_service::run_one(), io_service::poll() or io_service::poll_one().
An additional thread per io_service is used to emulate asynchronous host resolution. This thread is created on the first call to either ip::tcp::resolver::async_resolve() or ip::udp::resolver::async_resolve().

FreeBSD

Demultiplexing mechanism:

Uses kqueue for demultiplexing.

Threads:

Demultiplexing using kqueue is performed in one of the threads that calls io_service::run(), io_service::run_one(), io_service::poll() or io_service::poll_one().
An additional thread per io_service is used to emulate asynchronous host resolution. This thread is created on the first call to either ip::tcp::resolver::async_resolve() or ip::udp::resolver::async_resolve().

AIX

Demultiplexing mechanism:

Uses select for demultiplexing. This means that the number of file descriptors in the process cannot be permitted to exceed FD_SETSIZE.

Threads:

Demultiplexing using select is performed in one of the threads that calls io_service::run(), io_service::run_one(), io_service::poll() or io_service::poll_one().
An additional thread per io_service is used to emulate asynchronous host resolution. This thread is created on the first call to either ip::tcp::resolver::async_resolve() or ip::udp::resolver::async_resolve().

HP-UX

Demultiplexing mechanism:

Uses select for demultiplexing. This means that the number of file descriptors in the process cannot be permitted to exceed FD_SETSIZE.

Threads:

Demultiplexing using select is performed in one of the threads that calls io_service::run(), io_service::run_one(), io_service::poll() or io_service::poll_one().
An additional thread per io_service is used to emulate asynchronous host resolution. This thread is created on the first call to either ip::tcp::resolver::async_resolve() or ip::udp::resolver::async_resolve().

Tru64

Demultiplexing mechanism:

Uses select for demultiplexing. This means that the number of file descriptors in the process cannot be permitted to exceed FD_SETSIZE.

Threads:

Demultiplexing using select is performed in one of the threads that calls io_service::run(), io_service::run_one(), io_service::poll() or io_service::poll_one().
An additional thread per io_service is used to emulate asynchronous host resolution. This thread is created on the first call to either ip::tcp::resolver::async_resolve() or ip::udp::resolver::async_resolve().

Windows 95, 98 and Me

Demultiplexing mechanism:

Uses select for demultiplexing.

Threads:

Demultiplexing using select is performed in one of the threads that calls io_service::run(), io_service::run_one(), io_service::poll() or io_service::poll_one().
An additional thread per io_service is used to emulate asynchronous host resolution. This thread is created on the first call to either ip::tcp::resolver::async_resolve() or ip::udp::resolver::async_resolve().

Windows NT, 2000, XP, 2003 and Vista

Demultiplexing mechanism:

Uses overlapped I/O and I/O completion ports for all asynchronous socket operations except for asynchronous connect.
Uses select for emulating asynchronous connect.

Threads:

Demultiplexing using I/O completion ports is performed in all threads that call io_service::run(), io_service::run_one(), io_service::poll() or io_service::poll_one().
An additional thread per io_service is used for the select demultiplexing. This thread is created on the first call to async_connect().
An additional thread per io_service is used to emulate asynchronous host resolution. This thread is created on the first call to either ip::tcp::resolver::async_resolve() or ip::udp::resolver::async_resolve().