Event loop support Introduction

When WITH_POSIX_AVS_SOCKET option is disabled when compiling Anjay, WITH_EVENT_LOOP will normally be disabled as well. That means that the anjay_event_loop_run() and anjay_serve_any() functions will not be available, and applications will generally need to implement a Custom event loop instead.

However, as long as the underlying API provides a function reasonably similar to either select() or poll(), it is possible to enable the event loop functionality by providing a POSIX compatibility header and manually enabling WITH_EVENT_LOOP. Deciding between select() and poll()

Two equivalent implementations of the event loop are provided in Anjay - one uses the select() call, the other uses poll(). poll() is generally preferred due to known limitations of select(). On Unix-like systems, when using CMake to compile the library, one or the other implementation is chosen automatically based on whether poll() is available in the system.

The event loop uses these APIs directly because the avs_net layer does not provide abstraction over the concept of polling multiple sockets. It has been decided that this is a solution simpler than significantly extending the avs_net API.

The implementation based on select() requires the following APIs, reasonably similar to the ones defined in Unix-like systems, to be available:

  • sockfd_t type to represent the socket descriptor - normally a typedef to int

  • optional INVALID_SOCKET macro - automatically defined to -1 if not explicitly provided

  • fd_set type

  • FD_ZERO(), FD_SET() and FD_ISSET() operations, implemented as functions or macros

  • FD_SETSIZE constant

  • struct timeval with tv_sec and tv_usec fields

  • int select(nfds_t nfds, fd_set *readfds, fd_set *writefds, fd_set *exceptfds, struct timeval *timeout) function, or a macro that can be called as if it had this signature

Conversely, the implementation based on poll() requires the following:

  • sockfd_t type to represent the socket descriptor - normally a typedef to int

  • optional INVALID_SOCKET macro - automatically defined to -1 if not explicitly provided

  • struct pollfd with fd field of type sockfd_t, as well as events and revents fields of scalar types

  • POLLIN constant, compatible with the events field mentioned above

  • int poll(struct pollfds *fds, size_t nfds, int timeout_ms) function, or a macro that can be called as if it had this signature


The sockfd_t type is not standard on Unix-like systems. It has been introduced to allow for socket descriptor types other than int, found on some systems - e.g. SOCKET in Win32 is equivalent to uintptr_t.

One or the other implementation is chosen based on state of the AVS_COMMONS_NET_POSIX_AVS_SOCKET_HAVE_POLL compile-time definition. When using CMake for compiling, its value is detected; when manually populating the configuration headers, it can be configured in avs_commons_config.h.

You can also add #define or #undef for this macro in the POSIX compatibility header, explained below. Writing the POSIX compatibility header

The POSIX compatibility header mechanism has originally been conceived as a way of allowing the use of the default implementations of the networking API (as well as time API) on platforms that have APIs that are close to the Unix standard but have minor incompatible differences - examples include lwIP and Windows.

However, when the default networking layer is not in use, a variant of this header limited in scope can be used to provide the minimal API subset required for the event loop.

The POSIX compatibility header can be any custom header file, specified using the -DPOSIX_COMPAT_HEADER option on CMake command line, or via the AVS_COMMONS_POSIX_COMPAT_HEADER macro in avs_commons_config.h. It is utilized as #include AVS_COMMONS_POSIX_COMPAT_HEADER (when using CMake, quotes are added around the value provided on the command line), so please keep the include path configuration in mind or use absolute paths if feasible.

The header shall contain the necessary #include directives and declarations so that the requirements described above are met.

For example, a POSIX compatibility header for Zephyr may look like:

#include <net/socket.h>

typedef int sockfd_t;

#ifndef pollfd
#    define pollfd zsock_pollfd
#endif // pollfd

#ifndef poll
#    define poll zsock_poll
#endif // poll

#ifndef POLLIN
#endif // POLLIN

Note that neither include guards nor #pragma once is required in this file, although it is permitted to include such guards.


The POSIX compatibility header is also included in the file that implements avs_time_real_now() and avs_time_monotonic_now() if WITH_POSIX_AVS_TIME is enabled, so you may need to also add lines such as #include <sys/time.h> or consider implementing the Time API yourself.