ARES_FDS(3) Library Functions Manual ARES_FDS(3) NAME ares_fds - return file descriptors to select on (deprecated) SYNOPSIS #include <ares.h> int ares_fds(const ares_channel_t *channel, fd_set *read_fds, fd_set *write_fds) DESCRIPTION See the NOTES section on issues with this function and alternatives. The ares_fds(3) function retrieves the set of file descriptors which the calling application should select(2) on for reading and writing for the processing of name service queries pending on the name service channel identified by channel. Should not be used with ARES_OPT_EVENT_THREAD is passed to ares_init_options(3). File descriptors will be set in the file descriptor sets pointed to by read_fds and write_fds as appropriate. File descriptors already set in read_fds and write_fds will remain set; initialization of the file descrip- tor sets (using FD_ZERO) is the responsibility of the caller. RETURN VALUES ares_fds(3) returns a value that is one greater than the number of the highest socket set in either read_fds or write_fds. If no queries are ac- tive, ares_fds(3) returns 0. NOTES The select(2) call which takes the fd_set parameter has significant limita- tions which can impact modern systems. The limitations can vary from sys- tem to system, but in general if the file descriptor value itself is greater than 1024 (not the count but the actual value), this can lead to ares_fds(3) writing out of bounds which will cause a system crash. In mod- ern networking clients, it is not unusual to have file descriptor values above 1024, especially when a library is pulled in as a dependency into a larger project. c-ares does not attempt to detect this condition to prevent crashes due to both implementation-defined behavior in the OS as well as integrator-con- trollable tunables which may impact the limits. It is recommended to use ARES_OPT_EVENT_THREAD passed to ares_init_op- tions(3), or socket state callbacks (ARES_OPT_SOCK_STATE_CB) registered via ares_init_options(3) and use more modern methods to check for socket read- able/writable state such as poll(2), epoll(2), or kqueue(2). SEE ALSO ares_init_options(3), ares_timeout(3), ares_process(3) 23 July 1998 ARES_FDS(3)
NAME | SYNOPSIS | DESCRIPTION | RETURN VALUES | NOTES | SEE ALSO
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