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ARES_FDS(3) Library Functions Manual ARES_FDS(3)


ares_fds - return file descriptors to select on (deprecated)


#include <ares.h>
int ares_fds(ares_channel_t *channel,

fd_set *read_fds,
fd_set *write_fds)


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 descriptor sets (using FD_ZERO) is the responsibility of the caller.


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 active, ares_fds(3) returns 0.


The select(2) call which takes the fd_set parameter has significant limitations which can impact modern systems. The limitations can vary from system 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 modern 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-controllable tunables which may impact the limits.

It is recommended to use ARES_OPT_EVENT_THREAD passed to ares_init_options(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 readable/writable state such as poll(2), epoll(2), or kqueue(2).


ares_init_options(3), ares_timeout(3), ares_process(3)


Greg Hudson, MIT Information Systems
Copyright 1998 by the Massachusetts Institute of Technology.

23 July 1998