docs/ares_fds.3

.\"
.\" Copyright 1998 by the Massachusetts Institute of Technology.
.\" SPDX-License-Identifier: MIT
.\"
.TH ARES_FDS 3 "23 July 1998"
.SH NAME
ares_fds \- return file descriptors to select on. Deprecated.
.SH SYNOPSIS
.nf
#include <ares.h>

int ares_fds(ares_channel_t *\fIchannel\fP,
             fd_set *\fIread_fds\fP,
             fd_set *\fIwrite_fds\fP)
.fi
.SH DESCRIPTION
See the \fBNOTES\fP section on issues with this function and alternatives.

The \fBares_fds(3)\fP function retrieves the set of file descriptors which the
calling application should \fBselect(2)\fP on for reading and writing for the
processing of name service queries pending on the name service channel
identified by \fIchannel\fP.

File descriptors will be set in the file descriptor sets pointed to by
\fIread_fds\fP and \fIwrite_fds\fP as appropriate.  File descriptors already
set in \fIread_fds\fP and \fIwrite_fds\fP will remain set; initialization of
the file descriptor sets (using \fBFD_ZERO\fP) is the responsibility of the
caller.
.SH RETURN VALUES
\fBares_fds(3)\fP returns a value that is one greater than the number of the
highest socket set in either \fIread_fds\fP or \fIwrite_fds\fP.  If no queries
are active, \fBares_fds(3)\fP returns 0.

.SH NOTES
The \fBselect(2)\fP call which takes the \fIfd_set\fP 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
\fBares_fds(3)\fP 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 socket state callbacks (\fBARES_OPT_SOCK_STATE_CB\fP)
registered via \fBares_init_options(3)\fP, and use more modern methods to
check for socket readable/writable state such as \fBpoll(2)\fP, \fBepoll(2)\fP,
or \fBkqueue(2)\fP.
.SH SEE ALSO
.BR ares_timeout (3),
.BR ares_process (3)
.SH AUTHOR
Greg Hudson, MIT Information Systems
.br
Copyright 1998 by the Massachusetts Institute of Technology.