Can FD_SETSIZE be increased to more than 1024?

Solution Verified - Updated -

Environment

  • Red Hat Enterprise Linux 7
  • Red Hat Enterprise Linux 6

Issue

  • When using glibc select(), is it possible to increase FD_SETSIZE to more than 1024?
  • What is FD_SETSIZE?

Resolution

  • As per the glibc documentation:
Macro: int FD_SETSIZE

    The value of this macro is the maximum number of file descriptors that a fd_set object can hold information about. On systems with a fixed maximum number, FD_SETSIZE is at least that number. On some systems, including GNU, there is no absolute limit on the number of descriptors open, but this macro still has a constant value which controls the number of bits in an fd_set; if you get a file descriptor with a value as high as FD_SETSIZE, you cannot put that descriptor into an fd_set.
  • FD_SETSIZE is hardcoded to 1024 in /usr/include/bits/typesizes.h and can not be changed.
  • As a workaround, poll() or epoll() can be used instead of select().

Root Cause

  • It's the responsibility of application vendors to make sure alternatives are available in their code (such as using epoll/libevents) to select() what's limited by the FD_SETSIZE in order to make their application more flexible and robust.

Diagnostic Steps

  • Red Hat Enterprise Linux 7
$ rpm -qf /usr/include/bits/typesizes.h 
glibc-headers-2.17-157.el7_3.1.x86_64

$ vi /usr/include/bits/typesizes.h +/FD_SET
 85 /* Number of descriptors that can fit in an `fd_set'.  */
 86 #define __FD_SETSIZE            1024
  • Red Hat Enterprise Linux 6
$ rpm -qf /usr/include/bits/typesizes.h 
glibc-headers-2.12-1.209.el6_9.1.x86_64

$ vi /usr/include/bits/typesizes.h +/FD_SET
 62 /* Number of descriptors that can fit in an `fd_set'.  */
 63 #define __FD_SETSIZE            1024

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