Can FD_SETSIZE be increased to more than 1024?

Solution Verified - Updated -

Environment

  • Red Hat Enterprise Linux 10
  • Red Hat Enterprise Linux 9
  • Red Hat Enterprise Linux 8
  • Red Hat Enterprise Linux 7
  • Red Hat Enterprise Linux 6

Issue

  • What is FD_SETSIZE and what's used for?
  • When using GNU C Library's select(), is it possible to increase FD_SETSIZE to more than 1024?

Resolution

  • As per the GNU C Library 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, consider using poll() or epoll() interfaces, 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 or libevents to select() what's limited by the FD_SETSIZE in order to make their application more flexible and robust.

Diagnostic Steps

  • FD_SETSIZE in definied in the header file /usr/include/bits/typesizes.h, from glibc-headers package in RHEL9 and glibc-devel in RHEL10:

    # rpm -qf /usr/include/bits/typesizes.h --queryformat "%{NAME}\n"
glibc-headers

# grep -B1 "FD_SETSIZE" /usr/include/bits/typesizes.h
/* Number of descriptors that can fit in an `fd_set'.  */
#define __FD_SETSIZE      1024

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