Typo in <sys/select.h>?

Corinna Vinschen corinna-cygwin@cygwin.com
Wed Jul 6 15:07:14 GMT 2022

On Jul  6 14:26, Lavrentiev, Anton (NIH/NLM/NCBI) [C] via Cygwin wrote:
> >        WARNING: select() can monitor only file descriptors  numbers  that  are
> >        less  than  FD_SETSIZE (1024)-an unreasonably low limit for many modern
> Whoever wrote this, was wrong (they might have never consulted the actual kernel code, or were just
> blindsided by FD_SETSIZE being a predefined constant).  You can take a look at the kernel source code,
> if you don't believe me.
> This select() trick has been like that from the earliest days of Linux, and the behavior is carefully carried over.
> https://github.com/torvalds/linux/blob/master/fs/select.c#L625
> It's just an FYI :-)


You're right as far as the kernel is concerned.  The problem is apparently
in glibc.  From the same man page:

  POSIX allows an implementation to define an upper limit, advertised via
  the constant FD_SETSIZE, on the range of file descriptors that  can  be
  specified  in a file descriptor set.  The Linux kernel imposes no fixed
  limit, but the glibc implementation makes  fd_set  a  fixed-size  type,
  with  FD_SETSIZE  defined  as 1024, and the FD_*() macros operating ac‐
  cording to that limit.  To monitor file descriptors greater than  1023,
  use poll(2) or epoll(7) instead.

And that's still the case with glibc from current git master:


  /* Number of descriptors that can fit in an `fd_set'.  */
  #define __FD_SETSIZE            1024

That's defined unconditionally, just as this stuff from sys/select.h:


  /* fd_set for select and pselect.  */
  typedef struct
      /* XPG4.2 requires this member name.  Otherwise avoid the name
	 from the global namespace.  */
  #ifdef __USE_XOPEN
      __fd_mask fds_bits[__FD_SETSIZE / __NFDBITS];
  # define __FDS_BITS(set) ((set)->fds_bits)
      __fd_mask __fds_bits[__FD_SETSIZE / __NFDBITS];
  # define __FDS_BITS(set) ((set)->__fds_bits)
    } fd_set;

  /* Maximum number of file descriptors in `fd_set'.  */
  #define FD_SETSIZE              __FD_SETSIZE

However, discussing this shows how irrelevant the actual default value
of FD_SETSIZE is.  Per POSIX, it's just that, a default value.  In
interested process which thinks it needs higher fd numbers should make
sure to define FD_SETSIZE according to its requirements anyway.


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