On Jul 6 14:26, Lavrentiev, Anton (NIH/NLM/NCBI) [C] via Cygwin wrote:
> > DESCRIPTION
> > 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 :-)
:+1:
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:
bits/typesizes.h:
/* 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:
misc/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)
#else
__fd_mask __fds_bits[__FD_SETSIZE / __NFDBITS];
# define __FDS_BITS(set) ((set)->__fds_bits)
#endif
} 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.
Corinna
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