On mingw, getdtablesize() currently returns 512 but the dup2 test shows that
file descriptors up until 2047 can be used. (On Windows XP.)
This fixes getdtablesize() so that it returns 2048.
2009-08-23 Bruno Haible <br...@clisp.org>
Fix getdtablesize() on mingw.
* lib/getdtablesize.c (getdtablesize): Implement differently.
* lib/unistd.in.h (getdtablesize): Improve comment.
--- lib/getdtablesize.c.orig 2009-08-23 21:52:22.000000000 +0200
+++ lib/getdtablesize.c 2009-08-23 21:47:39.000000000 +0200
@@ -1,5 +1,5 @@
/* getdtablesize() function for platforms that don't have it.
- Copyright (C) 2008 Free Software Foundation, Inc.
+ Copyright (C) 2008-2009 Free Software Foundation, Inc.
Written by Bruno Haible <br...@clisp.org>, 2008.
This program is free software: you can redistribute it and/or modify
@@ -24,10 +24,40 @@
#include <stdio.h>
+/* Cache for the previous getdtablesize () result. */
+static int dtablesize;
+
int
getdtablesize (void)
{
- return _getmaxstdio ();
+ if (dtablesize == 0)
+ {
+ /* We are looking for the number N such that the valid file descriptors
+ are 0..N-1. It can be obtained through a loop as follows:
+ {
+ int fd;
+ for (fd = 3; fd < 65536; fd++)
+ if (dup2 (0, fd) == -1)
+ break;
+ return fd;
+ }
+ On Windows XP, the result is 2048.
+ The drawback of this loop is that it allocates memory for a libc
+ internal array that is never freed.
+
+ The number N can also be obtained as the upper bound for
+ _getmaxstdio (). _getmaxstdio () returns the maximum number of open
+ FILE objects. The sanity check in _setmaxstdio reveals the maximum
+ number of file descriptors. This too allocates memory, but it is
+ freed when we call _setmaxstdio with the original value. */
+ int orig_max_stdio = _getmaxstdio ();
+ unsigned int bound;
+ for (bound = 0x10000; _setmaxstdio (bound) < 0; bound = bound / 2)
+ ;
+ _setmaxstdio (orig_max_stdio);
+ dtablesize = bound;
+ }
+ return dtablesize;
}
#endif
--- lib/unistd.in.h.orig 2009-08-23 21:52:23.000000000 +0200
+++ lib/unistd.in.h 2009-08-23 21:51:22.000000000 +0200
@@ -328,7 +328,8 @@
#if @GNULIB_GETDTABLESIZE@
# if !...@have_getdtablesize@
-/* Return the maximum number of file descriptors in the current process. */
+/* Return the maximum number of file descriptors in the current process.
+ In POSIX, this is same as sysconf (_SC_OPEN_MAX). */
extern int getdtablesize (void);
# endif
#elif defined GNULIB_POSIXCHECK
