Index: configure.in
===================================================================
RCS file: /projects/cvsroot/pgsql/configure.in,v
retrieving revision 1.427
diff -u -r1.427 configure.in
--- configure.in	5 Oct 2005 17:11:45 -0000	1.427
+++ configure.in	8 Oct 2005 13:48:23 -0000
@@ -817,6 +817,7 @@
 PGAC_VAR_INT_TIMEZONE
 AC_FUNC_ACCEPT_ARGTYPES
 PGAC_FUNC_GETTIMEOFDAY_1ARG
+PGAC_FUNC_REAL_PREAD
 
 AC_CHECK_FUNCS([cbrt dlopen fcvt fdatasync getpeereid memmove poll pstat readlink setproctitle setsid sigprocmask symlink sysconf towlower utime utimes waitpid wcstombs])
 
Index: config/c-library.m4
===================================================================
RCS file: /projects/cvsroot/pgsql/config/c-library.m4,v
retrieving revision 1.31
diff -u -r1.31 c-library.m4
--- config/c-library.m4	24 Feb 2005 01:34:45 -0000	1.31
+++ config/c-library.m4	8 Oct 2005 13:48:23 -0000
@@ -297,3 +297,43 @@
 ])dnl AC_CACHE_VAL
 AC_MSG_RESULT([$pgac_cv_printf_arg_control])
 ])# PGAC_FUNC_PRINTF_ARG_CONTROL
+
+# PGAC_FUNC_REAL_PREAD
+# ------------------------------------------
+# Determine if pread exists and that it is not emulated
+#
+AC_DEFUN([PGAC_FUNC_REAL_PREAD],
+[AC_MSG_CHECKING([for verified non-emulated pread])
+AC_CACHE_VAL(pgac_cv_real_pread,
+[AC_TRY_RUN([#include <errno.h>
+#include <unistd.h>
+#include <asm/unistd.h>
+
+#if !defined(linux) || !defined(__GLIBC__)
+int main() { return 1; } /* Don't actually know... */
+#else
+/* Avoid conflict with system header */
+#define pread64 pg_pread
+static inline _syscall5(int,pread64,int,__fd, void *,__buf, size_t, __nbytes,__off_t, __offlow, __off_t, __offhigh);
+
+int main()
+{
+  char buffer[10];
+
+  int err = pread64(-1,buffer,1,1,0);
+
+  if( errno == EBADF )
+    return 0;
+
+  return 1;
+}
+#endif
+],
+[pgac_cv_real_pread=yes],
+[pgac_cv_real_pread=no],
+[pgac_cv_real_pread=cross])
+])dnl AC_CACHE_VAL
+AC_MSG_RESULT([$pgac_cv_real_pread])
+if test x"$pgac_cv_real_pread" = xyes ; then
+  AC_DEFINE(HAVE_REAL_PREAD, 1, [Define to 1 if you have a pread known not to be emulated.])
+fi])# PGAC_FUNC_REAL_PREAD
Index: src/backend/storage/file/fd.c
===================================================================
RCS file: /projects/cvsroot/pgsql/src/backend/storage/file/fd.c,v
retrieving revision 1.120
diff -u -r1.120 fd.c
--- src/backend/storage/file/fd.c	8 Aug 2005 03:11:49 -0000	1.120
+++ src/backend/storage/file/fd.c	8 Oct 2005 13:48:25 -0000
@@ -531,9 +531,11 @@
 	/* delete the vfd record from the LRU ring */
 	Delete(file);
 
+#ifndef HAVE_REAL_PREAD
 	/* save the seek position */
 	vfdP->seekPos = (long) lseek(vfdP->fd, 0L, SEEK_CUR);
 	Assert(vfdP->seekPos != -1L);
+#endif
 
 	/* close the file */
 	if (close(vfdP->fd))
@@ -604,6 +606,7 @@
 			++nfile;
 		}
 
+#ifndef HAVE_REAL_PREAD
 		/* seek to the right position */
 		if (vfdP->seekPos != 0L)
 		{
@@ -612,6 +615,7 @@
 			returnValue = (long) lseek(vfdP->fd, vfdP->seekPos, SEEK_SET);
 			Assert(returnValue != -1L);
 		}
+#endif
 	}
 
 	/*
@@ -1010,8 +1014,15 @@
 	returnCode = FileAccess(file);
 	if (returnCode < 0)
 		return returnCode;
+		
+#ifdef HAVE_REAL_PREAD
+	if( VfdCache[file].seekPos == FileUnknownPos )
+		ereport( ERROR, (errmsg("read() with unknown position")) );
 
+	returnCode = pread(VfdCache[file].fd, buffer, amount, VfdCache[file].seekPos );
+#else
 	returnCode = read(VfdCache[file].fd, buffer, amount);
+#endif
 	if (returnCode > 0)
 		VfdCache[file].seekPos += returnCode;
 	else
@@ -1036,8 +1047,14 @@
 		return returnCode;
 
 	errno = 0;
+#ifdef HAVE_REAL_PREAD
+	if( VfdCache[file].seekPos == FileUnknownPos )
+		ereport( ERROR, (errmsg("write() with unknown position")) );
+		
+	returnCode = pwrite(VfdCache[file].fd, buffer, amount, VfdCache[file].seekPos );
+#else
 	returnCode = write(VfdCache[file].fd, buffer, amount);
-
+#endif
 	/* if write didn't set errno, assume problem is no disk space */
 	if (returnCode != amount && errno == 0)
 		errno = ENOSPC;
@@ -1078,8 +1095,11 @@
 			   file, VfdCache[file].fileName,
 			   VfdCache[file].seekPos, offset, whence));
 
+/* Yes, not pretty. Fact is, with pread we always track the position ourselves */
+#ifndef HAVE_REAL_PREAD
 	if (FileIsNotOpen(file))
 	{
+#endif
 		switch (whence)
 		{
 			case SEEK_SET:
@@ -1101,6 +1121,7 @@
 				elog(ERROR, "invalid whence: %d", whence);
 				break;
 		}
+#ifndef HAVE_REAL_PREAD
 	}
 	else
 	{
@@ -1127,6 +1148,7 @@
 				break;
 		}
 	}
+#endif
 	return VfdCache[file].seekPos;
 }