[libfortran,committed] Use lseek() to figure out if a file is seekable

Submitted by Janne Blomqvist on June 11, 2011, 10:22 a.m.

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Message ID BANLkTi=qdhB72XqzvXsxhHGq7bi46zUKjA@mail.gmail.com
State New
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Commit Message

Janne Blomqvist June 11, 2011, 10:22 a.m.
Hi,

currently we try to figure out whether a fd is seekable by looking at
the mode returned by fstat(). Turns out this is not enough, as there
are some character special files that are seekable. Thus, change the
logic to try an lseek().

Committed as obvious.

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Index: ChangeLog
===================================================================
--- ChangeLog   (revision 174945)
+++ ChangeLog   (working copy)
@@ -1,3 +1,8 @@ 
+2011-06-11  Janne Blomqvist  <jb@gcc.gnu.org>
+
+       * io/unix.c (fd_to_stream): Figure out if a fd is seekable by
+       trying lseek().
+
 2011-06-10  Daniel Carrera  <dcarrera@gmail.com>

        * caf/mpi.c (_gfortran_caf_sync_all,
Index: io/unix.c
===================================================================
--- io/unix.c   (revision 174945)
+++ io/unix.c   (working copy)
@@ -952,15 +952,15 @@  fd_to_stream (int fd)

   if (S_ISREG (statbuf.st_mode))
     s->file_length = statbuf.st_size;
-  else if (S_ISBLK (statbuf.st_mode))
+  else
     {
-      /* Hopefully more portable than ioctl(fd, BLKGETSIZE64, &size)?  */
-      gfc_offset cur = lseek (fd, 0, SEEK_CUR);
+      /* Some character special files are seekable but most are not,
+        so figure it out by trying to seek.  On Linux, /dev/null is
+        an example of such a special file.  */
       s->file_length = lseek (fd, 0, SEEK_END);
-      lseek (fd, cur, SEEK_SET);
+      if (s->file_length > 0)
+       lseek (fd, 0, SEEK_SET);
     }
-  else
-    s->file_length = -1;

   if (!(S_ISREG (statbuf.st_mode) || S_ISBLK (statbuf.st_mode))
       || options.all_unbuffered