new module 'freadseek'

[Bruno Haible]

Here's the side-effecting companion of freadptr.

2008-02-29  Bruno Haible  <address@hidden>

        New module 'freadseek'.
        * modules/freadseek: New file.
        * lib/freadseek.h: New file.
        * lib/freadseek.c: New file.
        * MODULES.html.sh (File stream based Input/Output): Add freadseek.

========================== modules/freadseek ===============================
Description:
freadseek() function: Read and discard input from a stream.

Files:
lib/freadseek.h
lib/freadseek.c

Depends-on:
freadahead
lseek

configure.ac:

Makefile.am:
lib_SOURCES += freadseek.c

Include:
"freadseek.h"

License:
LGPL

Maintainer:
Bruno Haible

=========================== lib/freadseek.h ================================
/* Skipping input from a FILE stream.
   Copyright (C) 2007-2008 Free Software Foundation, Inc.

   This program is free software: you can redistribute it and/or modify
   it under the terms of the GNU General Public License as published by
   the Free Software Foundation; either version 3 of the License, or
   (at your option) any later version.

   This program is distributed in the hope that it will be useful,
   but WITHOUT ANY WARRANTY; without even the implied warranty of
   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
   GNU General Public License for more details.

   You should have received a copy of the GNU General Public License
   along with this program.  If not, see <http://www.gnu.org/licenses/>.  */

#include <stddef.h>
#include <stdio.h>

#ifdef __cplusplus
extern "C" {
#endif

/* Assuming the stream STREAM is open for reading:

   Read and discard OFFSET bytes from STREAM.

   freadseek (STREAM, OFFSET) is the same as
   fseek (STREAM, OFFSET, SEEK_CUR), except that the latter does not work
   on non-seekable input streams (such as pipes).

   Upon success, return 0.
   Upon premature end of stream, return 0 (like fseek does).
   Upon error, set the error indicator in the stream and return EOF.

   STREAM must not be wide-character oriented.  */

extern int freadseek (FILE *stream, size_t offset);

#ifdef __cplusplus
}
#endif
=========================== lib/freadseek.c ================================
/* Skipping input from a FILE stream.
   Copyright (C) 2007-2008 Free Software Foundation, Inc.

   This program is free software: you can redistribute it and/or modify
   it under the terms of the GNU General Public License as published by
   the Free Software Foundation; either version 3 of the License, or
   (at your option) any later version.

   This program is distributed in the hope that it will be useful,
   but WITHOUT ANY WARRANTY; without even the implied warranty of
   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
   GNU General Public License for more details.

   You should have received a copy of the GNU General Public License
   along with this program.  If not, see <http://www.gnu.org/licenses/>.  */

#include <config.h>

/* Specification.  */
#include "freadseek.h"

#include <stdlib.h>
#include <unistd.h>

#include "freadahead.h"

int
freadseek (FILE *fp, size_t offset)
{
  size_t buffered;
  int fd;

  if (offset == 0)
    return 0;

  /* Increment the in-memory pointer.  This is very cheap (no system calls).  */
  buffered = freadahead (fp);
  if (buffered > 0)
    {
      size_t increment = (buffered < offset ? buffered : offset);

      /* Keep this code in sync with freadahead and freadptr!  */
#if defined _IO_ferror_unlocked     /* GNU libc, BeOS */
      fp->_IO_read_ptr += increment;
#elif defined __sferror             /* FreeBSD, NetBSD, OpenBSD, MacOS X, 
Cygwin */
      fp->_p += increment;
      fp->_r -= increment;
#elif defined _IOERR                /* AIX, HP-UX, IRIX, OSF/1, Solaris, mingw 
*/
# if defined __sun && defined _LP64 /* Solaris/{SPARC,AMD64} 64-bit */
#  define fp_ ((struct { unsigned char *_ptr; \
                         unsigned char *_base; \
                         unsigned char *_end; \
                         long _cnt; \
                         int _file; \
                         unsigned int _flag; \
                       } *) fp)
      fp_->_ptr += increment;
      fp_->_cnt -= increment;
# else
      fp->_ptr += increment;
      fp->_cnt -= increment;
# endif
#elif defined __UCLIBC__            /* uClibc */
# ifdef __STDIO_BUFFERS
      fp->__bufpos += increment;
# else
      abort ();
# endif
#elif defined __QNX__               /* QNX */
      fp->_Next += increment;
#else
 #error "Please port gnulib freadseek.c to your platform! Look at the 
definition of getc, getc_unlocked on your system, then report this to 
bug-gnulib."
#endif

      offset -= increment;
      if (offset == 0)
        return 0;
    }

  /* Test whether the stream is seekable or not.  */
  fd = fileno (fp);
  if (fd >= 0 && lseek (fd, 0, SEEK_CUR) >= 0)
    {
      /* FP refers to a regular file.  fseek is most efficient in this case.  */
      return fseek (fp, offset, SEEK_CUR);
    }
  else
    {
      /* FP is a non-seekable stream, possibly not even referring to a file
         descriptor.  Read OFFSET bytes explicitly and discard them.  */
      char buf[4096];

      do
        {
          size_t count = (sizeof (buf) < offset ? sizeof (buf) : offset);
          if (fread (buf, 1, count, fp) < count)
            {
              if (ferror (fp))
                /* EOF, or error before or while reading.  */
                return EOF;
              else
                /* Encountered EOF.  */
                return 0;
            }
          offset -= count;
        }
      while (offset > 0);

      return 0;
   }
}