Eric Blake wrote:
> the real bug was that freadseek was incrementing the in-memory pointer
> beyond the bounds of the ungetc buffer).
True. But the use of freadahead was not entirely wrong. Because when the
stream is in ungetc state, what you don't want is:
- read the ungetc buffer (1 byte) with freadptr,
- use system calls like lseek and fseek for the rest.
What you want is:
- read the ungetc buffer (1 byte) with freadptr,
- read the main buffer (many hundred bytes) with freadptr,
- use system calls like lseek and fseek for the rest.
This implements this improvement.
2008-03-30 Bruno Haible <[EMAIL PROTECTED]>
Improve freadseek's efficiency after ungetc.
* lib/freadseek.c: Include freadahead.h.
(freadptrinc): New function, extracted from freadseek.
(freadseek): Use it in a loop. Use freadahead to determine the number
of loop iterations.
* modules/freadseek (Depends-on): Add freadahead.
(configure.ac): Require AC_C_INLINE.
*** lib/freadseek.c.orig 2008-03-30 12:18:51.000000000 +0200
--- lib/freadseek.c 2008-03-30 12:18:38.000000000 +0200
***************
*** 22,49 ****
#include <stdlib.h>
#include <unistd.h>
#include "freadptr.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).
*/
! if (freadptr (fp, &buffered) != NULL && buffered > 0)
! {
! size_t increment = (buffered < offset ? buffered : offset);
!
! /* Keep this code in sync with 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; \
--- 22,42 ----
#include <stdlib.h>
#include <unistd.h>
+ #include "freadahead.h"
#include "freadptr.h"
! /* Increment the in-memory pointer. INCREMENT must be at most the buffer size
! returned by freadptr().
! This is very cheap (no system calls). */
! static inline void
! freadptrinc (FILE *fp, size_t increment)
{
! /* Keep this code in sync with 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; \
***************
*** 53,79 ****
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. */
--- 46,108 ----
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
+ }
+
+ int
+ freadseek (FILE *fp, size_t offset)
+ {
+ size_t total_buffered;
+ int fd;
+
+ if (offset == 0)
+ return 0;
+
+ /* Seek over the already read and buffered input as quickly as possible,
+ without doing any system calls. */
+ total_buffered = freadahead (fp);
+ /* This loop is usually executed at most twice: once for ungetc buffer (if
+ present) and once for the main buffer. */
+ while (total_buffered > 0)
+ {
+ size_t buffered;
! if (freadptr (fp, &buffered) != NULL && buffered > 0)
! {
! size_t increment = (buffered < offset ? buffered : offset);
!
! freadptrinc (fp, increment);
! offset -= increment;
! if (offset == 0)
! return 0;
! total_buffered -= increment;
! if (total_buffered == 0)
! break;
! }
! /* Read one byte. If we were reading from the ungetc buffer, this
! switches the stream back to the main buffer. */
! if (fgetc (fp) == EOF)
! goto eof;
! offset--;
if (offset == 0)
return 0;
+ total_buffered--;
}
/* Test whether the stream is seekable or not. */
***************
*** 93,110 ****
{
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;
}
}
--- 122,140 ----
{
size_t count = (sizeof (buf) < offset ? sizeof (buf) : offset);
if (fread (buf, 1, count, fp) < count)
! goto eof;
offset -= count;
}
while (offset > 0);
return 0;
}
+
+ eof:
+ /* EOF, or error before or while reading. */
+ if (ferror (fp))
+ return EOF;
+ else
+ /* Encountered EOF. */
+ return 0;
}
*** modules/freadseek.orig 2008-03-30 12:18:52.000000000 +0200
--- modules/freadseek 2008-03-30 11:50:52.000000000 +0200
***************
*** 6,15 ****
--- 6,17 ----
lib/freadseek.c
Depends-on:
+ freadahead
freadptr
lseek
configure.ac:
+ AC_REQUIRE([AC_C_INLINE])
Makefile.am:
lib_SOURCES += freadseek.c
