最後(たぶん)
freadseek.c
/* Skipping input from a FILE stream.
Copyright (C) 2007-2014 Free Software Foundation, Inc.
(略)
/* Increment the in-memory pointer. INCREMENT must be at most the buffer size
returned by freadptr().
This is very cheap (no system calls). */
static void
freadptrinc (FILE *fp, size_t increment)
{
/* Keep this code in sync with freadptr! */
#if HAVE___FREADPTRINC /* musl libc */
__freadptrinc (fp, increment);
#elif defined _IO_ftrylockfile || __GNU_LIBRARY__ == 1 /* GNU libc, BeOS, Haiku, Linux libc5 */
fp->_IO_read_ptr += increment;
#elif defined __sferror || defined __DragonFly__ /* FreeBSD, NetBSD, OpenBSD, DragonFly, Mac OS X, Cygwin */
fp_->_p += increment;
fp_->_r -= increment;
#elif defined __EMX__ /* emx+gcc */
fp->_ptr += increment;
fp->_rcount -= increment;
#elif defined __minix /* Minix */
fp_->_ptr += increment;
fp_->_count -= increment;
#elif defined _IOERR /* AIX, HP-UX, IRIX, OSF/1, Solaris, OpenServer, mingw, NonStop Kernel */
fp_->_ptr += increment;
fp_->_cnt -= increment;
#elif defined __UCLIBC__ /* uClibc */
# ifdef __STDIO_BUFFERS
fp->__bufpos += increment;
# else
abort ();
# endif
#elif defined __QNX__ /* QNX */
fp->_Next += increment;
#elif defined __MINT__ /* Atari FreeMiNT */
fp->__bufp += increment;
#elif defined EPLAN9 /* Plan9 */
fp->rp += increment;
#elif defined SLOW_BUT_NO_HACKS /* users can define this */
#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. */
(略)
eof:
/* EOF, or error before or while reading. */
if (ferror (fp))
return EOF;
else
/* Encountered EOF. */
return 0;
}
freadprt.c
/* Retrieve information about a FILE stream.
Copyright (C) 2007-2014 Free Software Foundation, Inc.
(略)
const char *
freadptr (FILE *fp, size_t *sizep)
{
size_t size;
/* Keep this code in sync with freadahead! */
#if defined _IO_ftrylockfile || __GNU_LIBRARY__ == 1 /* GNU libc, BeOS, Haiku, Linux libc5 */
if (fp->_IO_write_ptr > fp->_IO_write_base)
return NULL;
size = fp->_IO_read_end - fp->_IO_read_ptr;
if (size == 0)
return NULL;
*sizep = size;
return (const char *) fp->_IO_read_ptr;
#elif defined __sferror || defined __DragonFly__ /* FreeBSD, NetBSD, OpenBSD, DragonFly, Mac OS X, Cygwin */
if ((fp_->_flags & __SWR) != 0 || fp_->_r < 0)
return NULL;
size = fp_->_r;
if (size == 0)
return NULL;
*sizep = size;
return (const char *) fp_->_p;
(略)
#elif defined SLOW_BUT_NO_HACKS /* users can define this */
/* This implementation is correct on any ANSI C platform. It is just
awfully slow. */
return NULL;
#else
#error "Please port gnulib freadptr.c to your platform! Look at the definition of fflush, fread, getc, getc_unlocked on your system, then report this to bug-gnulib."
#endif
}
ということで、cut が gawk より遅い云々はたぶんこの辺りが原因。
gawk はたしかバッファ一杯にごそっと読み込んでからレコードを切り出して、
それからごにょごにょやってるから。
あと、出力のときにもフィールド指定の具合によっては一バイトずつ読んでは…というのをやってたような。
デザインフィル ミドリ 2015年版フラットダイアリーライト 黒 2014年10月始まり A5 27362006
More Programming Pearls: Confessions of a Coder (ACM Press)
Programming Pearls (ACM Press)
