1 files changed, 145 insertions, 0 deletions
diff --git a/lib/freadseek.c b/lib/freadseek.c
new file mode 100644
index 0000000..7c5751f
--- /dev/null
+++ b/lib/freadseek.c
@@ -0,0 +1,145 @@
+/* Skipping input from a FILE stream.
+   Copyright (C) 2007-2023 Free Software Foundation, Inc.
+
+   This file is free software: you can redistribute it and/or modify
+   it under the terms of the GNU Lesser General Public License as
+   published by the Free Software Foundation; either version 2.1 of the
+   License, or (at your option) any later version.
+
+   This file 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 Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public License
+   along with this program.  If not, see <https://www.gnu.org/licenses/>.  */
+
+#include <config.h>
+
+/* Specification.  */
+#include "freadseek.h"
+
+#include <stdlib.h>
+#include <unistd.h>
+
+#include "freadahead.h"
+#include "freadptr.h"
+
+#include "stdio-impl.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 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_EOF_SEEN || defined _IO_ftrylockfile || __GNU_LIBRARY__ == 1
+  /* GNU libc, BeOS, Haiku, Linux libc5 */
+  fp->_IO_read_ptr += increment;
+#elif defined __sferror || defined __DragonFly__ || defined __ANDROID__
+  /* FreeBSD, NetBSD, OpenBSD, DragonFly, Mac OS X, Cygwin, Minix 3, Android */
+  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, UnixWare, mingw, MSVC, NonStop Kernel, OpenVMS */
+  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.  */
+  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 fseeko (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)
+            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;
+}