‘seek’ now accepts ‘SEEK_DATA’ and ‘SEEK_HOLE’ where supported.

* libguile/ports.c (scm_seek): Let SEEK_DATA and SEEK_HOLE through.
(scm_init_ice_9_ports): Define ‘SEEK_DATA’ and ‘SEEK_HOLE’.
* module/ice-9/ports.scm: Export ‘SEEK_DATA’ and ‘SEEK_HOLE’ when
defined.
* test-suite/tests/ports.test ("size of sparse file")
("SEEK_DATA while on data", "SEEK_DATA while in hole")
("SEEK_HOLE while in hole"): New tests.
* NEWS: Update.

NEWS

@@ -30,6 +30,13 @@ and 'never, with 'auto being the default.
 
 This speeds up copying large files a lot while saving the disk space.
 
+** 'seek' can now navigate holes in sparse files
+
+On systems that support it, such as GNU/Linux, the new SEEK_DATA and
+SEEK_HOLE values can now be passed to the 'seek' procedure to change
+file offset to the next piece of data or to the next hole in sparse
+files.  See "Random Access" in the manual for details.
+
 * Bug fixes
 
 ** (ice-9 suspendable-ports) incorrect UTF-8 decoding

doc/ref/api-io.texi

@@ -823,8 +823,26 @@ Seek from the current position.
 @defvar SEEK_END
 Seek from the end of the file.
 @end defvar
-If @var{fd_port} is a file descriptor, the underlying system
+
-call is @code{lseek}.  @var{port} may be a string port.
+On systems that support it, such as GNU/Linux, the following
+constants can be used for @var{whence} to navigate ``holes'' in
+sparse files:
+@defvar SEEK_DATA
+Seek to the next location in the file greater than or equal to
+@var{offset} containing data.  If @var{offset} points to data,
+then the file offset is set to @var{offset}.
+@end defvar
+@defvar SEEK_HOLE
+Seek to the next hole in the file greater than or equal to the
+@var{offset}.  If @var{offset} points into the middle of a hole,
+then the file offset is set to @var{offset}.  If there is no hole
+past @var{offset}, then the file offset is adjusted to the end of
+the file---i.e., there is an implicit hole at the end of any file.
+@end defvar
+
+If @var{fd_port} is a file descriptor, the underlying system call
+is @code{lseek} (@pxref{File Position Primitive,,, libc, The GNU C
+Library Reference Manual}).  @var{port} may be a string port.
 
 The value returned is the new position in @var{fd_port}.  This means
 that the current position of a port can be obtained using:

libguile/ports.c

@@ -1,4 +1,4 @@
-/* Copyright 1995-2001,2003-2004,2006-2019,2021
+/* Copyright 1995-2001,2003-2004,2006-2019,2021,2024
      Free Software Foundation, Inc.
 
    This file is part of Guile.
@@ -3713,8 +3713,25 @@ SCM_DEFINE (scm_seek, "seek", 3, 0, 0,
 	    "@defvar SEEK_END\n"
 	    "Seek from the end of the file.\n"
 	    "@end defvar\n"
-	    "If @var{fd_port} is a file descriptor, the underlying system\n"
+            "On systems that support it, such as GNU/Linux, the following\n"
-	    "call is @code{lseek}.  @var{port} may be a string port.\n"
+            "constants can be used for @var{whence} to navigate ``holes'' in\n"
+            "sparse files:\n"
+            "@defvar SEEK_DATA\n"
+            "Seek to the next location in the file greater than or equal to\n"
+            "@var{offset} containing data.  If @var{offset} points to data,\n"
+            "then the file offset is set to @var{offset}.\n"
+            "@end defvar\n"
+            "@defvar SEEK_HOLE\n"
+            "Seek to the next hole in the file greater than or equal to the\n"
+            "@var{offset}.  If @var{offset} points into the middle of a hole,\n"
+            "then the file offset is set to @var{offset}.  If there is no hole\n"
+            "past @var{offset}, then the file offset is adjusted to the end of\n"
+            "the file---i.e., there is an implicit hole at the end of any file.\n"
+            "@end defvar\n"
+            "\n"
+            "If @var{fd_port} is a file descriptor, the underlying system call\n"
+            "is @code{lseek} (@pxref{File Position Primitive,,, libc, The GNU C\n"
+            "Library Reference Manual}).  @var{port} may be a string port.\n"
             "\n"
 	    "The value returned is the new position in the file.  This means\n"
 	    "that the current position of a port can be obtained using:\n"
@@ -3728,7 +3745,14 @@ SCM_DEFINE (scm_seek, "seek", 3, 0, 0,
   fd_port = SCM_COERCE_OUTPORT (fd_port);
 
   how = scm_to_int (whence);
-  if (how != SEEK_SET && how != SEEK_CUR && how != SEEK_END)
+  if (how != SEEK_SET && how != SEEK_CUR && how != SEEK_END
+#ifdef SEEK_DATA
+      && how != SEEK_DATA
+#endif
+#ifdef SEEK_HOLE
+      && how != SEEK_HOLE
+#endif
+      )
     SCM_OUT_OF_RANGE (3, whence);
 
   if (SCM_OPPORTP (fd_port))
@@ -4151,6 +4175,14 @@ scm_init_ice_9_ports (void)
   scm_c_define ("SEEK_CUR", scm_from_int (SEEK_CUR));
   scm_c_define ("SEEK_END", scm_from_int (SEEK_END));
 
+  /* Support for sparse files (glibc).  */
+#ifdef SEEK_DATA
+  scm_c_define ("SEEK_DATA", scm_from_int (SEEK_DATA));
+#endif
+#ifdef SEEK_HOLE
+  scm_c_define ("SEEK_HOLE", scm_from_int (SEEK_HOLE));
+#endif
+
   scm_c_define ("%current-input-port-fluid", cur_inport_fluid);
   scm_c_define ("%current-output-port-fluid", cur_outport_fluid);
   scm_c_define ("%current-error-port-fluid", cur_errport_fluid);

module/ice-9/ports.scm

@@ -1,5 +1,5 @@
 ;;; Ports
-;;; Copyright (C) 2016,2019,2021 Free Software Foundation, Inc.
+;;; Copyright (C) 2016,2019,2021,2024 Free Software Foundation, Inc.
 ;;;
 ;;; This library is free software: you can redistribute it and/or modify
 ;;; it under the terms of the GNU Lesser General Public License as
@@ -153,6 +153,12 @@
 (load-extension (string-append "libguile-" (effective-version))
                 "scm_init_ice_9_ioext")
 
+(eval-when (load eval expand)
+  (when (defined? 'SEEK_DATA)
+    (module-export! (current-module) '(SEEK_DATA)))
+  (when (defined? 'SEEK_HOLE)
+    (module-export! (current-module) '(SEEK_HOLE))))
+
 
 
 (define (port-encoding port)

test-suite/tests/ports.test

@@ -1,8 +1,8 @@
 ;;;; ports.test --- Guile I/O ports.    -*- coding: utf-8; mode: scheme; -*-
 ;;;; Jim Blandy <jimb@red-bean.com> --- May 1999
 ;;;;
-;;;; 	Copyright (C) 1999, 2001, 2004, 2006, 2007, 2009, 2010,
+;;;; 	Copyright (C) 1999, 2001, 2004, 2006-2007, 2009-2015, 2017, 2019-2021,
-;;;;      2011, 2012, 2013, 2014, 2015, 2017, 2019, 2020, 2021 Free Software Foundation, Inc.
+;;;;      2024 Free Software Foundation, Inc.
 ;;;;
 ;;;; This library is free software; you can redistribute it and/or
 ;;;; modify it under the terms of the GNU Lesser General Public
@@ -185,6 +185,52 @@
     (close-port iport))
   (delete-file filename))
 
+(let* ((file (test-file))
+       (port (open-output-file file)))
+  (seek port 4096 SEEK_SET)
+  (display "bye." port)
+  (close-port port)
+
+  (pass-if-equal "size of sparse file"
+      4100
+    (stat:size (stat file)))
+
+  (pass-if-equal "SEEK_DATA while on data"
+      4096
+    (if (defined? 'SEEK_DATA)
+        (call-with-input-file file
+          (lambda (port)
+            (catch 'system-error
+              (lambda ()
+                (seek port 4096 SEEK_DATA))
+              (lambda _
+                (throw 'unresolved)))))
+        (throw 'unresolved)))
+
+  (pass-if-equal "SEEK_DATA while in hole"
+      4096
+    (if (defined? 'SEEK_DATA)
+        (call-with-input-file file
+          (lambda (port)
+            (catch 'system-error
+              (lambda ()
+                (seek port 10 SEEK_DATA))
+              (lambda _
+                (throw 'unresolved)))))
+        (throw 'unresolved)))
+
+  (pass-if-equal "SEEK_HOLE while in hole"
+      10
+    (if (defined? 'SEEK_HOLE)
+        (call-with-input-file file
+          (lambda (port)
+            (catch 'system-error
+              (lambda ()
+                (seek port 10 SEEK_HOLE))
+              (lambda _
+                (throw 'unresolved)))))
+        (throw 'unresolved))))
+
 ;;; unusual characters.
 (let* ((filename (test-file))
        (port (open-output-file filename)))