On 6/19/19 12:42 PM, Pádraig Brady wrote:
Maybe we should relax the cases we do read() for,
and try to seek in block/character special files,
falling back to read() where that fails?
Sure, that's easy enough. I installed the attached patch and am marking
this bug report as done.
From ea353844d8642b5533cbc713e0cec46addbf3907 Mon Sep 17 00:00:00 2001
From: Paul Eggert <egg...@cs.ucla.edu>
Date: Wed, 19 Jun 2019 18:46:57 -0700
Subject: [PATCH] od: use fseek on non-regular files
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Problem reported by Szőts Ákos (Bug#36291).
* NEWS: Mention this.
* src/od.c (skip): Try fseek even on files that do not have usable
sizes, falling back on fread if fseek fails.
---
NEWS | 3 +++
src/od.c | 8 ++++++--
2 files changed, 9 insertions(+), 2 deletions(-)
diff --git a/NEWS b/NEWS
index d30711bc6..fd0543351 100644
--- a/NEWS
+++ b/NEWS
@@ -36,6 +36,9 @@ GNU coreutils NEWS -*-
outline -*-
** New Features
+ od --skip-bytes now can use lseek even if the input is not a regular
+ file, greatly improving performance in some cases.
+
stat(1) now uses the statx() system call where available, which can
operate more efficiently by only retrieving requested attributes.
stat(1) also supports a new --cached= option to control cache
diff --git a/src/od.c b/src/od.c
index 1a89542ee..75a402004 100644
--- a/src/od.c
+++ b/src/od.c
@@ -1033,6 +1033,8 @@ skip (uintmax_t n_skip)
if (fstat (fileno (in_stream), &file_stats) == 0)
{
+ bool usable_size = usable_st_size (&file_stats);
+
/* The st_size field is valid for regular files.
If the number of bytes left to skip is larger than
the size of the current file, we can decrement n_skip
@@ -1040,8 +1042,7 @@ skip (uintmax_t n_skip)
when st_size is no greater than the block size, because
some kernels report nonsense small file sizes for
proc-like file systems. */
- if (usable_st_size (&file_stats)
- && ST_BLKSIZE (file_stats) < file_stats.st_size)
+ if (usable_size && ST_BLKSIZE (file_stats) < file_stats.st_size)
{
if ((uintmax_t) file_stats.st_size < n_skip)
n_skip -= file_stats.st_size;
@@ -1056,6 +1057,9 @@ skip (uintmax_t n_skip)
}
}
+ else if (!usable_size && fseeko (in_stream, n_skip, SEEK_CUR) == 0)
+ n_skip = 0;
+
/* If it's not a regular file with nonnegative size,
or if it's so small that it might be in a proc-like file system,
position the file pointer by reading. */
--
2.21.0
