On 25/02/11 17:29, Paul Eggert wrote: > On 02/25/2011 04:19 AM, Pádraig Brady wrote: > >> Attached is a proposed solution to this. > > My kneejerk reaction is that it tries to do too much inferring, > and ends up being more complicated than giving the user more control. > > If we're going to change the default to be not compatible with POSIX, > we need to give the user a way to get the POSIX behavior, something > that's less subtle than POSIXLY_CORRECT. I suggest that we add > a new option that is the inverse of "fullblock". We can call it > "partblock", say. > > Then, we can say that the default is "fullblock" normally, but it is > "partblock" if POSIXLY_CORRECT and if bs= is given and if no conversions > other than sync, noerror, and notrunk are given. > > Anyway, I'm just thinking out loud to some extent, and further > comments are welcome.
Hmm, it's better to be explicit but I think defaulting to "fullblock" is too risky. As an interim step at least, how about just warning as per the attached. cheers, Pádraig.
>From 987438bfe5f7a38b7736f0aaca9d8377b4281408 Mon Sep 17 00:00:00 2001 From: =?UTF-8?q?P=C3=A1draig=20Brady?= <[email protected]> Date: Fri, 25 Feb 2011 12:27:25 +0000 Subject: [PATCH] dd: suggest iflag=fullblock where appropriate * NEWS: Mention the change in behavior. * doc/coreutils.texi: Document when iflag=fullblock is auto suggested. src/dd.c (scan_args): Suggest O_FULLBLOCK when probably needed. --- NEWS | 6 ++++++ doc/coreutils.texi | 3 +++ src/dd.c | 32 ++++++++++++++++++++++++++++++++ 3 files changed, 41 insertions(+), 0 deletions(-) diff --git a/NEWS b/NEWS index a367d8d..e82bc02 100644 --- a/NEWS +++ b/NEWS @@ -8,6 +8,12 @@ GNU coreutils NEWS -*- outline -*- delimiter and an unbounded range like "-f1234567890-". [bug introduced in coreutils-5.3.0] +** Changes in behavior + + dd now suggests using iflag=fullblock with oflag=direct or conv=sync + where short reads can have adverse effects. This option is also + suggested if the user indicates a specific amount of data to read. + * Noteworthy changes in release 8.10 (2011-02-04) [stable] diff --git a/doc/coreutils.texi b/doc/coreutils.texi index ea35afe..da8e80b 100644 --- a/doc/coreutils.texi +++ b/doc/coreutils.texi @@ -8216,6 +8216,9 @@ Accumulate full blocks from input. The @code{read} system call may return early if a full block is not available. When that happens, continue calling @code{read} to fill the remainder of the block. +Using this flag is suggested when @samp{cio}, @samp{direct} +or @samp{conv=sync} are used, or when a specific amount of input +data is specfied. This flag can be used only with @code{iflag}. @end table diff --git a/src/dd.c b/src/dd.c index daddc1e..1a0e177 100644 --- a/src/dd.c +++ b/src/dd.c @@ -1075,6 +1075,38 @@ scanargs (int argc, char *const *argv) conversions_mask |= C_TWOBUFS; } + if (!(input_flags & O_FULLBLOCK) && !getenv ("POSIXLY_CORRECT")) + { + bool fb_warned = false; + /* Suggest 'fullblock' as one wouldn't want random + padding applied, when reading from a pipe for example. */ + if (conversions_mask & C_SYNC) + { + error (0, 0, _("warning: 'iflag=fullblock' is suggested when " + "padding input blocks")); + fb_warned = true; + } + /* Suggest 'fullblock' with 'direct' or 'cio' as again if reading from + a pipe, we're constrained in how we write to output. */ + if ((input_flags | output_flags) & (O_DIRECT | O_CIO)) + { + error (0, 0, _("warning: 'iflag=fullblock' is suggested when " + "writing with direct I/O constraints")); + fb_warned = true; + } + /* Suggest 'fullblock' if we're reading a specific number of blocks, + with a specific block size. */ + if (max_records && max_records != (uintmax_t) -1 && input_blocksize) + { + error (0, 0, _("warning: 'iflag=fullblock' is suggested when " + "reading a specific amount")); + fb_warned = true; + } + + if (fb_warned) + error (0, 0, _("Set POSIXLY_CORRECT to disable this warning")); + } + if (input_blocksize == 0) input_blocksize = DEFAULT_BLOCKSIZE; if (output_blocksize == 0) -- 1.7.4
