Re: [PATCH 22/23] ldd.1: Fix example command

["Michael Kerrisk (man-pages)" <mtk.manpages@gmail.com>]

Hi Emanuele and Alex,

On 8/8/21 10:41 AM, Alejandro Colomar wrote:
> Emanuele Torre via linux-man@:
> 
> [
> I was reading the man page for ldd(1)[1]; and I read this in the first
> paragraph of the DECRIPTION section:
> 
>  ldd prints the shared objects (shared libraries) required by each
>  program or shared object specified on the command line.  An
>  example of its use and output (using sed(1) to trim leading white
>  space for readability in this page) is the following:
> 
>      $ ldd /bin/ls | sed 's/^ */    /'
>          linux-vdso.so.1 (0x00007ffcc3563000)
>          libselinux.so.1 => /lib64/libselinux.so.1 (0x00007f87e5459000)
>          libcap.so.2 => /lib64/libcap.so.2 (0x00007f87e5254000)
>          libc.so.6 => /lib64/libc.so.6 (0x00007f87e4e92000)
>          libpcre.so.1 => /lib64/libpcre.so.1 (0x00007f87e4c22000)
>          libdl.so.2 => /lib64/libdl.so.2 (0x00007f87e4a1e000)
>          /lib64/ld-linux-x86-64.so.2 (0x00005574bf12e000)
>          libattr.so.1 => /lib64/libattr.so.1 (0x00007f87e4817000)
>          libpthread.so.0 => /lib64/libpthread.so.0 (0x00007f87e45fa000)
> 
> This is a little confusing though since that sed(1) command does not
> seem to work. (and also potentially misleading for someone who is trying
> figure out how to parse ldd(1)'s output.)
> 
> ldd(1) prepends a TAB character (0x09) to each line, not spaces:
> 
>  $ ldd /bin/ls | xxd | head -1
>  00000000: 096c 696e 7578 2d76 6473 6f2e 736f 2e31  .linux-vdso.so.1
> 
> I read ldd(1)'s source code[2] (it is part of glibc) and it seems to be
> a bash script that tries to use different rtld programs ( ld.so(8) )
> from an RTLDLIST.
> 
> Those, on my system, are:
> 
>  * /usr/lib/ld-linux.so.2
>  * /usr/lib64/ld-linux-x86-64.so.2
>  * /usr/libx32/ld-linux-x32.so.2
> 
> And they all seem to also be part of glibc.
> 
> I have tried to follow the git history of glibc to see when the switch
> from spaces to the TAB character occured, but, to me, it seems like
> glibc.git/elf/rtld.c has always used '\t'; at since
> 6a76c115150318eae5d02eca76f2fc03be7bd029[3] (358th commit since glibc
> started using the git repository repository - Nov 18th 1995): before
> that commit there are not any results for `git grep '\\t'` in the elf
> directory and I did not investigate further.
> 
> Still, at the time of that commit, glibc did not seem to have an ldd(1)
> utility.
> 
> Perhaps the man page is old and its original author was using and
> documenting an ldd(1) utility that was not part of glibc when he was
> writing it.
> 
> Anyhow, since I think that sed(1) command will not work on any system
> that uses, at least, the most recent version of glibc (because lld(1)
> and the ld.so(8) programs it depends on are all part of glibc), I think
> that that example should be changed to avoid confusions.
> 
> The output format of ldd(1) does not seem to be clearly defined, so I
> think this would be a good option:
> 
>  $ ldd /bin/ls | sed 's/^[[:space:]]*/    /'
> 
> NB: ^\s* should also work on most GNU/Linux systems, but \s is
>     non-standard or documented so I don not suggest using it in the man
>     page.
> 
> Another option could be to remove "the pipe to sed(1)" part and the note
> in parentheses that explains why it was used by the original author.
> 
> Cheers.
>  emanuele6
> 
> [1]: https://man7.org/linux/man-pages/man1/ldd.1.html
> [2]: https://sourceware.org/git/?p=glibc.git;a=blob;f=elf/ldd.bash.in;h=ba736464ac5e4a9390b1b6a39595035238250232;hb=5188a9d0265cc6f7235a8af1d31ab02e4a24853d
> [3]: https://sourceware.org/git/?p=glibc.git;a=commit;h=6a76c115150318eae5d02eca76f2fc03be7bd029
> 
> ///////
> 
>  $ uname -a
>  Linux t420 5.10.54-1-lts #1 SMP Wed, 28 Jul 2021 15:05:20 +0000
> x86_64 GNU/Linux
>  $ pacman -Qo ldd
>  /usr/bin/ldd is owned by glibc 2.33-5
>  $ pacman -Qo /usr/share/man/man1/ldd.1.gz
>  /usr/share/man/man1/ldd.1.gz is owned by man-pages 5.12-2
>  $ pacman -Qo /usr/lib/ld-linux.so.2
>  /usr/lib/ld-linux.so.2 is owned by lib32-glibc 2.33-5
>  $ pacman -Qo /usr/lib64/ld-linux-x86-64.so.2
>  /usr/lib/ld-linux-x86-64.so.2 is owned by glibc 2.33-5
>  $ pacman -F /usr/libx32/ld-linux-x32.so.2 || echo not available on arch linux.
>  not available on arch linux.
> ]
> 
> Reported-by: EmanueleTorre <torreemanuele6@gmail.com>
> Signed-off-by: Alejandro Colomar <alx.manpages@gmail.com>

Emanuele, thanks for all of the details.
Alex, thanks for the patch.

Applied.

Cheers,

Michael

> ---
>  man1/ldd.1 | 5 +----
>  1 file changed, 1 insertion(+), 4 deletions(-)
> 
> diff --git a/man1/ldd.1 b/man1/ldd.1
> index ed1ab3c05..8bb1410d5 100644
> --- a/man1/ldd.1
> +++ b/man1/ldd.1
> @@ -23,14 +23,11 @@ ldd \- print shared object dependencies
>  prints the shared objects (shared libraries) required by each program or
>  shared object specified on the command line.
>  An example of its use and output
> -(using
> -.BR sed (1)
> -to trim leading white space for readability in this page)
>  is the following:
>  .PP
>  .in +4n
>  .EX
> -$ \fBldd /bin/ls | sed \(aqs/^ */    /\(aq\fP
> +$ \fBldd /bin/ls\fP
>      linux\-vdso.so.1 (0x00007ffcc3563000)
>      libselinux.so.1 => /lib64/libselinux.so.1 (0x00007f87e5459000)
>      libcap.so.2 => /lib64/libcap.so.2 (0x00007f87e5254000)
> 


-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/