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

Michael Kerrisk (man-pages)
2021-08-08T22:33:01Z

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/