[man-pages] Misleading example in ldd(1).

[man-pages] Misleading example in ldd(1).

[Emanuele Torre]

Hello.

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.

Re: [man-pages] Misleading example in ldd(1).

[Alejandro Colomar (man-pages)]

Hi Emanuele,

On 8/6/21 4:26 PM, Emanuele Torre wrote:
> 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.

Thanks for the report!  I decided to remove the pipe for simplicity.

Cheers,

Alex


-- 
Alejandro Colomar
Linux man-pages comaintainer; https://www.kernel.org/doc/man-pages/
http://www.alejandro-colomar.es/