"useless" librt.so.1 dependency on Linux reported by dpkg-shlibdeps

[David Holmes]

On 19/09/2025 10:21 pm, Baesken, Matthias wrote:
> Hi David,  even  on RHEL   7.9   it works without  adding  -lrt to  linking , see the example   :

It is only pre glibc 2.17 that we need librt.so for.

David

> 
> gcc --version
> gcc (GCC) 4.8.5 20150623 (Red Hat 4.8.5-44)
> Copyright (C) 2015 Free Software Foundation, Inc.
> This is free software; see the source for copying conditions.  There is NO
> warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
> 
> gcc clock_gettime.c -o clock_gettime
> ./clock_gettime
> Elapsed time: 2.008803 seconds
> 
> more ./clock_gettime.c
> #include <stdio.h>
> #include <time.h>
> #include <unistd.h>
> 
> int main() {
>      struct timespec start, end;
>      double elapsed;
> 
>      // Get start time
>      clock_gettime(CLOCK_MONOTONIC, &start);
> 
>      // Simulate some work (sleep for 2 seconds)
>      sleep(2);
> 
>      // Get end time
>      clock_gettime(CLOCK_MONOTONIC, &end);
> 
>      // Calculate elapsed time in seconds (as a double)
>      elapsed = (end.tv_sec - start.tv_sec);
>      elapsed += (end.tv_nsec - start.tv_nsec) / 1e9;
> 
>      printf("Elapsed time: %.6f seconds\n", elapsed);
> 
>      return 0;
> }
> 
> 
> ldd --version
> ldd (GNU libc) 2.17
> 
> 
> 
> But  on ancient   SLES 11  (SUSE Linux 11)  with
> ldd --version
> ldd (GNU libc) 2.11.3
> 
> 
> I see the issue  ,  there I have to add   -lrt  :
> 
> gcc  clock_gettime.c  -o   clock_gettime
> /tmp/ccU07fSz.o: In function `main':
> clock_gettime.c:(.text+0x20): undefined reference to `clock_gettime'
> clock_gettime.c:(.text+0x40): undefined reference to `clock_gettime'
> collect2: ld returned 1 exit status
> 
> 
> gcc  clock_gettime.c  -o   clock_gettime -lrt
> ./clock_gettime
> Elapsed time: 2.000108 seconds
> 
> 
> So it seems we have to go back to SLES11   and RHEL 6  (!)   for this strange  librt  issue .
> 
> Best regards, Matthias
> 
> 
> 
> -----Original Message-----
> From: Baesken, Matthias
> Sent: Friday, 19 September 2025 09:42
> To: 'David Holmes' <david.holmes at oracle.com>; build-dev at openjdk.org; hotspot-dev <hotspot-dev-retn at openjdk.org>
> Cc: Langer, Christoph <christoph.langer at sap.com>
> Subject: RE: "useless" librt.so.1 dependency on Linux reported by dpkg-shlibdeps
> 
> Hi David, I found a RHEL  7.9   machine with an old glibc  :
> 
> ldd --version
> ldd (GNU libc) 2.17
> 
> But the gcc on this machine  is 4.8.5  , probably too  old   to build OpenJDK there to see  if this change breaks something .
> 
> Any idea what I should test  on this old machine  regarding librt.so.1   ?
> 
> 
> Best regards, Matthias
> 
> 
> 
>>>>>
>>>>> ( seems some versions of  RHEL 7  / SLES 12  still had those old glibc
>>>>> versions)
>>>>>
>>>>> Could we clean this up?
>>>>>
>>>>> Or is there still some need  to  support this in jdk26  ?
>>>>
>>>> I always find it very difficult to determine when checks like these
>>>> become unnecessary. I think RHEL 7 with glibc 2.17 is the oldest Linux
>>>> release we currently support.
>>>>
>>>> Given I wrote the above in April 2021, I would hope we can now move on,
>>>> but again I don't know how to readily determine that.
>>>
>>>
>>> I could create a removal PR and maybe you can test on the oldest platforms you want to support in release 26 ?
>>
>> I don't have access to such systems for testing.
>>
>> David
> 
>