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

[Baesken, Matthias]

Hi David,  even  on RHEL   7.9   it works without  adding  -lrt to  linking , see the example   :


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