Here is the regular and strace output on 2.4 kernel, more specifically
Linux 2.4.21-4.EL #1 Fri Oct 3 18:13:58 EDT 2003 i686 i686 i386 GNU/Linux.

$ test_threads2
Counter value functionCount1: 1
Counter value functionCount1: 2
Counter value functionCount1: 3
Counter value functionCount2: 4
Counter value functionCount2: 5
Counter value functionCount2: 6
Counter value functionCount2: 7
Counter value functionCount1: 8
Counter value functionCount1: 9
Counter value functionCount1: 10
Final count: 10
$

$ strace test_threads2
execve("/home/jmudd/test_threads2", ["test_threads2"], [/* 26 vars */]) = 0
rt_sigprocmask(SIG_UNBLOCK, [RT_1 RT_2], NULL, 8) = 0
set_thread_area({entry_number:-1 -> 6, base_addr:0x804e540, limit:1048575,
seg_32bit:1, contents:0, read_exec_only:0, limit_in_pages:1,
seg_not_present:0, useable:1}) = 0
set_tid_address(0x804e558)              = 25884
mmap2(NULL, 90112, PROT_NONE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0xb75ea000
mprotect(0xb75eb000, 86016, PROT_READ|PROT_WRITE) = 0
clone(child_stack=0xb75fff30,
flags=CLONE_VM|CLONE_FS|CLONE_FILES|CLONE_SIGHAND|CLONE_THREAD|CLONE_SYSVSEM|CLONE_SETTLS|CLONE_PARENT_SETTID|CLONE_CHILD_CLEARTID|CLONE_DETACHED,
parent_tidptr=0xb75fff58, {entry_number:6, base_addr:0xb75fff40,
limit:1048575, seg_32bit:1, contents:0, read_exec_only:0, limit_in_pages:1,
seg_not_present:0, useable:1}, child_tidptr=0xb75fff58) = 25885
mmap2(NULL, 90112, PROT_NONE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0xb75d4000
mprotect(0xb75d5000, 86016, PROT_READ|PROT_WRITE) = 0
clone(child_stack=0xb75e9f30,
flags=CLONE_VM|CLONE_FS|CLONE_FILES|CLONE_SIGHAND|CLONE_THREAD|CLONE_SYSVSEM|CLONE_SETTLS|CLONE_PARENT_SETTID|CLONE_CHILD_CLEARTID|CLONE_DETACHED,
parent_tidptr=0xb75e9f58, {entry_number:6, base_addr:0xb75e9f40,
limit:1048575, seg_32bit:1, contents:0, read_exec_only:0, limit_in_pages:1,
seg_not_present:0, useable:1}, child_tidptr=0xb75e9f58) = 25886
futex(0xb75fff58, FUTEX_WAIT, 25885, NULLCounter value functionCount1: 1
Counter value functionCount1: 2
Counter value functionCount1: 3
Counter value functionCount2: 4
Counter value functionCount2: 5
Counter value functionCount2: 6
Counter value functionCount2: 7
Counter value functionCount1: 8
Counter value functionCount1: 9
Counter value functionCount1: 10
) = 0
munmap(0xb75ea000, 90112)               = 0
munmap(0xb75d4000, 90112)               = 0
writev(1, [{"Final count: 10", 15}, {"\n", 1}], 2Final count: 10
) = 16
exit_group(0)                           = ?
$


>

On Fri, Mar 21, 2014 at 9:02 AM, John Mudd <johnbmudd@gmail.com> wrote:

> Thanks for your reply.
>
> I ran a test using an example thread app. I compiled the cond1.c example
> from http://www.yolinux.com/TUTORIALS/LinuxTutorialPosixThreads.htmlusing musl-gcc on Ubuntu 13.04, 2.6 kernel obviously. The executable runs
> on 2.6 as well as 2.4. I actually expected some kind of failure on 2.4. Did
> I pick a poor example or am I not understanding still?
>
>
> On Thu, Mar 20, 2014 at 7:14 PM, Rich Felker <dalias@aerifal.cx> wrote:
>
>> On Thu, Mar 20, 2014 at 04:58:49PM -0400, John Mudd wrote:
>> > Probably a dumb question. Is part of the value of musl that I can build
>> an
>> > app using musl on linux 2.6 and then run the binary on linux 2.4?
>>
>> The version of Linux you build on has no bearing on the binary that
>> comes out, so that's not a problem.
>>
>> However, Linux 2.4 is not officially supported since it lacks a lot of
>> functionality needed to provide a modern POSIX conforming environment.
>> The most notable is that it can't do threads. If you're ok with that,
>> the other problems might be small enough that you don't mind. I
>> remember some people in Freenode #musl trying out 2.4 recently and
>> finding that a few of the busybox applets didn't work right, though,
>> due to missing statfs64 syscall.
>>
>> This page has details on which kernel versions added which syscalls:
>>
>> http://man7.org/linux/man-pages/man2/syscalls.2.html
>>
>> so it may be helpful in evaluating if there's anything critical you'd
>> be missing. If a syscall has two versions, one with "64" on the end,
>> musl needs the one that ends in "64".
>>
>> This is definitely a topic we could attempt to document better if more
>> people are interested in trying to use 2.4.
>>
>> Rich
>>
>
>