What is the version number field in the output of the linux file command

Question

If I do the following command on my executable called "version", compiled on Fedora Core 11, I get this output

file version

version: ELF 32-bit LSB executable, Intel 80386, version 1 (SYSV), dynamically linked (uses shared libs), for GNU/Linux 2.6.18, not stripped

What's the significance of the 2.6.18 number towards the end, and is it any use in distinguishing to customers which version of some software they should download ?

From what I've looked at so far, this number is definitely not

1. The kernel version
2. The libc version
3. Anything to do with the lsb_release

I'd like to get some easy identifier to allow customers to know which binary release they should download, which they should ideally be able to identify by typing a command (like uname -a, although that obviously isn't the one in this case).

Thanks

Answer 1

It's the kernel version of the machine the binary was compiled on. If you use precompiled binaries from your distribution, it's the kernel version of a machine of the distribution vendor, probably in its compile farm.

It's relevant e.g. when considering syscalls. Say your binary uses the syscall no. X and you use a kernel which does not support X yet or worse has assigned syscall no. X to a different syscall.

The vanilla Linux Kernel User API is stable. That means every syscall available in Linux version A is available in Linux version B if A <=B. But it may happen that some developer releases his/her own development version of Linux (something like linux-2.6.18-xy) and s/he implements a new syscall. If s/he now compiles a binary using that kernel version, the binary gets tagged with that version. So, you are later on able to know that it may or may not work.

Btw, /usr/include/asm/unistd_32.h contains syscall numbers, excerpt:

[...]
#define __NR_restart_syscall      0
#define __NR_exit         1
#define __NR_fork         2
#define __NR_read         3
#define __NR_write        4
#define __NR_open         5
[...]