Why the /lib/libc.so.1 (linker/loader) is always mounted on Solaris 10 ? I have tried both mount and df output. It shows me /lib/libc.so.1 entry.
For both SPARC and x86 architectures, Solaris provides optimized C standard libraries. At boot time, the best suited for your machine, i.e. the one taking advantage of CPU specific instructions and features, is lofs mounted on top of the standard one.
Since Solaris 10, no static libc is provided so this dynamic libc, being the interface between the kernel and the userland, is a mandatory component of every program running on Solaris.
More details here.
One might ask why is this done with a lofs mount and not by a lightweight feature like a symlink.
The reason is a symlink is persistent, i.e. survives a reboot. Using a symlink might then render a system unusable should the hardware capabilities evolve or should should for some other reason the wrong library would have been linked to. Again, all Solaris commands are dynamically linked to libc.so. There has not been a libc.a since a long time.
Using a lofs mount ensure the first stage of system boot are done with using the safe default libc.so, and the optimized one is only selected at the right time and in particular allows a safe boot with all services disabled (-m milestone=none) not to be affected by a capabilities change.