Re: system call stubs in glibc

[Andreas Schwab <schwab at suse dot de>]

Urs Thuermann <urs@isnogud.escape.de> writes:

|> There are system calls which exist in the Linux kernel on some
|> architectures but not others.  For example time(2) and stime(2) exist
|> on all architectures but alpha and ia64.  I assume these are
|> implemented by glibc using gettimeofday(2) and settimeofday(2) on
|> alpha and ia64, correct?
|> 
|> If so, how does glibc know, which system calls on which architectures
|> exist, and when to implement them using other calls?  Is this
|> configured in some architecture-depend config file or does glibc check
|> itself for the availability of system calls like time(2) and stime(2)?

There are various files named syscalls.list under the sysdeps/unix tree.
They define the syscalls that _may_ be present.  The make-syscalls.sh
script goes through them and looks to see if the syscalls are actually
present and generates the file sysd-syscalls in the build directory
according to this information.  For the functions that are implemented as
syscalls there are usually fallback implementations,
eg. sysdeps/unix/time.c and sysdeps/unix/stime.c.  The build process
automatically picks them up when a syscall implementation is not
available.  Note that the fallbacks in sysdeps/generic are often only
stubs that always fail.

|> The reason I ask is, I'd like to know, if I would remove time() and
|> stime() from the kernel on my i386/Linux, would glibc, when configured
|> and compiled again running on that modified kernel, use
|> get/settimeofday() or would it fail when someone uses time() or
|> stime()?

This should work as expected.  You just have to make sure that __NR_time
and __NR_stime are not defined in <asm/unistd.h>.

Andeas.

-- 
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