System call vs POSIX call

[Nicholas Mc Guire]

On Thu, Aug 16, 2018 at 09:51:05PM +0530, Subhashini Rao Beerisetty wrote:
> Hi All,
> 
> I'm trying to get the difference between system call and POSIX call. System
> calls are user mode API's (open(), close(), ioctl(),...) gets the kernel
> service via software interrupt. What about POSIX calls and how it differs
> with the system call.? Can anyone clarify me on this.

glibc is the library that connects user-space libs/apps to the
system call interface (unistd.h basically) so if you do an
open() in user-space you are calling glibc and that can call
sys_open() but there are POSIX calls that will not necessarily
do a system call if they can handle the request with available
resources or with cached data. So there is no 1:1 mapping at
runtime from POSIX call to system call - and there can be multiple
POSIX calls that map to a single system call (e.g. printf -> write)

Note that you can do system calls directly with system() but that is
generaly not how you do it - you to through the glibc calls
which do some checks before invoking the actual system call.

Does that clarify it ?

thx!
hofrat