Re: Using unistd functions vs calling syscall straight in the code

[Murali Vijayaraghavan <vmurali@csail.mit.edu>]

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On Fri, Aug 10, 2012 at 11:16 PM, Szabolcs Nagy <nsz@port70.net> wrote:

> * Murali Vijayaraghavan <vmurali@csail.mit.edu> [2012-08-10 21:47:59
> +0900]:
> > You guys do have a unistd implementation which supposedly implements each
> > of the system calls. But you are not consistent with the use of these
> > functions to perform the unistd-implemented tasks. Wouldn't it be a lot
> > cleaner to call these functions instead of calling syscall / syscall_cp
> > directly from the other (top-level) functions? Was there some rationale
> or
> > is it just code evolution?
> >
>
> i don't understand the question
>
> can you show with an example what do you mean?
>
> calling a libc function is not the same as using a linux
> syscall, and there is usually a reason why one is used
> instead of the other..
>
> (the first has posix semantics the second has whatever
> semantics linux have, even if these happen to be compatible
> then the first one creates an extra call and an extra
> internal dependency when static linking is used)
>


For example, I could have implemented src/stdio/__stdio_read.c using
src/unistd/readv.c's readv function instead of calling
syscall/syscall_cp(SYS_readv, ...) in lines 20 and 24. I believe unistd is
the POSIX compatibility layer (correct me if I am wrong). So shouldn't the
C standard library, namely stdio functions like scanf eventually use the
unistd functions instead of using the syscall directly?

This would have made my job easier because I could have just modified this
POSIX compability layer instead of scanning through the C standard library
functions and changing them one by one. Remember I have multiple special
instructions to perform each IO task instead of a single system call
instruction, since it's easier to implement hardware simulator that way - I
can get the function type simply by decoding the instruction rather than
reading some register.

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