Re: -ffreestanding option of GCC

[Ian Lance Taylor <iant at google dot com>]

ali hagigat <hagigatali@gmail.com> writes:

> Thank you Ian for the reply. If -ffreestanding is not specified ,
> compiler may optimize the code based on considerations? without making
> sure if a function is built-in or it has been defined by a standard
> library?
> What are these modifications exactly?

The optimizations which gcc implements in the default hosted environment
are fairly extensive.  In general, in a hosted environment, gcc can
assume that there is a complete library available that meets the
conditions of the language standard being used (the default for C is
gnu90, the GNU extension to the ISO C90 standard).  That means, for
example, it can optimize printf("hi\n") into puts("hi").  There are many
such optimizations.  When -ffreestanding is used, gcc does not assume
that you are using a standard library environment.


> -ffreestanding
> A freestanding environment is one in which the standard
> library may not exist, and program startup may not necessarily be at main.
> --------------------------------------------------------------------------------
> I have copied part of gcc manual. The fact that program startup may
> not necessarily be at main, is not a special feature which is created
> by -ffreestanding option! If we do not specify any option or use
> -fhosted, program startup may not necessarily be at main too!!
> Why the manual mentions the issue?

Your statement is incorrect.  The C90 standard specifies that a program
starts at main, and on some (but not most) targets gcc does take
advantage of that fact to run certain initialization code at that time.
If you use -ffreestanding, gcc does not assume that the program starts
at main.

Ian