6 Using the GNU Compiler Collection (GCC)
The ISO C standard defines (in clause 4) two classes of conforming implementation. A
conforming hosted implementation supports the whole standard including all the library
facilities; a conforming freestanding implementation is only required to provide certain
library facilities: those in <float.h>, <limits.h>, <stdarg.h>, and <stddef.h>; since
AMD1, also those in <iso646.h>; since C99, also those in <stdbool.h> and <stdint.h>;
and since C11, also those in <stdalign.h> and <stdnoreturn.h>. In addition, complex
types, added in C99, are not required for freestanding implementations.
The standard also defines two environments for programs, a freestanding environment,
required of all implementations and which may not have library facilities beyond those
required of freestanding implementations, where the handling of program startup and ter-
mination are implementation-defined; and a hosted environment, which is not required,
in which all the library facilities are provided and startup is through a function int main
(void) or int main (int, char *[]). An OS kernel is an example of a program running
in a freestanding environment; a program using the facilities of an operating system is an
example of a program running in a hosted environment.
GCC aims towards being usable as a conforming freestanding implementation, or as the
compiler for a conforming hosted implementation. By default, it acts as the compiler for a
hosted implementation, defining __STDC_HOSTED__ as 1 and presuming that when the names
of ISO C functions are used, they have the semantics defined in the standard. To make it act
as a conforming freestanding implementation for a freestanding environment, use the option
‘-ffreestanding’; it then defines __STDC_HOSTED__ to 0 and does not make assumptions
about the meanings of function names from the standard library, with exceptions noted
below. To build an OS kernel, you may well still need to make your own arrangements for
linking and startup. See Section 3.4 [Options Controlling C Dialect], page 33.
GCC does not provide the library facilities required only of hosted implementations, nor
yet all the facilities required by C99 of freestanding implementations on all platforms. To
use the facilities of a hosted environment, you need to find them elsewhere (for example, in
the GNU C library). See Section 12.5 [Standard Libraries], page 740.
Most of the compiler support routines used by GCC are present in ‘libgcc’, but there
are a few exceptions. GCC requires the freestanding environment provide memcpy, memmove,
memset and memcmp. Finally, if __builtin_trap is used, and the target does not implement
the trap pattern, then GCC emits a call to abort.
For references to Technical Corrigenda, Rationale documents and information concerning
the history of C that is available online, see http://gcc.gnu.org/readings.html
2.2 C++ Language
GCC supports the original ISO C++ standard published in 1998, and the 2011 and 2014
revisions.
The original ISO C++ standard was published as the ISO standard (ISO/IEC 14882:1998)
and amended by a Technical Corrigenda published in 2003 (ISO/IEC 14882:2003). These
standards are referred to as C++98 and C++03, respectively. GCC implements the majority
of C++98 (export is a notable exception) and most of the changes in C++03. To select
this standard in GCC, use one of the options ‘-ansi’, ‘-std=c++98’, or ‘-std=c++03’; to
obtain all the diagnostics required by the standard, you should also specify ‘-pedantic’ (or
‘-pedantic-errors’ if you want them to be errors rather than warnings).
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