Pragma Once; Pragma System_Header; Gcc Compatibility Extensions - Analog Devices VISUALDSP++ 3.5 Manual

C/C++ Compiler Language Extensions

#pragma once

The
#pragma once
tells the compiler that the header is written in such a way that including it
several times has the same effect as including it once. For example,
#pragma once
#ifndef FILE_H
#define FILE_H
... contents of header file ...
#endif
In this example, the
compiler recognizes the
not reopen a header that uses it.

#pragma system_header

The
#pragma system_header
with VisualDSP++. The pragma tells the compiler that every function and
variable declared in the file (but not in files included in the file) is the vari-
able or function with that name from the VDSP++ library.
The compiler will take advantage of any special knowledge it has of the
behavior of the library.

GCC Compatibility Extensions

The compiler provides compatibility with the C dialect accepted by ver-
sion 3.2 of the GNU C Compiler. Many of these features are available in
the C99 ANSI Standard. A brief description of the extensions is included
in this section. For more information, refer to the following web address:
http://gcc.gnu.org/onlinedocs/gcc-3.2.1/gcc/C-Extensions.html#C%20Extensions
1-138
, which should appear at the beginning of a header file,
#pragma once
#ifndef/#define/#endif
identifies an
VisualDSP++ 3.5 C/C++ Compiler and Library Manual
is actually optional because the
idiom and will
file as the file supplied
include
for ADSP-219x DSPs