ARM Compiler toolchain v4.1 for µVision Using the Compiler

Compiler Coding Practices
5.60 New language features of C99
The 1999 C99 standard introduces several new language features, including:
• Some features similar to extensions to C90 offered in the GNU compiler, for example,
macros with a variable number of arguments.
Note
The implementations of extensions to C90 in the GNU compiler are not always
compatible with the implementations of similar features in C99.
• Some features available in C++, such as // comments and the ability to mix declarations
and statements.
• Some entirely new features, for example complex numbers, restricted pointers and
designated initializers.
• New keywords and identifiers.
• Extended syntax for the existing C90 language.
A selection of new features in C99 that might be of interest to developers using them for the first
time are documented.
Note
C90 is compatible with Standard C++ in the sense that the language specified by the standard is
a subset of C++, except for a few special cases. New features in the C99 standard mean that C99
is no longer compatible with C++ in this sense.
Some examples of special cases where the language specified by the C90 standard is not a subset
of C++ include support for // comments and merging of the typedef and structure tag
namespaces. For example, in C90 the following code expands to x = a / b - c; because /*
hello world */ is deleted, but in C++ and C99 it expands to x = a - c; because everything
from // to the end of the line is deleted:
x = a //* hello world */ b
- c;
The following code demonstrates how typedef and the structure tag are treated differently
between C (90 and 99) and C++ because of their merged namespaces:
typedef int a;
{
struct a { int x, y; };
printf("%d\n", sizeof(a));
}
In C 90 and C99, this code defines two types with separate names whereby a is a typedef for
int and struct a is a structure type containing two integer data types. sizeof(a) evaluates to
sizeof(int).
In C++, a structure type can be addressed using only its tag. This means that when the definition
of struct a is in scope, the name a used on its own refers to the structure type rather than the
typedef, so in C++ sizeof(a) is greater than sizeof(int).
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