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Preface Overview of the Compiler Getting Started with the Compiler Compiler Features Compiler Coding Practices The compiler as an optimizing compiler Compiler optimization for code size versus speed Compiler optimization levels and the debug view Selecting the target processor at compile time Enabling FPU for bare-metal Optimization of loop termination in C code Loop unrolling in C code Compiler optimization and the volatile keyword Code metrics Code metrics for measurement of code size and data Stack use in C and C++ Benefits of reducing debug information in objects Methods of reducing debug information in objects a Guarding against multiple inclusion of header file Methods of minimizing function parameter passing o Returning structures from functions through regist Functions that return the same result when called Comparison of pure and impure functions Recommendation of postfix syntax when qualifying f Inline functions Compiler decisions on function inlining Automatic function inlining and static functions Inline functions and removal of unused out-of-line Automatic function inlining and multifile compilat Restriction on overriding compiler decisions about Compiler modes and inline functions Inline functions in C++ and C90 mode Inline functions in C99 mode Inline functions and debugging Types of data alignment Advantages of natural data alignment Compiler storage of data objects by natural byte a Relevance of natural data alignment at compile tim Unaligned data access in C and C++ code The __packed qualifier and unaligned data access i Unaligned fields in structures Performance penalty associated with marking whole Unaligned pointers in C and C++ code Unaligned Load Register (LDR) instructions generat Comparisons of an unpacked struct, a __packed stru Compiler support for floating-point arithmetic Default selection of hardware or software floating Example of hardware and software support differenc Vector Floating-Point (VFP) architectures Limitations on hardware handling of floating-point Implementation of Vector Floating-Point (VFP) supp Compiler and library support for half-precision fl Half-precision floating-point number format Compiler support for floating-point computations a Types of floating-point linkage Compiler options for floating-point linkage and co Floating-point linkage and computational requireme Processors and their implicit Floating-Point Units Integer division-by-zero errors in C code Software floating-point division-by-zero errors in About trapping software floating-point division-by Identification of software floating-point division Software floating-point division-by-zero debugging New language features of C99 New library features of C99 // comments in C99 and C90 Compound literals in C99 Designated initializers in C99 Hexadecimal floating-point numbers in C99 Flexible array members in C99 __func__ predefined identifier in C99 inline functions in C99 long long data type in C99 and C90 Macros with a variable number of arguments in C99 Mixed declarations and statements in C99 New block scopes for selection and iteration state _Pragma preprocessing operator in C99 Restricted pointers in C99 Additional library functions in C99 Complex numbers in C99 Boolean type and in C99 Extended integer types and functions in floating-point environment access in C99 snprintf family of functions in C99 type-generic math macros in C99 wide character I/O functions in C99 How to prevent uninitialized data from being initi Compiler Diagnostic Messages Using the Inline and Embedded Assemblers of the AR Compiler Command-line Options Language Extensions Compiler-specific Features C and C++ Implementation Details What is Semihosting? Via File Syntax Summary Table of GNU Language Extensions Standard C Implementation Definition Standard C++ Implementation Definition C and C++ Compiler Implementation Limits

New language features of C99

4.59 New language features of C99

The 1999 C99 standard introduces several new language features.

These new features include:
  • Some features similar to extensions to C90 offered in the GNU compiler, for example, macros with a variable number of arguments.


    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.


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 first 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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