C99 - Design

Design

C99 is, for the most part, backward compatible with C89 but is stricter in some ways.

In particular, a declaration that lacks a type specifier no longer has int implicitly assumed. The C standards committee decided that it was of more value for compilers to diagnose inadvertent omission of the type specifier than to silently process legacy code that relied on implicit int. In practice, compilers are likely to display a warning, then assume int and continue translating the program.

C99 introduced several new features, many of which had already been implemented as extensions in several compilers:

  • inline functions
  • intermingled declarations and code: variable declaration is no longer restricted to file scope or the start of a compound statement (block), similar to C++
  • several new data types, including long long int, optional extended integer types, an explicit boolean data type, and a complex type to represent complex numbers
  • variable-length arrays
  • support for one-line comments beginning with //, as in BCPL or C++
  • new library functions, such as snprintf
  • new header files, such as stdbool.h, complex.h, tgmath.h, and inttypes.h
  • type-generic math functions
  • improved support for IEEE floating point
  • designated initializers
  • compound literals
  • support for variadic macros (macros with a variable number of arguments)
  • restrict qualification allows more aggressive code optimization
  • universal character names, which allows user variables to contain other characters than the standard character set

Parts of the C99 standard are included in the current version of the C++ standard, C++11, including integer types, header files, and library functions. Variable-length arrays are not among these included parts because C++'s Standard Template Library already includes similar functionality.

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