C99 (previously C9X, formally ISO/IEC 9899:1999) is a past version of the C programming language open standard. It extends the previous version (C90) with new features for the language and the standard library, and helps implementations make better use of available computer hardware, such as IEEE 754-1985 floating-point arithmetic, and compiler technology. The C11 version of the C programming language standard, published in 2011, updates C99.

History

After ANSI produced the official standard for the C programming language in 1989, which became an international standard in 1990, the C language specification remained relatively static for some time, while C++ continued to evolve, largely during its own standardization effort. Normative Amendment 1 created a new standard for C in 1995, but only to correct some details of the 1989 standard and to add more extensive support for international character sets. The standard underwent further revision in the late 1990s, leading to the publication of ISO/IEC 9899:1999 in 1999, which was adopted as an ANSI standard in May 2000. The language defined by that version of the standard is commonly referred to as "C99". The international C standard is maintained by the working group ISO/IEC JTC1/SC22/WG14.

Design

C99 is, for the most part, backward compatible with C89, but it is stricter in some ways. In particular, a declaration that lacks a type specifier no longer has 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. In practice, compilers are likely to display a warning, then assume and continue translating the program. C99 introduced several new features, many of which had already been implemented as extensions in several compilers: Parts of the C99 standard are included in the current version of the C++ standard, including integer types, headers, and library functions. Variable-length arrays are not among these included parts because C++'s Standard Template Library already includes similar functionality.

IEEE 754 floating-point support

A major feature of C99 is its numerics support, and in particular its support for access to the features of IEEE 754-1985 (also known as IEC 60559) floating-point hardware present in the vast majority of modern processors (defined in "Annex F IEC 60559 floating-point arithmetic"). Platforms without IEEE 754 hardware can also implement it in software. On platforms with IEEE 754 floating point: tends to limit the risk of rounding errors affecting numerically unstable expressions (see IEEE 754 design rationale) and is the designed default method for x87 hardware, but yields unintuitive behavior for the unwary user; was the default evaluation method originally used in K&R C, which promoted all floats to double in expressions; and is also commonly used and specifies a strict "evaluate to type" of the operands. (For gcc, is the default on 32 bit x86, and is the default on 64 bit x86-64, but can be specified on x86-64 with option -mfpmath=387.) Before C99, compilers could round intermediate results inconsistently, especially when using x87 floating-point hardware, leading to compiler-specific behaviour; such inconsistencies are not permitted in compilers conforming to C99 (annex F).

Example

The following annotated example C99 code for computing a continued fraction function demonstrates the main features: Footnotes:

Version detection

A standard macro is defined with value to indicate that C99 support is available. As with the macro for C90, can be used to write code that will compile differently for C90 and C99 compilers, as in this example that ensures that is available in either case (by replacing it with in C90 to avoid linker errors).

Implementations

Most C compilers provide support for at least some of the features introduced in C99. Historically, Microsoft has been slow to implement new C features in their Visual C++ tools, instead focusing mainly on supporting developments in the C++ standards. However, with the introduction of Visual C++ 2013 Microsoft implemented a limited subset of C99, which was expanded in Visual C++ 2015.

Future work

Since ratification of the 1999 C standard, the standards working group prepared technical reports specifying improved support for embedded processing, additional character data types (Unicode support), and library functions with improved bounds checking. Work continues on technical reports addressing decimal floating point, additional mathematical special functions, and additional dynamic memory allocation functions. The C and C++ standards committees have been collaborating on specifications for threaded programming. The next revision of the C standard, C11, was ratified in 2011. The C standards committee adopted guidelines that limited the adoption of new features that have not been tested by existing implementations. Much effort went into developing a memory model, in order to clarify sequence points and to support threaded programming.