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C99 (previouslyC9X, formallyISO/IEC 9899:1999) is a past version of theC programming languageopen standard.^[1] It extends the previous version (C90) with new features for the language and thestandard library, and helps implementations make better use of available computer hardware, such asIEEE 754-1985floating-point arithmetic, and compiler technology.^[2] TheC11 version of the C programming language standard, published in 2011, updates C99.

AfterANSI 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, whileC++ 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 theworking groupISO/IEC JTC1/SC22/WG14.

C99 is, for the most part, backward compatible with C89, but it is stricter in some ways.^[3]

In particular, a declaration that lacks a type specifier no longer hasint 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 implicitint. In practice, compilers are likely to display a warning, then assumeint and continue translating the program.

C99 introduced several new features, many of which had already been implemented as extensions in several compilers:^[4]

• inline functions
• intermingled declarations and code:variable declaration is no longer restricted to file scope or the start of a compound statement (block)
• several newdata types, includinglong long int, optional extended integer types, an explicitBoolean data type, and acomplex type to representcomplex numbers
• variable-length arrays (although subsequently relegated inC11 to a conditional feature that implementations are not required to support)
• flexible array members
• support for one-linecomments beginning with//, as inBCPL,C++ andJava
• new library functions, such assnprintf
• newheaders, such as<stdbool.h>,<complex.h>,<tgmath.h>, and<inttypes.h>
• type-generic math (macro) functions, in<tgmath.h>, which select amath libraryfunction based uponfloat,double, orlong double arguments, etc.
• improved support forIEEE floating point
• designated initializers. For example, initializing a structure by field names:struct point p = { .x = 1, .y = 2 };^[5]
• compound literals. For instance, it is possible to construct structures in function calls:function((struct x) {1, 2})^[6]
• support forvariadic macros (macros with a variable number of arguments)
• restrict qualification allows more aggressive codeoptimization, removing compile-time array access advantages previously held byFORTRAN over ANSI C^[7]
• universal character names, which allows user variables to contain other characters than the standard character set: four-digit\u0040 or eight-digit hexadecimal sequences\U0001f431
• keywordstatic in array indices in parameter declarations^[8]

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

A major feature of C99 is its numerics support, and in particular its support for access to the features ofIEEE 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.^[2]

On platforms with IEEE 754 floating point:

FLT_EVAL_METHOD == 2 tends to limit the risk ofrounding errors affecting numerically unstable expressions (seeIEEE 754 design rationale) and is the designed default method forx87 hardware, but yields unintuitive behavior for the unwary user;^[9]FLT_EVAL_METHOD == 1 was the default evaluation method originally used inK&R C, which promoted all floats to double in expressions; andFLT_EVAL_METHOD == 0 is also commonly used and specifies a strict "evaluate to type" of the operands. (Forgcc,FLT_EVAL_METHOD == 2 is the default on 32 bit x86, andFLT_EVAL_METHOD == 0 is the default on 64 bit x86-64, butFLT_EVAL_METHOD == 2 can be specified on x86-64 with option -mfpmath=387.) Before C99, compilers could round intermediate results inconsistently, especially when usingx87 floating-point hardware, leading to compiler-specific behaviour;^[10] such inconsistencies are not permitted in compilers conforming to C99 (annex F).

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

#include<stdio.h>#include<math.h>#include<float.h>#include<fenv.h>#include<tgmath.h>#include<stdbool.h>#include<assert.h>doublecompute_fn(doublez)// [1]{#pragma STDC FENV_ACCESS ON// [2]assert(FLT_EVAL_METHOD==2);// [3]if(isnan(z))// [4]puts("z is not a number");if(isinf(z))puts("z is infinite");longdoubler=7.0-3.0/(z-2.0-1.0/(z-7.0+10.0/(z-2.0-2.0/(z-3.0))));// [5, 6]feclearexcept(FE_DIVBYZERO);// [7]boolraised=fetestexcept(FE_OVERFLOW);// [8]if(raised)puts("Unanticipated overflow.");returnr;}intmain(void){#ifndef __STDC_IEC_559__puts("Warning: __STDC_IEC_559__ not defined. IEEE 754 floating point not fully supported.");// [9]#endif#pragma STDC FENV_ACCESS ON#ifdef TEST_NUMERIC_STABILITY_UPfesetround(FE_UPWARD);// [10]#elif TEST_NUMERIC_STABILITY_DOWNfesetround(FE_DOWNWARD);#endifprintf("%.7g\n",compute_fn(3.0));printf("%.7g\n",compute_fn(NAN));return0;}

Footnotes:

1. Compile with:gcc-std=c99-mfpmath=387-otest_c99_fptest_c99_fp.c-lm
2. As the IEEE 754 status flags are manipulated in this function, this #pragma is needed to avoid the compiler incorrectly rearranging such tests when optimising. (Pragmas are usually implementation-defined, but those prefixed withSTDC are defined in the C standard.)
3. C99 defines a limited number of expression evaluation methods: the current compilation mode can be checked to ensure it meets the assumptions the code was written under.
4. The special values such asNaN and positive or negative infinity can be tested and set.
5. long double is defined as IEEE 754 double extended or quad precision if available. Using higher precision than required for intermediate computations can minimizeround-off error^[11] (thetypedefdouble_t can be used for code that is portable under allFLT_EVAL_METHODs).
6. The main function to be evaluated. Although it appears that some arguments to this continued fraction, e.g., 3.0, would lead to a divide-by-zero error, in fact the function is well-defined at 3.0 and division by 0 will simply return a +infinity that will then correctly lead to a finite result: IEEE 754 is defined not to trap on such exceptions by default and is designed so that they can very often be ignored, as in this case. (IfFLT_EVAL_METHOD is defined as 2 then all internal computations including constants will be performed in long double precision; ifFLT_EVAL_METHOD is defined as 0 then additional care is need to ensure this, including possibly additional casts and explicit specification of constants as long double.)
7. As the raised divide-by-zero flag is not an error in this case, it can simply be dismissed to clear the flag for use by later code.
8. In some cases, other exceptions may be regarded as an error, such as overflow (although it can in fact be shown that this cannot occur in this case).
9. __STDC_IEC_559__ is to be defined only if "Annex F IEC 60559 floating-point arithmetic" is fully implemented by the compiler and the C library (users should be aware that this macro is sometimes defined while it should not be).
10. The default rounding mode is round to nearest (with the even rounding rule in the halfway cases) for IEEE 754, but explicitly setting the rounding mode toward + and - infinity (by definingTEST_NUMERIC_STABILITY_UP etc. in this example, when debugging) can be used to diagnose numerical instability.^[12] This method can be used even ifcompute_fn() is part of a separately compiled binary library. But depending on the function, numerical instabilities cannot always be detected.

A standard macro__STDC_VERSION__ is defined with value199901L to indicate that C99 support is available. As with the__STDC__ macro for C90,__STDC_VERSION__ can be used to write code that will compile differently for C90 and C99 compilers, as in this example that ensures thatinline is available in either case (by replacing it withstatic in C90 to avoid linker errors).

#if __STDC_VERSION__ >= 199901L/* "inline" is a keyword */#else# define inline static#endif

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 theirVisual C++ tools, instead focusing mainly on supporting developments in the C++ standards.^[13] However, with the introduction of Visual C++ 2013 Microsoft implemented a limited subset of C99, which was expanded in Visual C++ 2015.^[14]

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 improvedbounds checking. Work continues on technical reports addressing decimalfloating point, additional mathematicalspecial functions, and additionaldynamic memory allocation functions. The C and C++ standards committees have been collaborating on specifications forthreaded programming.

The next revision of the C standard,C11, was ratified in 2011.^[41] 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 amemory model, in order to clarifysequence points and to supportthreaded programming.

• C++23,C++20,C++17,C++14,C++11,C++03,C++98, versions of the C++ programming language standard
• Compatibility of C and C++
• C++ Technical Report 1
• Floating point, for further discussion of usage of IEEE 754 hardware

• N1256 (final draft of C99 standard plus TC1, TC2, TC3); WG14; 2007. (HTML and ASCII versions)
• ISO/IEC 9899:1999 (official C99 standard);ISO; 1999.
• Rationale for C99; WG14; 2003.
• Cheng, Harry (1 March 2002)."C99 & Numeric computing".Dr. Dobb's Journal.
• Seebach, Peter (24 March 2004)."Open source development using C99".developerWorks.IBM.

• C Language Working Group 14 (WG14) Documents
• C9X Charter - WG14
• New things in C9X
• Features of C99

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