C99 (previously known as C9X) is an informal name for ISO/IEC 9899:1999, a past version of the C programming language standard.[1] 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.[2] The C11 version of the C programming language standard, published in 2011, updates C99.

Cover of the C99 standards document



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.



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 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:[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 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 (although subsequently relegated in C11 to a conditional feature that implementations are not required to support)
  • flexible array members
  • support for one-line comments beginning with //, as in BCPL, C++ and Java
  • new library functions, such as snprintf
  • new headers, such as <stdbool.h>, <complex.h>, <tgmath.h>, and <inttypes.h>
  • type-generic math (macro) functions, in <tgmath.h>, which select a math library function based upon float, double, or long double arguments, etc.
  • improved support for IEEE 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 for variadic macros (macros with a variable number of arguments)
  • restrict qualification allows more aggressive code optimization, removing compile-time array access advantages previously held by FORTRAN 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
  • keyword static in array indices in parameter declarations[8]

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.[2]

On platforms with IEEE 754 floating point:

  • float is defined as IEEE 754 single precision, double is defined as double precision, and long double is defined as IEEE 754 extended precision (e.g., Intel 80-bit double extended precision on x86 or x86-64 platforms), or some form of quad precision where available; otherwise, it is double precision.
  • The four arithmetic operations and square root are correctly rounded as defined by IEEE 754.
    FLT_EVAL_METHOD float double long double
    0 float double long double
    1 double double long double
    2 long double long double long double
  • Expression evaluation is defined to be performed in one of three well-defined methods, indicating whether floating-point variables are first promoted to a more precise format in expressions: FLT_EVAL_METHOD == 2 indicates that all internal intermediate computations are performed by default at high precision (long double) where available (e.g., 80 bit double extended), FLT_EVAL_METHOD == 1 performs all internal intermediate expressions in double precision (unless an operand is long double), while FLT_EVAL_METHOD == 0 specifies each operation is evaluated only at the precision of the widest operand of each operator. The intermediate result type for operands of a given precision are summarized in the adjacent table.

FLT_EVAL_METHOD == 2 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;[9] FLT_EVAL_METHOD == 1 was the default evaluation method originally used in K&R C, which promoted all floats to double in expressions; and FLT_EVAL_METHOD == 0 is also commonly used and specifies a strict "evaluate to type" of the operands. (For gcc, FLT_EVAL_METHOD == 2 is the default on 32 bit x86, and FLT_EVAL_METHOD == 0 is the default on 64 bit x86-64, but FLT_EVAL_METHOD == 2 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;[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>

double compute_fn(double z)  // [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");

        long double r = 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]

        bool raised = fetestexcept(FE_OVERFLOW);  // [8]

        if (raised)
                puts("Unanticipated overflow.");

        return r;

int main(void)
        #ifndef __STDC_IEC_559__
        puts("Warning: __STDC_IEC_559__ not defined. IEEE 754 floating point not fully supported."); // [9]

        #pragma STDC FENV_ACCESS ON

        fesetround(FE_UPWARD);                   // [10]

        printf("%.7g\n", compute_fn(3.0));
        printf("%.7g\n", compute_fn(NAN));

        return 0;


  1. Compile with: gcc -std=c99 -mfpmath=387 -o test_c99_fp test_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 with STDC 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 as NaN 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 minimize round-off error[11] (the typedef double_t can be used for code that is portable under all FLT_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. (If FLT_EVAL_METHOD is defined as 2 then all internal computations including constants will be performed in long double precision; if FLT_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 defining TEST_NUMERIC_STABILITY_UP etc. in this example, when debugging) can be used to diagnose numerical instability.[12] This method can be used even if compute_fn() is part of a separately compiled binary library. But depending on the function, numerical instabilities cannot always be detected.

Version detection


A standard macro __STDC_VERSION__ is defined with value 199901L 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 that inline is available in either case (by replacing it with static in C90 to avoid linker errors).

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



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.[13] However, with the introduction of Visual C++ 2013 Microsoft implemented a limited subset of C99, which was expanded in Visual C++ 2015.[14]

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.[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 a memory model, in order to clarify sequence points and to support threaded programming.

See also



  1. ^ "ISO/IEC 9899:1999 - Programming languages - C". Iso.org. 8 December 2011. Retrieved 8 April 2014.
  2. ^ a b "IEEE 754 Support in C99" (PDF). grouper.ieee.org. Archived from the original (PDF) on 28 October 2017. Retrieved 15 July 2021.
  3. ^ "Standards - Using the GNU Compiler Collection (GCC)". Gcc.gnu.org. Retrieved 8 April 2014.
  4. ^ "C Dialect Options - Using the GNU Compiler Collection (GCC)". Gcc.gnu.org. 6 May 2009. Retrieved 8 April 2014.
  5. ^ "Using the GNU Compiler Collection (GCC): Designated Initializers". gnu.org. Retrieved 18 September 2019.
  6. ^ "Using the GNU Compiler Collection (GCC): Compound Literals". gnu.org. Retrieved 31 January 2016.
  7. ^ Ulrich Drepper (23 October 2007). "What every programmer should know about memory". LWN.net. Retrieved 3 April 2015.
  8. ^ ISO/IEC 9899:1999 specification, TC3 (PDF). p. 119, § Function declarators (including prototypes) para. 7.
  9. ^ Doug Priest (1997). "Differences Among IEEE 754 Implementations".
  10. ^ Jack Woehr (1 November 1997). "A conversation with William Kahan".
  11. ^ William Kahan (11 June 1996). "The Baleful Effect of Computer Benchmarks upon Applied Mathematics, Physics and Chemistry" (PDF).
  12. ^ William Kahan (11 January 2006). "How Futile are Mindless Assessments of Roundoff in Floating-Point Computation?" (PDF).
  13. ^ Peter Bright (29 June 2013). "C99 acknowledged at last as Microsoft lays out its path to C++14". Ars Technica. Retrieved 9 January 2015.
  14. ^ a b c Brenner, Pat. "What's New for Visual C++ in Visual Studio 2015". Microsoft Developer Network. Retrieved 27 April 2015.
  15. ^ "Using the x86 Open64 Compiler Suite" (PDF). Developer.amd.com. Archived (PDF) from the original on 24 January 2022. Retrieved 2 March 2022.
  16. ^ "cc65 - a freeware C compiler for 6502 based systems". Retrieved 14 September 2011.
  17. ^ "C/C++ interpreter Ch C99 features". SoftIntegration, Inc. 15 February 2008. Retrieved 15 February 2008.
  18. ^ "Clang Compiler User's Manual". Retrieved 14 October 2017.
  19. ^ "The CompCert C verified compiler documentation and user's manual (Version 3.10)". 19 November 2021. Retrieved 3 March 2022.
  20. ^ "libfirm homepage". Retrieved 4 February 2014.
  21. ^ "C Language Implementation - Digital Mars". Retrieved 14 September 2011.
  22. ^ "Status of C99 features in GCC". Free Software Foundation, Inc. 28 July 2021. Retrieved 13 August 2021.
  23. ^ "Status of C99 features in GCC 4.6". Free Software Foundation, Inc. 23 May 2013. Retrieved 23 May 2013.
  24. ^ "Status of C99 features in GCC 4.7". Free Software Foundation, Inc. 23 May 2013. Retrieved 23 May 2013.
  25. ^ "Semantics of Floating Point Math in GCC". 20 July 2018. Retrieved 12 August 2018.
  26. ^ "IBM C for AIX, V6.0 Now Supports the C99 Standard". 2 July 2002. Retrieved 31 January 2016.
  27. ^ "IBM - XL C/C++ for AIX". Retrieved 31 January 2016.
  28. ^ "IBM Rational Logiscope support for C99 standard - United States". 24 February 2012. Retrieved 31 January 2016.
  29. ^ "Reader Q&A: What about VC++ and C99?". Sutter’s Mill. 3 May 2012. Retrieved 31 January 2016.
  30. ^ "A.27 Use of C99 Variable Length Arrays". Microsoft. Retrieved 31 January 2016.
  31. ^ "Microsoft to C99 Developers: Use ISO C++". InfoQ. Retrieved 31 January 2016.
  32. ^ "C99 library support in Visual Studio 2013". Microsoft. 19 July 2013. Retrieved 31 January 2016.
  33. ^ "C++11/14 STL Features, Fixes, And Breaking Changes In VS 2013". Blogs.msdn.com. 28 June 2013. Retrieved 8 April 2014.
  34. ^ "Announcing full support for a C/C++ conformant preprocessor in MSVC". Microsoft. 27 March 2020. Retrieved 17 September 2020.
  35. ^ "C99 compliance in Open Watcom". Archived from the original on 3 May 2015. Retrieved 25 September 2015.
  36. ^ "Pelles C Overview". January 2013. Archived from the original on 13 March 2022. Retrieved 2 March 2022.
  37. ^ "Sun Studio 12: C Compiler 5.9 Readme". Sun Microsystems, Inc. 31 May 2007. Retrieved 23 September 2012.
  38. ^ "Tiny C Compiler Reference Documentation". Retrieved 31 January 2016.
  39. ^ According to the project's TODO list complex types are the only missing C99 feature. Variable Length Arrays have been added in TCC 0.9.26 [1]
  40. ^ "TCC : Tiny C Compiler". Retrieved 31 January 2016.
  41. ^ "Standards - Using the GNU Compiler Collection (GCC)". Gcc.gnu.org. Retrieved 8 April 2014.

Further reading

  • 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
Preceded by C language standards Succeeded by