Editing C99 (section)

==Design==
[[File:1999 ISO C Concepts.png|thumb]]
C99 is, for the most part, backward compatible with C89, but it is stricter in some ways.<ref>{{cite web|url=https://gcc.gnu.org/onlinedocs/gcc-3.4.6/gcc/Standards.html |title=Standards - Using the GNU Compiler Collection (GCC) |publisher=Gcc.gnu.org |access-date=2014-04-08}}</ref>

In particular, a declaration that lacks a type specifier no longer has <code>int</code> 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 <code>int</code>. In practice, compilers are likely to display a warning, then assume <code>int</code> and continue translating the program.

C99 introduced several new features, many of which had already been implemented as extensions in several compilers:<ref>{{cite web|url=https://gcc.gnu.org/onlinedocs/gcc/C-Dialect-Options.html |title=C Dialect Options - Using the GNU Compiler Collection (GCC) |publisher=Gcc.gnu.org |date=2009-05-06 |access-date=2014-04-08}}</ref>

* [[inline function]]s
* intermingled declarations and code: [[Variable (programming)|variable]] declaration is no longer restricted to file scope or the start of a compound statement (block)
* several new [[data type]]s, including <code>long long int</code>, optional extended integer types, an explicit [[Boolean data type]], and a <code>complex</code> type to represent [[complex number]]s
* [[variable-length array]]s (although subsequently relegated in [[C11 (C standard revision)|C11]] to a conditional feature that implementations are not required to support)
* [[flexible array member]]s
* support for one-line [[comment (programming)|comments]] beginning with <code>//</code>, as in [[BCPL]], [[C++]] and [[Java (programming language)|Java]]
* new library functions, such as <code>snprintf</code>
* new [[header file|headers]], such as <code><[[stdbool.h]]></code>, <code><[[complex.h]]></code>, <code><[[tgmath.h]]></code>, and <code><[[inttypes.h]]></code>
* type-generic math (macro) functions, in <code><tgmath.h></code>, which select a [[math library]] [[library function|function]] based upon <code>float</code>, <code>double</code>, or <code>long double</code> arguments, etc.
* improved support for [[IEEE floating point]]
* designated initializers. For example, initializing a structure by field names: <code>struct point p = { .x = 1, .y = 2 };</code><ref>{{cite web |url=https://gcc.gnu.org/onlinedocs/gcc-3.3/gcc/Designated-Inits.html |title=Using the GNU Compiler Collection (GCC): Designated Initializers |website=gnu.org |access-date=18 September 2019}}</ref>
* compound literals. For instance, it is possible to construct structures in function calls: <code>function((struct x) {1, 2})</code><ref>{{cite web |url=https://gcc.gnu.org/onlinedocs/gcc-3.3/gcc/Compound-Literals.html |title=Using the GNU Compiler Collection (GCC): Compound Literals |website=gnu.org |access-date=31 January 2016}}</ref>
* support for [[variadic macro]]s (macros with a variable number of arguments)
* <code>[[restrict]]</code> qualification allows more aggressive code [[optimization (computer science)|optimization]], removing compile-time array access advantages previously held by [[FORTRAN]] over ANSI C<ref>{{cite web |url=https://lwn.net/Articles/255364/ |title=What every programmer should know about memory |date=2007-10-23 |access-date=2015-04-03 |author=Ulrich Drepper |publisher=[[LWN.net]]}}</ref>
* universal character names, which allows user variables to contain other characters than the standard character set: four-digit {{code|\u0040}} or eight-digit hexadecimal sequences {{code|\U0001f431}}
* keyword <code>static</code> in array indices in parameter declarations<ref>{{cite book | url=https://www.open-std.org/jtc1/sc22/wg14/www/docs/n1256.pdf | title=ISO/IEC 9899:1999 specification, TC3 | at=p. 119, § 6.7.5.3 ''Function declarators (including prototypes)'' para. 7}}</ref>

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.