Editing c/c99

{{C standards}}
'''C99''' is a past version of the [[C programming language]] standard which was ratified by ISO in 1999 and became '''ISO/IEC 9899:1999'''. The same standard was later also adopted by ANSI on May 22, 2000. C99 cancels and replaces the first edition, [[C89]], and [[Normative Addendum 1|NA1]].

C99 brought the first major updates to the language in a decade. The new standard introduced restricted pointers, variable-length arrays, flexible array members, complex numbers support, type-generic math, <code>long long int</code>, extended identifiers, hexadecimal floating-point constants, compound literals, designated initializers, single line, <code>//</code> comments, extended integer type, the ability to mix declarations and code, variadic macros, new math functions, inline functions, boolean types, _Pragma and standard pragmas, and VA_COPY. The standard also removed implicit function declaration.

== Version detection ==
C99 can be detected via the __STDC_VERSION__ mandatory macro which must equal <code>199901L</code>. 

<source lang="C">
#if __STDC_VERSION__ >= 199901L
    /* C99 support */
#endif
</source>

== New headers ==
C99 introduced 6 new [[C standard library|standard headers]]: [[tgmath.h - C|<tgmath.h>]], [[stdint.h - C|<stdint.h>]], [[stdbool.h - C|<stdbool.h>]], [[inttypes.h - C|<inttypes.h>]], [[fenv.h - C|<fenv.h>]], and [[complex.h - C|<complex.h>]].

== Restricted pointers ==
{{main|Restricted pointers - C|l1=Restricted pointers}}

C99 introduced the concept of restricted pointer to the C language through the introduction of the '''[[restrict - C|restrict]]''' [[Reserved keywords - C|keyword]]. Given two pointers, if they do not point to two distinct objects, they are said to be ''aliases''. The ''restrict'' keyword establishes a special association between the pointer and the object it accesses, guaranteeing all accesses to the object it points to occur through that pointer or expressions based on that pointer.

Since multiple pointers can point to the same object, compilers are often unable to make certain optimizations that require them to know that only a specific pointer has access to the object it points to. The restrict keyword was designed to aid such compiler issues. Consequently, various functions such as memcpy(), strcpy(), and strcat() have had their signatures changed to:

<source lang="C">
void *memcpy(void * restrict s1, const void * restrict s2, size_t n);
char *strcpy(char * restrict s1, const char * restrict s2);
char *strcat(char * restrict s1, const char * restrict s2);
</source>

== Variable-length array ==
{{main|Variable-length array - C|l1=Variable length arrays}}

'''Variable-length array''' ('''VLA''') are arrays of automatic storage whose size is determined at run-time. C99 introduced support for variable-length arrays. The length of the array does not change throughout the duration of the object's lifetime.

<source lang="C">
int foo(size_t len)
{
    int bar[len];
    something(bar);
    return bar[0];
}
</source>

== Complex numbers ==
{{main|Complex numbers - C|l1=Complex numbers}}

C99 brought support for complex numbers including a set of functions for dealing with complex numbers which can be found in [[complex.h - C|<complex.h>]]. For example,

<source lang="C">
#include <stdio.h>
#include <complex.h>
int main()
{
        double complex val = 4 * I;
        val = cpow(val, 2);
        printf("%f + %fi\n", creal(val), cimag(val));
        return 0;
}
</source>

Which outputs
 -16.000000 + 0.000000i

== Type-generic math ==
{{main|tgmath.h - C|l1=<tgmath.h>}}

C99 introduce the [[tgmath.h - C|<tgmath.h>]] header which provide [[generic selection - C|type-generic macros]] that determine the function depending on the arguments provided.

== Extended identifiers ==
{{main|Extended identifiers - C|l1=Extended identifiers}}

C99 brought support for extended identifiers and extended characters. For example:

<source lang="C">
#include <stdio.h>
void שלום() { puts("Hello"); }
int main()
{
        שלום();
        \u05E9\u05DC\u05D5\u05DD();
        return 0;
}
</source>

== Compound literal ==
{{main|Compound literals - C|l1=compound literals}}

A compound literal is a postfix expression that provides an unnamed object whose value is given by the initializer list. Such expressions may be const as well. Compound literals take the form <code>(type-name){initializer-list}</code>. For example,

<source lang="C">
#include <stdio.h>
#include <string.h>
int main()
{
        char *p = (char[]){"string"};
        p[2] = '@';
        p = (char[100]){0};
        strcpy(p, "example");
        puts(p);
        return 0;
}
</source>

== Designated initializers ==
{{main|designated initializers - C|l1=designated initializers}}

'''Designated initializers''' are a feature added in C99 that allows a particular element to be initialized. Designated initializers are supported for [[Arrays - C|arrays]], [[Structures - C|structs]], and [[Unions - C|unions]]. For example:

<source lang="C">
#include <stdio.h>
#include <string.h>
enum bosses { aquamentus, dodongo, manhandla, gleeok };
struct boss
{
    const char *name;
    const char *weapon[5];
}
boss_list[] =
{
   [aquamentus] = { .weapon = { [1] = "bombs", [0] = "sword", [2] = 0 }, .name = "aquamentus"},
   [dodongo]    = { .weapon = { [0] = "bombs", [1] = "sword", [2] = 0 }, .name = "dodongo" },
   [manhandla]  = { .weapon = { [0] = "bombs", [1] = "sword", [2] = 0 }, .name = "manhandla" },
   [gleeok]     = { .weapon = { [0] = "bombs", [2] = 0, [1] = "sword" }, .name = "gleeok"}
};
int main()
{
    printf("%s -> %s\n", boss_list[manhandla].name, boss_list[manhandla].weapon[0]);
    return 0;
}
</source>

== Single-line comments ==
C99 introduced the <code>//</code> notation for single-line comments. Single-line comments start with two [[slashes]] and terminate at the end of the line.
<source lang="C">// this is a comment</source>

== Extended integer types ==
{{main|Extended integer types - C|l1=Extended integer types}}

Starting with C99, extended integer types were added to the C programming language. Extended integer types, which come in pairs of signed and unsigned types, are defined by the implementation in a manner they choose. They must obey the same standard integer rules but their names is up to the implementation, for example:

<source lang="C">
long long long int x = 333333;
__int128 y = 123;
</source>

== Mixed declarations and code ==
C99 relaxed the restrictions that required declarations to be at the beginning of a code block. Starting with C99, declarations and code may be mixed.

== Variadic macros ==
C99 added support for variadic macros. The new <code>__VA_ARGS__</code> preprocessor keyword must only be used within the replacement list of a function-like macro that uses the ellipsis notation in the arguments. For example:

<source lang="C">
#define debug(...) fprintf(stderr, __VA_ARGS__)
debug("Hello World!\n");
debug("Hello %s!\n", "Bob");
</source>

== inline functions ==
{{main|inline functions - C|l1=inline functions}}

Support for inline functions were added in C99.

== boolean types ==
{{main|stdbool.h - C|l1=<stdbool.h>}}

C99 introduced a new boolean type with the <code>_Bool</code> keyword. A set of convenient macros are defined in the [[stdbool.h - C|<stdbool.h>]] header.

<source lang="C">
#include <stdio.h>
#include <stdbool.h>
int main(void)
{
    bool foo = true;
    if (foo)
        puts("It's true!");
    return 0;
}
</source>

== Compiler support ==
Support for C99 has been a slow ongoing effort.

{| class="wikitable"
|-
| style="background:#CCCCFF;text-align:center;" colspan="4" | C99 Compiler Support
|-
! Compiler Name !! Support Level !! style="text-align:center;" | Enforcement Switch !! Note
|-
| ACK || style="background:#FF6666;text-align:center;" | No Support || ||
|-
| Clang || style="background:#99FF66;text-align:center;" | Complete || style="text-align:center;" | -std=c99 || Supports everything<ref>[http://p99.gforge.inria.fr/c99-conformance/c99-conformance-clang-3.2.html C99 Conformance CLANG Test]</ref>
|-
| GCC || style="background:#99FF66;text-align:center;" | Complete || style="text-align:center;" | -std=c99 || Supports everything<ref>[http://p99.gforge.inria.fr/c99-conformance/c99-conformance-gcc-4.7.html C99 Conformance GCC Test]</ref>
|-
| TCC || style="background:#FFCC66;text-align:center;" | Partial || || Missing lots of features<ref>[http://p99.gforge.inria.fr/c99-conformance/c99-conformance-tcc-0.9.25.html C99 Conformance TCC Test]</ref>
|-
| ICC || style="background:#FFCC66;text-align:center;" | Partial || style="text-align:center;" | -std=c99 || Broken extended identifier support, broken inline function support<ref>[http://p99.gforge.inria.fr/c99-conformance/c99-conformance-icc-12.0.0.html C99 Conformance ICC Test]</ref>
|-
| Visual C++ || style="background:#FFCC66;text-align:center;" | Partial ||   || Missing type-generic math<ref>[http://blogs.msdn.com/b/vcblog/archive/2013/07/19/c99-library-support-in-visual-studio-2013.aspx C99 library support in Visual Studio 2013]</ref>
|}

== References ==
{{reflist}}

== External links ==
* [http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1124.pdf C99 with Technical Corrigendum included]

[[Category:C standards]]