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 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, long long int, extended identifiers, hexadecimal floating-point constants, compound literals, designated initializers, single line, // 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 199901L.

#if __STDC_VERSION__ >= 199901L
    /* C99 support */
#endif

New headers

C99 introduced 6 new standard headers: <tgmath.h>, <stdint.h>, <stdbool.h>, <inttypes.h>, <fenv.h>, and <complex.h>.

Restricted pointers

Main article: Restricted pointers

C99 introduced the concept of restricted pointer to the C language through the introduction of the restrict 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:

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);

Variable-length array

Main article: 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.

int foo(size_t len)
{
    int bar[len];
    something(bar);
    return bar[0];
}

Complex numbers

Main article: 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>. For example,

#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;
}

Which outputs

-16.000000 + 0.000000i

Compiler support

Support for C99 has been a slow ongoing effort.

C99 Compiler Support
Compiler Name	Support Level	Enforcement Switch	Note
ACK	No Support
Clang	Complete	-std=c99	No extended identifier support
GCC	Complete	-std=c99	Supports everything
TCC	Partial		Missing lots of features
ICC	Partial	-std=c99	Broken extended identifier support, broken inline function support
Visual C++	Partial		Missing type-generic math