ANSI C and ISO C

During the late 1970s and 1980s, versions of C were implemented for a wide variety of mainframe computers, minicomputers, and microcomputers, including the IBM PC, as its popularity began to increase significantly.

In 1983, the American National Standards Institute (ANSI) formed a committee, X3J11, to establish a standard specification of C. In 1989, the standard was ratified as ANSI X3.159-1989 "Programming Language C." This version of the language is often referred to as ANSI C, Standard C, or sometimes C89.

In 1990, the ANSI C standard (with a few minor modifications) was adopted by the International Organization for Standardization (ISO) as ISO/IEC 9899:1990. This version is sometimes called C90. Therefore, the terms "C89" and "C90" refer to essentially the same language.

One of the aims of the C standardization process was to produce a superset of K&R C, incorporating many of the unofficial features subsequently introduced. However, the standards committee also included several new features, such as function prototypes (borrowed from C++), void pointers, support for international character sets and locales, and preprocessor enhancements. The syntax for parameter declarations was also augmented to include the C++ style:

int main(int argc, char **argv)
{
...
}
although the K&R interface

int main(argc, argv)
int argc;
char **argv;
{
...
}
continued to be permitted, for compatibility with existing source code.

C89 is supported by current C compilers, and most C code being written nowadays is based on it. Any program written only in Standard C and without any hardware-dependent assumptions will run correctly on any platform with a conforming C implementation, within its resource limits. Without such precautions, programs may compile only on a certain platform or with a particular compiler, due, for example, to the use of non-standard libraries, such as GUI libraries, or to a reliance on compiler- or platform-specific attributes such as the exact size of data types and byte endianness.

In cases where code must be compilable by either standard-conforming or K&R C-based compilers, the __STDC__ macro can be used to split the code into Standard and K&R sections to take advantage of features available only in Standard C.

#ifdef __STDC__
extern int getopt(int,char * const *,const char *);
#else
extern int getopt();
#endif
In the above example, a compiler which has defined the __STDC__ macro (as mandated by the C standard) only interprets the line following the ifdef command. In other, nonstandard compilers which don't define the macro, only the line following the else command is interpreted.


C99
Note: C99 is also the name of a C compiler for the Texas Instruments TI-99/4A home computer. Aside from being a C compiler, it is otherwise unrelated.
After the ANSI standardization process, the C language specification remained relatively static for some time, whereas C++ continued to evolve, largely during its own standardization effort. Normative Amendment 1 created a new standard for the C language in 1995, but only to correct some details of the C89 standard and to add more extensive support for international character sets. However, the standard underwent further revision in the late 1990s, leading to the publication of ISO 9899:1999 in 1999. This standard is commonly referred to as "C99." It was adopted as an ANSI standard in May 2000.


New features
C99 introduced several new features, many of which had already been implemented as extensions in several compilers:

inline functions
variable declaration no longer restricted to file scope or the start of a compound statement
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
support for one-line comments beginning with //, as in BCPL or C++
new library functions, such as snprintf
new header files, such as stdbool.h and inttypes.h
type-generic math functions (tgmath.h)
improved support for IEEE floating point
designated initializers
compound literals
support for variadic macros (macros of variable arity)
restrict qualification to allow more aggressive code optimization

Upward-compatibility with C90
C99 is for the most part upward-compatible with C90, but is stricter in some ways; 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 diagnose the omission but also assume int and continue translating the program.


Support by major compilers
GCC and other C compilers now support many of the new features of C99. However, there has been less support from vendors such as Microsoft and Borland that have mainly focused on C++, since C++ provides similar functionality improvement.

GCC, despite its extensive C99 support, is still not a completely compliant implementation; several key features are missing or don't work correctly.[6]

According to Sun Microsystems, Sun Studio Compiler Suite (which is freely downloadable) now supports the full C99 standard.[7]


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.

#if __STDC_VERSION__ >= 199901L
/* "inline" is a keyword */
#else
# define inline /* nothing */
#endif