In 1972 Dennis M. Ritchie worked out the C programming language forfurther development of the UNIX operating system.The idea was to implement only the C compiler for differentplatforms, and implement most part of the operating systemin the new programming language to simplify the portability betweendifferent architectures.It follows that C is very eligible for (but not limited to) writingoperating systems and low-level applications.

The C language did not have a specification or standardized version fora long time.It went through a lot of changes and improvements for ages.In 1978, Brian W. Kernighan and Dennis M. Ritchie published thefirst book about C under the title "The C Programming Language".We can think of this book as the first specification of the language.This version is often referred as K&R; C after the names of the authors.Sometimes it is referred as C78, as well, after the publishing year ofthe first edition of the book.

It is important to notice, that the instruction set of the language islimited to the most fundamental elements for simplicity.Handling of the standard I/O and such common functions are implemented inthe libraries shipped with the compiler.As these functions are also widely used, it was demanded to include intothe description what requisites the library should conform to, not juststrictly the language itself.Accordingly, the aforementioned standards cover the library elements, as well.The elements of this standard library is still not enough for morecomplicated tasks.In this case the provided system calls of the given operating system can beused.To not lose the portability by using these system calls, the POSIX(Portable Operating System Interface) standard evolved.It describes what functions should be available to keep portability.Note, that POSIX is not a C standard, but an operating system standardand thus is beyond the scope of this manual.The standards discussed below are all C standards and only coverthe C programming language and the accompanying library.

After the publication of the book mentioned before,the American National Standards Institute (ANSI) started to work onstandardizing the language, and they announced ANSI X3.159-1989in 1989.It is usually referred as ANSI C or C89.The main difference in this standard were the function prototypes,which is a new way of declaring functions.With the old-style function declarations, the compiler was unable tocheck the sanity of the actual parameters at a function call.The old syntax was highly error-prone because incompatible parameterswere hard to detect in the program code and the problem only showed upat run-time.

In 1990, the International Organization for Standardization (ISO) adoptedthe ANSI standard as ISO/IEC 9899:1990 in 1990.This is also referred as ISO C or C90.It only contains negligible minor modifications against ANSI C,so the two standards often considered to be fully equivalent.This was a very important milestone in the history of the C language, but thedevelopment of the language did not stop.

The ISO C standard was later extended with an amendment asISO/IEC 9899 AM1 in 1995.This contained, for example, the wide-character support in wchar.h andwctype.h.Two corrigenda were also published: Technical Corrigendum 1 asISO/IEC 9899 TCOR1 in 1995 and Technical Corrigendum 2 as ISO/IEC 9899 TCOR1in 1996.The continuous development and growth made it necessary to work out a newstandard, which contains the new features and fixes the known defects anddeficiencies of the language.As a result, ISO/IEC 9899:1999 was born in 1999.Similarly to the other standards, this is referred after thepublication year as C99.The improvements include the following:

• Inline functions
• Support for variable length arrays
• New high-precision integer type named long long int, and other integer typesdefined in stdint.h
• New boolean data type implemented in stdbool.h
• One line comments taken from the C++ language
• Some new preprocessor features
• New variables can be declared anywhere, not just in the beginning of theprogram or program blocks
• No implicit int type

Since then new standards have not been published, but the C language is stillevolving.New and useful features have been showed up in the most famousC compiler: GNU C.Most of the UNIX-like operating systems use GNU C as a system compiler,but those addition in GNU C should not be considered asstandard features.

ANSI,X3.159-1989,

ISO/IEC,9899:1990, Programming languages -- C,

ISO/IEC,9899 AM1,

ISO/IEC,9899 TCOR1, Programming languages -- C, Technical Corrigendum 1,

ISO/IEC,9899 TCOR2, Programming languages -- C, Technical Corrigendum 2,

ISO/IEC,9899:1999, Programming languages -- C,