c90(7) manual page

C is a general purpose programming language, which has a strong connection with the UNIX operating system and its derivatives, since the vast majority of those systems were written in the C language. The C language contains some basic ideas from the BCPL language through the B language written by Ken Thompson in 1970 for the DEC PDP-7 machines. The development of the UNIX operating system was started on a PDP-7 machine in assembly language, but it made very difficult to port the existing code to other systems.

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

The C language did not have a specification or standardized version for a long time. It went through a lot of changes and improvements for ages. In 1978, Brian W. Kernighan and Dennis M. Ritchie published the first 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 of the first edition of the book.

It is important to notice, that the instruction set of the language is limited to the most fundamental elements for simplicity. Handling of the standard I/O and such common functions are implemented in the libraries shipped with the compiler. As these functions are also widely used, it was demanded to include into the description what requisites the library should conform to, not just strictly the language itself. Accordingly, the aforementioned standards cover the library elements, as well. The elements of this standard library is still not enough for more complicated tasks. In this case the provided system calls of the given operating system can be used. 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 standard and thus is beyond the scope of this manual. The standards discussed below are all C standards and only cover the C programming language and the accompanying library.

After the publication of the book mentioned before, the American National Standards Institute (ANSI) started to work on standardizing the language, and they announced ANSI X3.159-1989 in 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 to check the sanity of the actual parameters at a function call. The old syntax was highly error-prone because incompatible parameters were hard to detect in the program code and the problem only showed up at run-time.

In 1990, the International Organization for Standardization (ISO) adopted the 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 the development of the language did not stop.

The ISO C standard was later extended with an amendment as ISO/IEC 9899 AM1 in 1995. This contained, for example, the wide-character support in wchar.h and wctype.h. Two corrigenda were also published: Technical Corrigendum 1 as ISO/IEC 9899 TCOR1 in 1995 and Technical Corrigendum 2 as ISO/IEC 9899 TCOR1 in 1996. The continuous development and growth made it necessary to work out a new standard, which contains the new features and fixes the known defects and deficiencies of the language. As a result, ISO/IEC 9899:1999 was born in 1999. Similarly to the other standards, this is referred after the publication 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 types defined 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 the program or program blocks
No implicit int type

Since then new standards have not been published, but the C language is still evolving. New and useful features have been showed up in the most famous C 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 as standard features.

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