C(7)

NAME
c, c78, c89, c90, c99, c11 - The C programming language

DESCRIPTION
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 this choice made it 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 parts of the operating system in the new programming language to simplify the portability between different architectures. It follows that C is very well adapted 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 to as “K&R C” after the names of the authors. Sometimes it is referred to 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 similar 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 are 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 (for Unix)) 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 in 1989 they announced ANSI X3.159-1989. It is usually referred to as ANSI C or C89. The main difference in this standard were the function prototypes, which was a new way of declaring functions. With the old-style function declarations, the compiler was unable to check the sanity of the actual parameters of 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. This is also referred to as ISO C or C90. It only contains negligible minor modifications against ANSI C, so the two standards are 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 TCOR2 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 to after the publication year as C99. The improvements include the following:

•   Inline functions.

•   Support for variable length arrays.

•   New large-range integer type named long long int, and other integer types described in stdint(3) and inttypes(3).

•   New boolean data type; see stdbool(3).

•   One-line comments taken from the C++ language.

•   Some new preprocessor features.

•   A predefined identifier __func__ and a restrict type qualifier.

•   Declarations are allowed after statements, not just in the beginning of the program or program blocks.

•   No implicit int type.

In 2011, another revision of ISO/IEC 9899 was published, nicknamed C11, adding features such as:

•   Compile-time assertions.

•   Type-generic expressions.

•   Unnamed struct and union members.

In 2017, another revision of ISO/IEC 9899 was published, nicknamed C17, containing corrections to C11, but no new features.

Since then no new standards have been published, but the C language is still evolving.

Most of the UNIX-like operating systems use GNU C as a system compiler, but the various extensions of GNU C, such as attribute(3) or typeof(3), should not be considered standard features.

SEE ALSO
c89(1), c99(1), c11(1), cc(1), cdefs(3) Brian W. Kernighan, Dennis M. Ritchie, The C Programming Language, 1988, Second Edition, 40th printing, Prentice Hall.

STANDARDS
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.

ISO/IEC, 9899:1999 TCOR1, Programming languages -- C, Technical Corrigendum 1.

ISO/IEC, 9899:1999 TCOR2, Programming languages -- C, Technical Corrigendum 2.

ISO/IEC, 9899:1999 TCOR3, Programming languages -- C, Technical Corrigendum 3.

ISO/IEC, 9899:2011, Programming languages -- C.

HISTORY
This manual page first appeared in FreeBSD 9.0 and NetBSD 6.0.

AUTHORS
This manual page was written by Gabor Kovesdan <gabor@FreeBSD.org>.