System programming

Language С is the general purpose programming language. Occurrence of

him is closely connected to creation of operational system UNIX for he was developed specially for this system and on him the most part of system software UNIX is written. However language С is not focused on work only in one operational system or by any concrete machine.

designs for the organization of cycles and the conditional transitions

providing an opportunity of a spelling of flexible and well structured programs;

the big set of operators and operations, many of which suppose direct translation in a machine code;

variety of primitive types of the data, including an opportunity of management in length of integer and material variables;

presence of means for designing new aggregated types of the data;

machine addresses through the device of indexes;
presence of the preprocessor,

allowing to make change to the text of the program directly ahead of its compilation;
block structure of a program code, an opportunity of work with various classes of memory and means for modular programming

to number compiling programming languages. It means, that preparation for

performance of the С-program includes the following stages:
input of the initial text of the program in a file by means of any editor of texts (the name of a file, as a rule, has standard expansion);
compilation of the program, i.e. transformation of her description in the source language in a semantic equivalent on a computer language, named the objective module (the name of a file in which the result of compilation is located, usually has expansion "OBJ");
construction of the loading module ready to performance from objective modules, including modules from external libraries (the file containing the ready program, has a name with expansion "EXE").

big letters of the latin alphabet A B C D E

F G H I J K L M N O P Q R S T U V W X Y Z - small letters of the latin alphabet a b c d e f g h i j k l m n o p q r s t u v w x y z
decimal figures 0123456789

% and ' () * +,-./:; ? [\] ^ _

{|} ~

Names of objects programs (identifiers) Identifiers in language С are sequences of letters and the figures, beginning with the letter, and the symbol of underlining (_) is considered by the compiler as the letter. The big and small letters of the latin alphabet are considered various. Length of the identifier be formal can any.

predetermined identifiers having special values. Names of any objects of

the program should not coincide with the following reserved key words:
types of the data char float short typedef void double int signed union enum long struct unsigned
classes of memory auto extern register static

default for return while case

do goto sizeof continue else if switch
special const far huge pascal cdecl fortran near volatile

sequences of symbols ignored by the compiler. Comments have the

following form:
/* Characters */
where Characters there is an any combination of any printed symbols ASCII.

be numbers (the whole and material), symbols and lines of

symbols which is allowable to use in the program in sense of their values. Value of any constant cannot be changed during work of the program. Special instructions of language enable to give to constants of any type symbolical names.

to understand the actions which are carried out by the

program above objects determined in her as operations. For a designation of operations some standard combinations of special symbols from the alphabet of language are used. The objects of the program participating in operation, refer to as operands. Any combination of one or greater number of operands and symbols of the operations, giving a unique value, forms expression. Any expression, coming to an end a semicolon, is the operator. The set of all operations of language С is convenient for dividing into eight following groups:

++--
operations of the relation ==! = =
logic operations !

|| &&
bit-by-bit operations ~ <>> & | ^
operations of giving = + = - = * = / = % = <<=>> = and = ^ = | =

and associations ->
other operations ?:, syzeof (type specifier) ()

С is any expression, coming to an end a semicolon.

In particular, separately worth semicolon is interpreted by the compiler as the zero - operator. The most significant group of operators is formed with instructions (operators) of management of process of performance of the program. To them concern:

(; ; ) statement;
while (expression) statement;

do statement while (expression);

if (expression)

statement1
switch (expression) {> .. .. .. > .. .. .. >}

continue return
Here expression there is an any correct expression

of language С (C++), and through statement it is designated simple or compound the operator whom, in particular, can be and any operator of management (see lower). The last means, that the investment of operators of management each other is allowable. Elements of designs of language, prisoners in angular brackets (<>), are not obligatory and can be lowered. One or several operators made in braces, the compound operator or the block form. The semicolon after a closing bracket thus is not put.

the lines of the program determining names and the characteristics

of functions, variable, types and the symbolical constants used in the program.
General structure of the program
Any program in language С represents set of the functions which are carrying out the basic work on realization of some algorithm. Each of these functions, in turn, is an independent set of descriptions and the operators made between heading of function and her end. That function with which performance of the program begins, refers to as the main function. She should have the predetermined name main ().