C language

C is an imperative (procedural) systems implementation language. It was designed to be compiled using a relatively straightforward compiler, to provide low-level access to memory, to provide language constructs that map efficiently to machine instructions, and to require minimal run-time support. C was therefore useful for many applications that had formerly been coded in assembly language.

Despite its low-level capabilities, the language was designed to encourage cross-platform programming. A standards-compliant and portably written C program can be compiled for a very wide variety of computer platforms and operating systems with few changes to its source code. The language has become available on a very wide range of platforms, from embedded microcontrollers to supercomputers.

The C language also exhibits the following more specific characteristics:

There are a small, fixed number of keywords, including a full set of flow of control primitives: for, if, while, switch, and do..while. There is basically one namespace, and user-defined names are not distinguished from keywords by any kind of sigil.

There are a large number of arithmetical and logical operators, such as +, +=, ++, &, ~, etc.

More than one assignment may be performed in a single statement.

Function return values can be ignored when not needed.

Typing is static, but weakly enforced: all data has a type, but implicit conversions can be performed; for instance, characters can be used as integers.

Declaration syntax mimics usage context. C has no "define" keyword; instead, a statement beginning with the name of a type is taken as a declaration. There is no "function" keyword; instead, a function is indicated by the parentheses of an argument list.

User-defined (typedef) and compound types are possible.
- Heterogeneous aggregate data types (struct) allow related data elements to be accessed, for example assigned, as a unit.
- Array indexing is a secondary notion, defined in terms of pointer arithmetic. Unlike structs, arrays are not first-class objects; they cannot be assigned or compared using single built-in operators. There is no "array" keyword, in use or definition; instead, square brackets indicate arrays syntactically, e.g. month.
- Enumerated types are possible with the enum keyword. They are not tagged, and are freely interconvertible with integers.
- Strings are not a separate data type, but are conventionally implemented as null-terminated arrays of characters.

Low-level access to computer memory is possible by converting machine addresses to typed pointers.

Procedures (subroutines not returning values) are a special case of function, with a dummy return type void.

Functions may not be defined within the lexical scope of other functions.

Function and data pointers permit ad hoc run-time polymorphism.

A preprocessor performs macro definition, source code file inclusion, and conditional compilation.

There is a basic form of modularity: files can be compiled separately and linked together, with control over which functions and data objects are visible to other files via static and extern attributes.