Data structures

C has a static weak typing type system that shares some similarities with that of other ALGOL descendants such as Pascal. There are built-in types for integers of various sizes, both signed and unsigned, floating-point numbers, characters, and enumerated types (enum). C99 added a boolean datatype. There are also derived types including arrays, pointers, records (struct), and untagged unions (union).
C is often used in low-level systems programming where escapes from the type system may be necessary. The compiler attempts to ensure type correctness of most expressions, but the programmer can override the checks in various ways, either by using a type cast to explicitly convert a value from one type to another, or by using pointers or unions to reinterpret the underlying bits of a value in some other way.

[edit] Pointers
C supports the use of pointers, a very simple type of reference that records, in effect, the address or location of an object or function in memory. Pointers can be dereferenced to access data stored at the address pointed to, or to invoke a pointed-to function. Pointers can be manipulated using assignment and also pointer arithmetic. The run-time representation of a pointer value is typically a raw memory address (perhaps augmented by an offset-within-word field), but since a pointer's type includes the type of the thing pointed to, expressions including pointers can be type-checked at compile time. Pointer arithmetic is automatically scaled by the size of the pointed-to data type. (See Array↔pointer interchangeability below.) Pointers are used for many different purposes in C. Text strings are commonly manipulated using pointers into arrays of characters. Dynamic memory allocation, which is described below, is performed using pointers. Many data types, such as trees, are commonly implemented as dynamically allocated struct objects linked together using pointers. Pointers to functions are useful for callbacks from event handlers.
A null pointer is a pointer value that points to no valid location (it is often represented by address zero). Dereferencing a null pointer is therefore meaningless, typically resulting in a run-time error. Null pointers are useful for indicating special cases such as no next pointer in the final node of a linked list, or as an error indication from functions returning pointers.
Void pointers (void *) point to objects of unknown type, and can therefore be used as "generic" data pointers. Since the size and type of the pointed-to object is not known, void pointers cannot be dereferenced, nor is pointer arithmetic on them allowed, although they can easily be (and in many contexts implicitly are) converted to and from any other object pointer type.

[edit] Arrays
Array types in C are always one-dimensional and, traditionally, of a fixed, static size specified at compile time. (The more recent C99 standard also allows a form of variable-length arrays.) However, it is also possible to allocate a block of memory (of arbitrary size) at run-time, using the standard library's malloc function, and treat it as an array. C's unification of arrays and pointers (see below) means that true arrays and these dynamically-allocated, simulated arrays are virtually interchangeable. Since arrays are always accessed (in effect) via pointers, array accesses are typically not checked against the underlying array size, although the compiler may provide bounds checking as an option. Array bounds violations are therefore possible and rather common in carelessly written code (see the "Criticism" article), and can lead to various repercussions: illegal memory accesses, corruption of data, buffer overrun, run-time exceptions, etc.
C does not have a special provision for declaring multidimensional arrays, but rather relies on recursion within the type system to declare arrays of arrays, which effectively accomplishes the same thing. The index values of the resulting "multidimensional array" can be thought of as increasing in row-major order.

[edit] Array↔pointer interchangeability
A distinctive (but potentially confusing) feature of C is its treatment of arrays and pointers. The array-subscript notation x[i] can also be used when x is a pointer; the interpretation (using pointer arithmetic) is to access the (i+1)th of several adjacent data objects pointed to by x, counting the object that x points to (which is x[0]) as the first element of the array.
Formally, x[i] is equivalent to *(x + i). Since the type of the pointer involved is known to the compiler at compile time, the address that x + i points to is not the address pointed to by x incremented by i bytes, but rather incremented by i multiplied by the size of an element that x points to. The size of these elements can be determined with the operator sizeof by applying it to any dereferenced element of x, as in n = sizeof *x or n = sizeof x[0].
Furthermore, in most expression contexts (a notable exception is sizeof array), the name of an array is automatically converted to a pointer to the array's first element; this implies that an array is never copied as a whole when named as an argument to a function, but rather only the address of its first element is passed. Therefore, although C's function calls use pass-by-value semantics, arrays are in effect passed by reference.
The number of elements in a declared array a can be determined as sizeof a / sizeof a[0].
An interesting demonstration of the interchangeability of pointers and arrays is shown below. The four assignments are equivalent and each is valid C code. Note how the last line contains the strange code i[x] = 1;, which has the index variable i apparently interchanged with the array variable x. This last line might be found in obfuscated C C code./* x designates an array */
x[i] = 1;
*(x + i) = 1;
*(i + x) = 1;
i[x] = 1; /* strange, but correct: i[x] is equivalent to *(i + x) */
However, there is a distinction to be made between arrays and pointer variables. Even though the name of an array is in most expression contexts converted to a pointer (to its first element), this pointer does not itself occupy any storage. Consequently, you cannot change what an array "points to", and it is impossible to assign to an array. (Arrays may however be copied using the memcpy function, for example.)

[edit] Memory management
One of the most important functions of a programming language is to provide facilities for managing memory and the objects that are stored in memory. C provides three distinct ways to allocate memory for objects:
Static memory allocation: space for the object is provided in the binary at compile-time; these objects have an extent (or lifetime) as long as the binary which contains them is loaded into memory
Automatic memory allocation: temporary objects can be stored on the stack, and this space is automatically freed and reusable after the block in which they are declared is exited
Dynamic memory allocation: blocks of memory of arbitrary size can be requested at run-time using library functions such as malloc from a region of memory called the heap; these blocks persist until subsequently freed for reuse by calling the library function free
These three approaches are appropriate in different situations and have various tradeoffs. For example, static memory allocation has no allocation overhead, automatic allocation may involve a small amount of overhead, and dynamic memory allocation can potentially have a great deal of overhead for both allocation and deallocation. On the other hand, stack space is typically much more limited and transient than either static memory or heap space, and dynamic memory allocation allows allocation of objects whose size is known only at run-time. Most C programs make extensive use of all three.
Where possible, automatic or static allocation is usually preferred because the storage is managed by the compiler, freeing the programmer of the potentially error-prone chore of manually allocating and releasing storage. Unfortunately, many data structures can grow in size at runtime, and since static allocations (and automatic allocations in C89 and C90) must have a fixed size at compile-time, there are many situations in which dynamic allocation must be used. Prior to the C99 standard, variable-sized arrays were a common example of this (see "malloc" for an example of dynamically allocated arrays).

[edit] Libraries
The C programming language uses libraries as its primary method of extension. In C, a library is a set of functions contained within a single "archive" file. Each library typically has a header file, which contains the prototypes of the functions contained within the library that may be used by a program, and declarations of special data types and macro symbols used with these functions. In order for a program to use a library, it must include the library's header file, and the library must be linked with the program, which in many cases requires compiler flags (e.g., -lm, shorthand for "math library").
The most common C library is the C standard library, which is specified by the ISO and ANSI C standards and comes with every C implementation. ("Freestanding" [embedded] C implementations may provide only a subset of the standard library.) This library supports stream input and output, memory allocation, mathematics, character strings, and time values.
Another common set of C library functions are those used by applications specifically targeted for Unix and Unix-like systems, especially functions which provide an interface to the kernel. These functions are detailed in various standards such as POSIX and the Single UNIX Specification.
Since many programs have been written in C, there are a wide variety of other libraries available. Libraries are often written in C because C compilers generate efficient object code; programmers then create interfaces to the library so that the routines can be used from higher-level languages like Java, Perl, and Python.