C has a
formal grammar specified by the C standard. Line endings are generally not significant in C; however, line boundaries do have significance during the preprocessing phase. Comments may appear either between the delimiters /* and */, or (since C99) following // until the end of the line. Comments delimited by /* and */ do not nest, and these sequences of characters are not interpreted as comment delimiters if they appear inside
string or character literals. C source files contain declarations and function definitions. Function definitions, in turn, contain declarations and
statements. Declarations either define new types using keywords such as struct, union, and enum, or assign types to and perhaps reserve storage for new variables, usually by writing the type followed by the variable name. Keywords such as char and int specify built-in types. Sections of code are enclosed in braces ({ and }, sometimes called "curly brackets") to limit the scope of declarations and to act as a single statement for control structures. As an imperative language, C uses
statements to specify actions. The most common statement is an
expression statement, consisting of an expression to be evaluated, followed by a semicolon; as a
side effect of the evaluation,
functions may be called and
variables assigned new values. To modify the normal sequential execution of statements, C provides several control-flow statements identified by reserved keywords.
Structured programming is supported by if ... [else] conditional execution and by do ... while, while, and for iterative execution (looping). The for statement has separate initialization, testing, and reinitialization expressions, any or all of which can be omitted. break and continue can be used within the loop. Break is used to leave the innermost enclosing loop statement and continue is used to skip to its reinitialisation. There is also a non-structured
goto statement, which branches directly to the designated
label within the function.
switch selects a case to be executed based on the value of an integer expression. Different from many other languages, control-flow will
fall through to the next case unless terminated by a break. Expressions can use a variety of built-in operators and may contain function calls. The order in which arguments to functions and operands to most operators are evaluated is unspecified. The evaluations may even be interleaved. However, all side effects (including storage to variables) will occur before the next "
sequence point"; sequence points include the end of each expression statement, and the entry to and return from each function call. Sequence points also occur during evaluation of expressions containing certain operators (&&, ||,
?: and the
comma operator). This permits a high degree of object code optimization by the compiler, but requires C programmers to take more care to obtain reliable results than is needed for other programming languages. Kernighan and Ritchie say in the Introduction of
The C Programming Language: "C, like any other language, has its blemishes. Some of the operators have the wrong precedence; some parts of the syntax could be better." The C standard did not attempt to correct many of these blemishes, because of the impact of such changes on already existing software.
Character set The basic C source character set includes the following characters: • Lowercase and uppercase letters of the
ISO basic Latin alphabet: a–z, A–Z • Decimal digits: 0–9 • Graphic characters: ! " # % & ' ( ) * + , - . / : ; ? [ \ ] ^ _ { | } ~ •
Whitespace characters:
space,
horizontal tab,
vertical tab,
form feed,
newline The
newline character indicates the end of a text line; it need not correspond to an actual single character, although for convenience C treats it as such. The POSIX standard mandates a
portable character set which adds a few characters (notably "@") to the basic C source character set. Both standards do not prescribe any particular value encoding—
ASCII and
EBCDIC both comply with these standards, since they include at least those basic characters, even though they use different encoded values for those characters. Additional multi-byte encoded characters may be used in
string literals, but they are not entirely
portable. Since
C99 multi-national Unicode characters can be embedded portably within C source text by using \uXXXX or \UXXXXXXXX encoding (where X denotes a hexadecimal character). The basic C execution character set contains the same characters, along with representations for the
null character,
alert,
backspace, and
carriage return. (‡ indicates an alternative spelling alias for a C23 keyword) • _Alignas ‡ • _Alignof ‡ • _Atomic • _Generic • _Noreturn • _Static_assert ‡ • _Thread_local ‡ C23 reserved fifteen more words: • alignas • alignof • bool • constexpr • false • nullptr • static_assert • thread_local • true • typeof • typeof_unqual • _BitInt • _Decimal32 • _Decimal64 • _Decimal128 Most of the recently reserved words begin with an underscore followed by a capital letter, because identifiers of that form were previously reserved by the C standard for use only by implementations. Since existing program source code should not have been using these identifiers, it would not be affected when C implementations started supporting these extensions to the programming language. Some standard headers do define more convenient synonyms for underscored identifiers. Some of those words were added as keywords with their conventional spelling in C23 and the corresponding macros were removed. Prior to C89, entry was reserved as a keyword. In the second edition of their book
The C Programming Language, which describes what became known as C89, Kernighan and Ritchie wrote, "The ... [keyword] entry, formerly reserved but never used, is no longer reserved." and "The stillborn entry keyword is withdrawn."
Operators C supports a rich set of
operators, which are symbols used within an
expression to specify the manipulations to be performed while evaluating that expression. C has operators for: •
arithmetic:
+,
-,
*,
/,
% •
assignment: = •
augmented assignment: •
bitwise logic: ~, &, |, ^ •
bitwise shifts: <<, >> •
Boolean logic: !, &&, || •
conditional evaluation:
? : • equality testing:
==,
!= •
calling functions: ( ) •
increment and decrement: ++, -- •
member selection: ., -> • object size:
sizeof • type:
typeof, typeof_unqual
since C23 •
order relations: <, <=, >, >= •
reference and dereference: &, *, [ ] • sequencing:
, •
subexpression grouping: ( ) •
type conversion: (
typename) C uses the operator = (used in mathematics to express equality) to indicate assignment, following the precedent of
Fortran and
PL/I, but unlike
ALGOL and its derivatives. C uses the operator == to test for equality. The similarity between the operators for assignment and equality may result in the accidental use of one in place of the other, and in many cases the mistake does not produce an error message (although some compilers produce warnings). For example, the conditional expression if (a == b + 1) might mistakenly be written as if (a = b + 1), which will be evaluated as true unless the value of a is 0 after the assignment. The C
operator precedence is not always intuitive. For example, the operator == binds more tightly than (is executed prior to) the operators & (bitwise AND) and | (bitwise OR) in expressions such as x & 1 == 0, which must be written as (x & 1) == 0 if that is the coder's intent.
Data types The
type system in C is
static and
weakly typed, which makes it similar to the type system of
ALGOL descendants such as
Pascal. There are built-in types for integers of various sizes, both signed and unsigned,
floating-point numbers, and enumerated types (enum). Integer type char is often used for single-byte characters. C99 added a
Boolean data type. There are also derived types including
arrays,
pointers,
records (
struct), and
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 data object in some other way. Some find C's declaration syntax unintuitive, particularly for
function pointers. (Ritchie's idea was to declare identifiers in contexts resembling their use: "
declaration reflects use".) C's
usual arithmetic conversions allow for efficient code to be generated, but can sometimes produce unexpected results. For example, a comparison of signed and unsigned integers of equal width requires a conversion of the signed value to unsigned. This can generate unexpected results if the signed value is negative.
Pointers C supports the use of
pointers, a type of
reference that records 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 or
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. Pointers are used for many purposes in C.
Text strings are commonly manipulated using pointers into arrays of characters.
Dynamic memory allocation is performed using pointers; the result of a malloc is usually
cast to the data type of the data to be stored. Many data types, such as
trees, are commonly implemented as dynamically allocated struct objects linked together using pointers. Pointers to other pointers are often used in multi-dimensional arrays and arrays of struct objects. Pointers to functions (
function pointers) are useful for passing functions as arguments to
higher-order functions (such as
qsort or
bsearch), in
dispatch tables, or as
callbacks to
event handlers. Array bounds violations are therefore possible and can lead to various repercussions, including illegal memory accesses, corruption of data,
buffer overflows, and run-time exceptions. C does not have a special provision for declaring
multi-dimensional 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 "multi-dimensional array" can be thought of as increasing in
row-major order. Multi-dimensional arrays are commonly used in numerical algorithms (mainly from applied
linear algebra) to store matrices. The structure of the C array is well suited to this particular task. However, in early versions of C the bounds of the array must be known fixed values or else explicitly passed to any subroutine that requires them, and dynamically sized arrays of arrays cannot be accessed using double indexing. (A workaround for this was to allocate the array with an additional "row vector" of pointers to the columns.) C99 introduced "variable-length arrays" which address this issue. The following example using modern C (C99 or later) shows allocation of a two-dimensional array on the heap and the use of multi-dimensional array indexing for accesses (which can use bounds-checking on many C compilers): int func(int n, int m) { float (*p)[n][m] = malloc(sizeof *p); if (p == NULL) { return -1; } for (int i = 0; i And here is a similar implementation using C99's
Auto VLA feature: int func(int n, int m) { // Caution: checks should be made to ensure n * m * sizeof(float) does NOT exceed limitations for auto VLAs and is within available size of stack. float p[n][m]; // auto VLA is held on the stack, and sized when the function is invoked for (int i = 0; i
Array–pointer interchangeability The subscript notation x[i] (where x designates a pointer) is
syntactic sugar for *(x+i). Taking advantage of the compiler's knowledge of the pointer type, the address that x + i points to is not the base address (pointed to by x) incremented by i bytes, but rather is defined to be the base address incremented by i multiplied by the size of an element that x points to. Thus, x[i] designates the i+1th element of the array. Furthermore, in most expression contexts (a notable exception is as operand of
sizeof), an expression of array type 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 function calls in C use
pass-by-value semantics, arrays are in effect passed by
reference. The total size of an array x can be determined by applying sizeof to an expression of array type. The size of an element can be determined by applying the operator sizeof to any dereferenced element of an array A, as in n = sizeof A[0]. Thus, the number of elements in a declared array A can be determined as sizeof A / sizeof A[0]. Note, that if only a pointer to the first element is available as it is often the case in C code because of the automatic conversion described above, the information about the full type of the array and its length are lost.
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 principal 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 realloc or free. These three approaches are appropriate in different situations and have various trade-offs. For example, static memory allocation has little allocation overhead, automatic allocation may involve slightly more overhead, and dynamic memory allocation can potentially have a great deal of overhead for both allocation and deallocation. The persistent nature of static objects is useful for maintaining state information across function calls, automatic allocation is easy to use but stack space is typically much more limited and transient than either static memory or heap space, and dynamic memory allocation allows convenient 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 simplest because the storage is managed by the compiler, freeing the programmer of the potentially error-prone chore of manually allocating and releasing storage. However, many data structures can change in size at run time, and since static allocations (and automatic allocations before C99) must have a fixed size at compile time, there are many situations in which dynamic allocation is necessary. Prior to the C99 standard, variable-sized arrays were a common example of this. (See the article on
C dynamic memory allocation for an example of dynamically allocated arrays.) Unlike automatic allocation, which can fail at run time with uncontrolled consequences, the dynamic allocation functions return an indication (in the form of a null pointer value) when the required storage cannot be allocated. (Static allocation that is too large is usually detected by the
linker or
loader, before the program can even begin execution.) Unless otherwise specified, static objects contain zero or null pointer values upon program startup. Automatically and dynamically allocated objects are initialized only if an initial value is explicitly specified; otherwise they initially have indeterminate values (typically, whatever
bit pattern happens to be present in the
storage, which might not even represent a valid value for that type). If the program attempts to access an uninitialized value, the results are undefined. Many modern compilers try to detect and warn about this problem, but both
false positives and false negatives can occur. Heap memory allocation has to be synchronized with its actual usage in any program to be reused as much as possible. For example, if the only pointer to a heap memory allocation goes out of scope or has its value overwritten before it is deallocated explicitly, then that memory cannot be recovered for later reuse and is essentially lost to the program, a phenomenon known as a
memory leak. Conversely, it is possible for memory to be freed but referenced subsequently, leading to unpredictable results. Typically, the failure symptoms appear in a portion of the program unrelated to the code that causes the error, making it difficult to diagnose the failure. Such issues are ameliorated in languages with
automatic garbage collection.
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. 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 "link the 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 (implementations which target limited environments such as
embedded systems may provide only a subset of the standard library). This library supports stream input and output, memory allocation, mathematics, character strings, and time values. Several separate standard headers (for example, stdio.h) specify the interfaces for these and other standard library facilities. 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.
File handling and streams File input and output (I/O) is not part of the C language itself but instead is handled by libraries (such as the C standard library) and their associated header files (e.g. stdio.h). File handling is generally implemented through high-level I/O which works through
streams. A stream is from this perspective a data flow that is independent of devices, while a file is a concrete device. The high-level I/O is done through the association of a stream to a file. In the C standard library, a
buffer (a memory area or queue) is temporarily used to store data before it is sent to the final destination. This reduces the time spent waiting for slower devices, for example a
hard drive or
solid-state drive. Low-level I/O functions are not part of the standard C library but are generally part of "bare metal" programming (programming that is independent of any
operating system such as most
embedded programming). With few exceptions, implementations include low-level I/O. == Language tools ==