C programming, Language reference

Table of keywords

ANSI (American National Standards Institute) C (C89)/ISO C (C90)

Very old compilers may not recognize some or all of the C89 keywords const, enum, signed, void, volatile, as well as any later standards' keywords.

ISO C (C99)

These are supported in most new compilers.

ISO C (C11)

These are supported only in some newer compilers

Although not technically a keyword, C99-capable preprocessors/compilers additionally recognize the special preprocessor operator _Pragma, which acts as an alternate form of the #pragma directive that can be used from within macro expansions. For example, the following code will cause some compilers (incl. GCC, Clang) to emit a diagnostic message:

#define EMIT_MESSAGE(str) EMIT_PRAGMA(message(str))
#define EMIT_PRAGMA(content) _Pragma(#content)
EMIT_MESSAGE("Hello, world!")

Some compilers use a slight variant syntax; in particular, MSVC supports __pragma instead of _Pragma.

Specific compilers may also—in a non-standards-compliant mode, or with additional syntactic markers like extension—treat some other words as keywords, including asm, cdecl, far, fortran, huge, interrupt, near, pascal, or typeof. However, they typically allow these keywords to be overridden by declarations when operating in standards-compliant modes (e.g., by defining a variable named typeof), in order to avoid introducing incompatibilities with existing programs. In order to ensure the compiler can maintain access to extension features, these compilers usually have an additional set of proper keywords beginning with two underscores (__). For example, GCC treats asm, __asm, and asm somewhat identically, but the latter two are always guaranteed to have the expected meaning since they can't be overridden.

Many of the newly introduced keywords—namely, those beginning with an underscore and capital letter like _Noreturn or _Imaginary—are intended to be used only indirectly in most situations. Instead, the programmer should prefer the use of standard headers such as or , which typically use the preprocessor to establish an all-lower-case variant of the keyword (e.g., complex or noreturn). These headers serve the purpose of enabling C and C++ code, as well as code targeting different compilers or language versions, to interoperate more cleanly. For example, by including , the tokens bool, true, and false can be used identically in either C99 or C++ without having to explicitly use _Bool in C99 or bool in C++.

See also the list of reserved identifiers [1].

Table of operators

Operators in the same row of this table have the same precedence and the order of evaluation is decided by the associativity (left-to-right or right-to-left). Operators closer to the top of this table have higher precedence than those in a subsequent group.

Table of data types

Signedness is implementation-defined.
Any encoding of 8 bits or less (e.g. ASCII) can be used to store characters.
Integer operations can be performed portably only for the range 0 ~ 127.
All bits contribute to the value of the char , i.e. there are no "holes" or "padding" bits. | — |
| signed char | same as char | Characters stored like for type char .
Can store integers in the range -127 ~ 127 portably [ 3 ] . | — |
| unsigned char | same as char | Characters stored like for type char .
Can store integers in the range 0 ~ 255 portably. | — |
| short | ≥ 16, ≥ size of char | Can store integers in the range -32767 ~ 32767 portably [ 4 ] .
Used to reduce memory usage (although the resulting executable may be larger and probably slower as compared to using int . | short int , signed short , signed short int |
| unsigned short | same as short | Can store integers in the range 0 ~ 65535 portably.
Used to reduce memory usage (although the resulting executable may be larger and probably slower as compared to using int . | unsigned short int |
| int | ≥ 16, ≥ size of short | Represents the "normal" size of data the processor deals with (the word-size); this is the integral data-type used normally.
Can store integers in the range -32767 ~ 32767 portably [ 4 ] . | signed , signed int |
| unsigned int | same as int | Can store integers in the range 0 ~ 65535 portably. | unsigned |
| long | ≥ 32, ≥ size of int | Can store integers in the range -2147483647 ~ 2147483647 portably [ 5 ] . | long int , signed long , signed long int |
| unsigned long | same as long | Can store integers in the range 0 ~ 4294967295 portably. | unsigned long int |
| float | ≥ size of char | Used to reduce memory usage when the values used do not vary widely.
The floating-point format used is implementation defined and need not be the IEEE single-precision format.
unsigned cannot be specified. | — |
| double | ≥ size of float | Represents the "normal" size of data the processor deals with; this is the floating-point data-type used normally.
The floating-point format used is implementation defined and need not be the IEEE double-precision format.
unsigned cannot be specified. | — |

Used by the "j" length modifier in the C programming/File IO#Formatted output functions: the printf family of functions . | — |

Pointers to different types may have different representations, which means they could also be of different sizes. So they are not convertible to one another.
Even in an implementation which guarantess all data pointers to be of the same size, function pointers and data pointers are in general incompatible with each other.
For functions taking variable number of arguments, the arguments passed must be of appropriate type, so even 0 must be cast to the appropriate type in such function-calls. | — |
| type [ integer [ 8 ] ] (array) | ≥ integer × size of type | The brackets ( [] ) follow the identifier name in a declaration.
In a declaration which also initializes the array (including a function parameter declaration), the size of the array (the integer ) can be omitted.
type [] is not the same as type * . Only under some circumstances one can be converted to the other. | — |
| type ( comma-delimited list of types/declarations ) (function) | — | Functions declared without any storage class are extern .
The parentheses ( () ) follow the identifier name in a declaration, e.g. a 2-arg function pointer: int (* fptr) ( int arg1, int arg2) . | — |

Character sets

Programs written in C can read and write any character set, provided the libraries that support them are included/used.

The source code for C programs, however, is usually limited to the ASCII character set.

In a file containing source code, the end of a line is sometimes, depending on the operating system it was created on not a newline character but compilers treat the end of each line as if it were a single newline character.

Virtually all compilers allow the $, @, and ` characters in string constants. Many compilers also allow literal multibyte Unicode characters, but they are not portable.

Certain characters must be escaped with a backslash to represent themselves in a string or character constant. These are:

Additionally, some compilers allow these characters:

• \r Carriage return
• \a Alert (audible bell)
• \b Backspace

\xhh, where the 'h' characters are hexadecimal digits, is used to represent arbitrary bytes (including \x00, the zero byte).

\uhhhh or \Uhhhhhhhh, where the 'h' characters are hexadecimal digits, is used to portably represent Unicode characters.

References

• ↑ http://publib.boulder.ibm.com/infocenter/comphelp/v7v91/topic/com.ibm.vacpp7a.doc/language/ref/clrc02reserved_identifiers.htm list of reserved identifiers
• ↑ Very old compilers may not recognize the unary + operator.
• ↑ -128 can be stored in two's-complement machines (i.e. most machines in existence). Very old compilers may not recognize the signed keyword
• ↑ a b -32768 can be stored in two's-complement machines (i.e. most machines in existence). Very old compilers may not recognize the signed keyword
• ↑ -2147483648 can be stored in two's-complement machines (i.e. most machines in existence). Very old compilers may not recognize the signed keyword
• ↑ -9223372036854775808 can be stored in two's-complement machines (i.e. most machines in existence)
• ↑ The precedences in a declaration are:

• ↑ The standards do NOT place any restriction on the size/type of the integer, it's implementation dependent. The only mention in the standards is a reference that an implementation may have limits to the maximum size of memory block which can be allocated, and as such the limit on integer will be size_of_max_block/sizeof(type)

Where this page came from

This page was imported from Wikibooks. From “C programming” on Wikibooks, by its contributors, under CC BY-SA 4.0. Changed here: set as a page; navigation and edit links left out; each image under its own licence, credited in its caption.

Nobody has written it yet — it is the source material at a new address, which is why search engines are asked to skip it and why no one earns from it. It is up for grabs: take it on, and it is yours to rewrite and to earn from.