# Data Types

# Fixed Width Integer Types (since C99)

The header <stdint.h> provides several fixed-width integer type definitions. These types are optional and only provided if the platform has an integer type of the corresponding width, and if the corresponding signed type has a two's complement representation of negative values.

See the remarks section for usage hints of fixed width types.

/* commonly used types include */
uint32_t u32 = 32; /* exactly 32-bits wide */

uint8_t u8 = 255;  /* exactly 8-bits wide */

int64_t i64 = -65  /* exactly 64 bit in two's complement representation */

# Interpreting Declarations

A distinctive syntactic peculiarity of C is that declarations mirror the use of the declared object as it would be in a normal expression.

The following set of operators with identical precedence and associativity are reused in declarators, namely:

  • the unary * "dereference" operator which denotes a pointer;
  • the binary [] "array subscription" operator which denotes an array;
  • the (1+n)-ary () "function call" operator which denotes a function;
  • the () grouping parentheses which override the precedence and associativity of the rest of the listed operators.

The above three operators have the following precedence and associativity:

Operator Relative Precedence Associativity
[] (array subscription) 1 Left-to-right
() (function call) 1 Left-to-right
* (dereference) 2 Right-to-left

When interpreting declarations, one has to start from the identifier outwards and apply the adjacent operators in the correct order as per the above table. Each application of an operator can be substituted with the following English words:

Expression Interpretation
thing[X] an array of size X of...
thing(t1, t2, t3) a function taking t1, t2, t3 and returning...
*thing a pointer to...

It follows that the beginning of the English interpretation will always start with the identifier and will end with the type that stands on the left-hand side of the declaration.

# Examples

char *names[20];

[] takes precedence over *, so the interpretation is: names is an array of size 20 of a pointer to char.

char (*place)[10];

In case of using parentheses to override the precedence, the * is applied first: place is a pointer to an array of size 10 of char.

int fn(long, short);

There is no precedence to worry about here: fn is a function taking long, short and returning int.

int *fn(void);

The () is applied first: fn is a function taking void and returning a pointer to int.

int (*fp)(void);

Overriding the precedence of (): fp is a pointer to a function taking void and returning int.

int arr[5][8];

Multidimensional arrays are not an exception to the rule; the [] operators are applied in left-to-right order according to the associativity in the table: arr is an array of size 5 of an array of size 8 of int.

int **ptr;

The two dereference operators have equal precedence, so the associativity takes effect. The operators are applied in right-to-left order: ptr is a pointer to a pointer to an int.

# Multiple Declarations

int fn(void), *ptr, (*fp)(int), arr[10][20], num;

The declared objects in the above example are:

  • fn: a function taking void and returning int;
  • ptr: a pointer to an int;
  • fp: a pointer to a function taking int and returning int;
  • arr: an array of size 10 of an array of size 20 of int;
  • num: int.

# Alternative Interpretation

Because declarations mirror use, a declaration can also be interpreted in terms of the operators that could be applied over the object and the final resulting type of that expression. The type that stands on the left-hand side is the final result that is yielded after applying all operators.

/*
 * Subscripting "arr" and dereferencing it yields a "char" result.
 * Particularly: *arr[5] is of type "char".
 */
char *arr[20];

/*
 * Calling "fn" yields an "int" result.
 * Particularly: fn('b') is of type "int".
 */
int fn(char);

/*
 * Dereferencing "fp" and then calling it yields an "int" result.
 * Particularly: (*fp)() is of type "int".
 */
int (*fp)(void);

/*
 * Subscripting "strings" twice and dereferencing it yields a "char" result.
 * Particularly: *strings[5][15] is of type "char"
 */
char *strings[10][20];

# Integer types and constants

Signed integers can be of these types (the int after short, or long is optional):

signed char c = 127; /* required to be 1 byte, see remarks for further information. */
signed short int si = 32767; /* required to be at least 16 bits. */
signed int i = 32767; /* required to be at least 16 bits */
signed long int li = 2147483647; /* required to be at least 32 bits. */

signed long long int li = 2147483647; /* required to be at least 64 bits */

Each of these signed integer types has an unsigned version.

unsigned int i = 65535;
unsigned short = 2767;
unsigned char = 255;

For all types but char the signed version is assumed if the signed or unsigned part is omitted. The type char constitutes a third character type, different from signed char and unsigned char and the signedness (or not) depends on the platform.

Different types of integer constants (called literals in C jargon) can be written in different bases, and different width, based on their prefix or suffix.

/* the following variables are initialized to the same value: */
int d = 42;   /* decimal constant (base10) */
int o = 052;  /* octal constant (base8) */
int x = 0xaf; /* hexadecimal constants (base16) */
int X = 0XAf; /* (letters 'a' through 'f' (case insensitive) represent 10 through 15) */

Decimal constants are always signed. Hexadecimal constants start with 0x or 0X and octal constants start just with a 0. The latter two are signed or unsigned depending on whether the value fits into the signed type or not.

/* suffixes to describe width and signedness : */
long int i = 0x32; /* no suffix represent int, or long int */
unsigned int ui = 65535u; /* u or U represent unsigned int, or long int */
long int li = 65536l; /* l or L represent long int */

Without a suffix the constant has the first type that fits its value, that is a decimal constant that is larger than INT_MAX is of type long if possible, or long long otherwise.

The header file <limits.h> describes the limits of integers as follows. Their implementation-defined values shall be equal or greater in magnitude (absolute value) to those shown below, with the same sign.

Macro Type Value
CHAR_BIT smallest object that is not a bit-field (byte) 8
SCHAR_MIN signed char -127 / -(27 - 1)
SCHAR_MAX signed char +127 / 27 - 1
UCHAR_MAX unsigned char 255 / 28 - 1
CHAR_MIN char see below
CHAR_MAX char see below
SHRT_MIN short int -32767 / -(215 - 1)
SHRT_MAX short int +32767 / 215 - 1
USHRT_MAX unsigned short int 65535 / 216 - 1
INT_MIN int -32767 / -(215 - 1)
INT_MAX int +32767 / 215 - 1
UINT_MAX unsigned int 65535 / 216 - 1
LONG_MIN long int -2147483647 / -(231 - 1)
LONG_MAX long int +2147483647 / 231 - 1
ULONG_MAX unsigned long int 4294967295 / 232 - 1
Macro Type Value
LLONG_MIN long long int -9223372036854775807 / -(263 - 1)
LLONG_MAX long long int +9223372036854775807 / 263 - 1
ULLONG_MAX unsigned long long int 18446744073709551615 / 264 - 1

If the value of an object of type char sign-extends when used in an expression, the value of CHAR_MIN shall be the same as that of SCHAR_MIN and the value of CHAR_MAX shall be the same as that of SCHAR_MAX . If the value of an object of type char does not sign-extend when used in an expression, the value of CHAR_MIN shall be 0 and the value of CHAR_MAX shall be the same as that of UCHAR_MAX.

The C99 standard added a new header, <stdint.h>, which contains definitions for fixed width integers. See the fixed width integer example for a more in-depth explanation.

# Floating Point Constants

The C language has three mandatory real floating point types, float, double, and long double.

float f = 0.314f;        /* suffix f or F denotes type float */
double d = 0.314;        /* no suffix denotes double */
long double ld = 0.314l; /* suffix l or L denotes long double */

/* the different parts of a floating point definition are optional */
double x = 1.; /* valid, fractional part is optional */
double y = .1; /* valid, whole-number part is optional */

/* they can also defined in scientific notation */
double sd = 1.2e3; /* decimal fraction 1.2 is scaled by 10^3, that is 1200.0 */

The header <float.h> defines various limits for floating point operations.

Floating point arithmetic is implementation defined. However, most modern platforms (arm, x86, x86_64, MIPS) use IEEE 754 floating point operations.

C also has three optional complex floating point types that are derived from the above.

# String Literals

A string literal in C is a sequence of chars, terminated by a literal zero.

char* str = "hello, world"; /* string literal */

/* string literals can be used to initialize arrays */
char a1[] = "abc"; /* a1 is char[4] holding {'a','b','c','\0'} */
char a2[4] = "abc"; /* same as a1 */
char a3[3] = "abc"; /* a1 is char[3] holding {'a','b','c'}, missing the '\0' */

String literals are not modifiable (and in fact may be placed in read-only memory such as .rodata). Attempting to alter their values results in undefined behaviour.

char* s = "foobar";
s[0] = 'F'; /* undefined behaviour */

/* it's good practice to denote string literals as such, by using `const` */
char const* s1 = "foobar";
s1[0] = 'F'; /* compiler error! */

Multiple string literals are concatenated at compile time, which means you can write construct like these.

/* only two narrow or two wide string literals may be concatenated */
char* s = "Hello, " "World";

/* since C99, more than two can be concatenated */
/* concatenation is implementation defined */
char* s1 = "Hello" ", " "World";

/* common usages are concatenations of format strings */
char* fmt = "%" PRId16; /* PRId16 macro since C99 */

String literals, same as character constants, support different character sets.

/* normal string literal, of type char[] */
char* s1 = "abc";

/* wide character string literal, of type wchar_t[] */
wchar_t* s2 = L"abc";

/* UTF-8 string literal, of type char[] */
char* s3 = u8"abc";

/* 16-bit wide string literal, of type char16_t[] */
char16_t* s4 = u"abc";

/* 32-bit wide string literal, of type char32_t[] */
char32_t* s5 = U"abc";

# Remarks

  • While char is required to be 1 byte, 1 byte is not required to be 8 bits (often also called an octet), even though most of modern computer platforms define it as 8 bits. The implementation's number of bits per char is provided by the CHAR_BIT macro, defined in <limits.h>. POSIX does require 1 byte to be 8 bits.
  • Fixed width integer types should be use sparsely, C's built-in types are designed to be natural on every architecture, the fixed width types should only be used if you explicitly need a specifically sized integer (for example for networking).