# integer constant

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Allows values of integer type to be used in expressions directly.

### Syntax

An integer constant is a non-lvalue expression of the form

decimal-constant integer-suffix(optional) | (1) |
octal-constant integer-suffix(optional) | (2) |
hex-constant integer-suffix(optional) | (3) |

where

- decimal-constant is a non-zero decimal digit (`1`, `2`, `3`, `4`, `5`, `6`, `7`, `8`, `9`), followed by zero or more decimal digits (`0`, `1`, `2`, `3`, `4`, `5`, `6`, `7`, `8`, `9`)
- octal-constant is the digit zero (`0`) followed by zero or more octal digits (`0`, `1`, `2`, `3`, `4`, `5`, `6`, `7`)
- hex-constant is the character sequence `0x` or the character sequence `0X` followed by one or more hexadecimal digits (`0`, `1`, `2`, `3`, `4`, `5`, `6`, `7`, `8`, `9`, `a`, `A`, `b`, `B`, `c`, `C`, `d`, `D`, `e`, `E`, `f`, `F`)
- integer-suffix, if provided, may contain one or both of the following (if both are provided, they may appear in any order:

  - unsigned-suffix (the character `u` or the character `U`)
  - long-suffix (the character `l` or the character `L`) or the long-long-suffix (the character sequence `ll` or the character sequence `LL`) (since C99)

### Explanation

1) Decimal integer constant (base 10, the first digit is the most significant).
2) Octal integer constant (base 8, the first digit is the most significant).
3) Hexadecimal integer constant (base 16, the first digit is the most significant, the letters 'a' through 'f' represent the decimal values 10 through 15).

The following variables are initialized to the same value:

    int d = 42;
    int o = 052;
    int x = 0x2a;
    int X = 0X2A;

### The type of the integer constant

The type of the integer constant is the first type in which the value can fit, from the list of types which depends on which numeric base and which integer-suffix was used.

Types allowed for integer constants
suffix | decimal bases | hexadecimal or octal bases
no suffix | int long int unsigned long int (until C99) long long int (since C99) | int unsigned int long int unsigned long int long long int(since C99) unsigned long long int(since C99)
`u` or `U` | unsigned int unsigned long int unsigned long long int(since C99) | unsigned int unsigned long int unsigned long long int(since C99)
`l` or `L` | long int unsigned long int(until C99) long long int(since C99) | long int unsigned long int long long int(since C99) unsigned long long int(since C99)
both `l`/`L` and `u`/`U` | unsigned long int unsigned long long int(since C99) | unsigned long int unsigned long long int(since C99)
`ll` or `LL` | long long int(since C99) | long long int(since C99) unsigned long long int(since C99)
both `ll`/`LL` and `u`/`U` | unsigned long long int(since C99) | unsigned long long int(since C99)

If the value of the integer constant is too big to fit in any of the types allowed by suffix/base combination and the compiler supports extended integer types (such as __int128), the constant may be given the extended integer type; otherwise, the program is ill-formed.

### Notes

Letters in the integer constants are case-insensitive: `0xDeAdBaBeU` and `0XdeadBABEu` represent the same number (one exception is the long-long-suffix, which is either `ll` or `LL`, never `lL` or `Ll`)

There are no negative integer constants. Expressions such as -1 apply the unary minus operator to the value represented by the constant, which may involve implicit type conversions.

When used in a controlling expression of #if or #elif, all signed integer constants act as if they have type intmax_t and all unsigned integer constants act as if they have type uintmax_t.

Integer constants may be used in integer constant expressions.

Due to maximal munch, hexadecimal integer constants ending in `e` and `E`, when followed by the operators `+` or `-`, must be separated from the operator with whitespace or parentheses in the source:

    int x = 0xE+2;   // error
    int y = 0xa+2;   // OK
    int z = 0xE +2;  // OK
    int q = (0xE)+2; // OK

Otherwise, a single invalid preprocessing number token is formed, which causes further analysis to fail.

### Example

Run this code

    #include <stdio.h>
    #include <inttypes.h>
    int main(void)
    {
        printf("123 = %d\n", 123);
        printf("0123 = %d\n", 0123);
        printf("0x123 = %d\n", 0x123);
        printf("12345678901234567890ull = %llu\n", 12345678901234567890ull);
        // the type is a 64-bit type (unsigned long long or possibly unsigned long)
        // even without a long suffix
        printf("12345678901234567890u = %"PRIu64"\n", 12345678901234567890u );

    //  printf("%lld\n", -9223372036854775808); // ERROR
    // the value 9223372036854775808 cannot fit in signed long long, which is the
    // biggest type allowed for unsuffixed decimal integer constant

        printf("%llu\n", -9223372036854775808ull );
        // unary minus applied to unsigned value subtracts it from 2^64,
        // this gives unsigned 9223372036854775808

        printf("%lld\n", -9223372036854775807ull - 1);
        // correct way to form signed value -9223372036854775808
    }

Output:

    123 = 123
    0123 = 83
    0x123 = 291
    12345678901234567890ull = 12345678901234567890
    12345678901234567890u = 12345678901234567890
    9223372036854775808
    -9223372036854775808

### References

- C11 standard (ISO/IEC 9899:2011):

  - 6.4.4.1 Integer constants (p: 62-64)

- C99 standard (ISO/IEC 9899:1999):

  - 6.4.4.1 Integer constants (p: 54-56)

- C89/C90 standard (ISO/IEC 9899:1990):

  - 3.1.3.2 Integer constants

### See also

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