integer constant

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

Syntax

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

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

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.

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

#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
}

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

References