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Logical operators - cppreference.com

Logical operators apply standard boolean algebra operations to their operands.

The logical NOT expression has the form

where

The logical NOT operator has type int. Its value is ​0​ if expression evaluates to a value that compares unequal to zero. Its value is 1 if expression evaluates to a value that compares equal to zero. (so !E is the same as (0==E))

#include <stdbool.h>
#include <stdio.h>
#include <ctype.h>
int main(void)
{
    bool b = !(2+2 == 4); // not true
    printf("!(2+2==4) = %s\n", b ? "true" : "false");
 
    int n = isspace('a'); // non-zero if 'a' is a space, zero otherwise
    int x = !!n; // "bang-bang", common C idiom for mapping integers to [0,1]
                 // (all non-zero values become 1)
    char *a[2] = {"non-space", "space"};
    puts(a[x]); // now x can be safely used as an index to array of 2 strings
}

Output:

!(2+2==4) = false
non-space

The logical AND expression has the form

where

The logical-AND operator has type int and the value 1 if both lhs and rhs compare unequal to zero. It has the value ​0​ otherwise (if either lhs or rhs or both compare equal to zero).

There is a sequence point after the evaluation of lhs. If the result of lhs compares equal to zero, then rhs is not evaluated at all (so-called short-circuit evaluation)

#include <stdbool.h>
#include <stdio.h>
int main(void)
{
    bool b = 2+2==4 && 2*2==4; // b == true
 
    1 > 2 && puts("this won't print");
 
    char *p = "abc";
    if(p && *p) // common C idiom: if p is not null
                // AND if p does not point at the end of the string
    {           // (note that thanks to short-circuit evaluation, this
                //  will not attempt to dereference a null pointer)
    // ...      // ... then do some string processing
    }
}

The logical OR expression has the form

where

The logical-OR operator has type int and the value 1 if either lhs or rhs compare unequal to zero. It has value ​0​ otherwise (if both lhs and rhs compare equal to zero).

There is a sequence point after the evaluation of lhs. If the result of lhs compares unequal to zero, then rhs is not evaluated at all (so-called short-circuit evaluation)

#include <stdbool.h>
#include <stdio.h>
#include <string.h>
#include <errno.h>
int main(void)
{
    bool b = 2+2 == 4 || 2+2 == 5; // true
    printf("true or false = %s\n", b ? "true" : "false");
 
    // logical OR can be used simialar to perl's "or die", as long as rhs has scalar type
    fopen("test.txt", "r") || printf("could not open test.txt: %s\n", strerror(errno));
}

Possible output:

true or false = true
could not open test.txt: No such file or directory

• C11 standard (ISO/IEC 9899:2011):

• 6.5.3.3 Unary arithmetic operators (p: 89)

• 6.5.13 Logical AND operator (p: 99)

• 6.5.14 Logical OR operator (p: 99)

• C99 standard (ISO/IEC 9899:1999):

• 6.5.3.3 Unary arithmetic operators (p: 79)

• 6.5.13 Logical AND operator (p: 89)

• 6.5.14 Logical OR operator (p: 89)

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

• 3.3.3.3 Unary arithmetic operators

• 3.3.13 Logical AND operator

• 3.3.14 Logical OR operator

Operator precedence

a = b
a += b
a -= b
a *= b
a /= b
a %= b
a &= b
a |= b
a ^= b
a <<= b
a >>= b

++a
--a
a++
a--

+a
-a
a + b
a - b
a * b
a / b
a % b
~a
a & b
a | b
a ^ b
a << b
a >> b

!a
a && b
a || b

a == b
a != b
a < b
a > b
a <= b
a >= b

a[b]
*a
&a
a->b
a.b

a(...)
a, b
(type) a
a ? b : c
sizeof

_Alignof

alignof

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