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cintegerzero

In C, will !0 always return 1?

This program anecdotally shows that the integer value of ! 0 is 1:

#include <stdio.h>

int main( int argc, char* argv[] ) {
  printf( "%d\n", ! 0 );
  return 0;
}

$ gcc --version && gcc -g ./main.c && ./a.out
gcc (Debian 6.3.0-18+deb9u1) 6.3.0 20170516
Copyright (C) 2016 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

1
$

Is this guaranteed by any standard, or could ! 0 ever evaluate to any other nonzero integer value?

Solution

  • It is guaranteed to evaluate to 1.

    6.5.3.3 Unary arithmetic operators
    ...
    5     The result of the logical negation operator ! is 0 if the value of its operand compares unequal to 0, 1 if the value of its operand compares equal to 0. The result has type int. The expression !E is equivalent to (0==E).
    C 2011 Online Draft