/tmp>cat gcc-warn-assign.c
int main(void)
{
        int i,x,y;
        i=x=y=0;
        /* unintentional assignment inside if condition */
        if (x=42) i++;
        if (x=y) i++;
        /* intentional assignment inside if condition, marked with extra parentheses */
        if ((x=42)) i++;
        if ((x=y)) i++;
        return i;
}

/tmp>gcc -Wall -pedantic gcc-warn-assign.c
gcc-warn-assign.c: In function ‘main’:
gcc-warn-assign.c:6:2: warning: suggest parentheses around assignment used as truth value
gcc-warn-assign.c:7:2: warning: suggest parentheses around assignment used as truth value

/tmp>gcc --version
gcc (GCC) 4.5.2
Copyright (C) 2010 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.

/tmp>