Using the equality operator == to compare two strings for equality in C

Question

int main (int argc, **argv)
{
       if (argv[1] == \"-hello\")
            printf(\"True\\n\");
       else
            printf(\"False\\n\");
}

Answer 1

Because C strings dont exist as such. They are char arrays ending in a \0.

The equality operator == will test that the pointer to the first element of the array are the same. It wont compare lexicographically.

On the other hand "-hello" == "-hello" may return non zero, but that doesn't mean that the == operator compares lexicographycally. That's due to other facts.

If you want to compare lexicographycally, you can always

#define STR_EQ(s1,s2)    \
   strcmp(s1,s2) == 0

Reading harder I see that you tagged as c++. So you could

 std::string arg1 ( argv[1] );

 if (arg1 == "-hello"){
    // yeahh!!!
 }
 else{
    //awwwww
 }

Answer 2

Strings are not native types in C. What you are comparing in that example are two pointers. One to your first argument, and the other is a static character array with the contents of "-hello".

You really want to use strncmp or something similar.

Answer 3

You can't compare strings in C with ==, because the C compiler does not really have a clue about strings beyond a string-literal.

The compiler sees a comparison with a char* on either side, so it does a pointer comparison (which compares the addresses stored in the pointers)