C equality of constant strings

Question

Is the following comparison guaranteed to be true?

"hello world"=="hello world";

Also, is the following always guaranteed to be false?

char a[] = "hello world";
a == "hello world";

Answer 1

To be clear - in both cases you are comparing pointers, not the actual string contents.

for

"hello world"=="hello world";

it is permitted that the comparison be true or false. The C standard says in 6.4.5 "String literals":

It is unspecified whether these arrays are distinct provided their elements have the appropriate values.

So the standard allows the storage for the literals to be the same or different.

For

char a[] = "hello world";
a == "hello world";

the comparison will always be false since the address of the array a must be different than the address of the string literal.

Answer 2

In C you have to use a function that compare the strings for you. Doing it straigh away will only tell you if two strings are in the same place on the memory. so

char a[] = "hello world";
char b[] = a;

the making

a == b;

will give you true because both a and b have point to the same place, or string, in the memory.

If you want to compare two strings you'll have to use strcmp() that returns 0 if the strings are equal.

if( strcmp(a, b) == 0 )
    printf("True\n");
else
    printf("False\n");

To use it you'll need to include the library string.h.

#include <string.h>

Answer 3

Comparing string contents is not allowed through == operator in C. You should use

strcmp(str1,str2);

In this case you are comparing pointers. So

"hello world" == "hello world";

may or may not be TRUE.

But the second case is always FALSE.

char a[] = "hello world";
a == "hello world";

Answer 4

== operator in C can be used to compare primitive data types. Since String are not primitive in C,you can't use the == to compare them. The best option is to use the strcmp() function by including string.h library.