麻烦C else，如果和scanf

Question

I am new to C and I'm making my first "decent" program. I am facing trouble with my code.

char username[] = "root";
char usernametry[10];

scanf("%s",usernametry);

if (usernametry == username)
{
    printf("Welcome ROOT user\n");
}
else
{
    printf("Try again\n");
}

Whenever I execute my program, everything works; however, when I try to log in and type root as the username, it returns Try again . Can someone suggest why this is happening? I am very new to C so I apologize for my lack of C knowledge.

Answer 1

usernametry and username are arrays of type char . When you do usernametry == username , you are not comparing if two strings are equal, you are comparing the memory address of the first char in usernametry with the memory address of the first char in username . In order to compare if two strings are equal, you should use strcmp . Also, consider replacing %s in your scanf with %9s so that usernametry will never exceed 9 char s (which would have been a buffer overflow, as the 10th char needs to be '\\0' , the null character, which is used to terminate C-style strings).

Answer 2

You can't compare strings using ==, this is not a that-high-level-language. See strcmp() for comparing strings.

Incidentally, what if someone enters a name longer than 10 characters? You're drifting towards a buffer overflow bug in your code ;-)

Answer 3

use strcmp() functions to compare 2 strings instead of ==

use of == is wrong because in this case you compare address of usernametry and username and of course they are always differents.

if(strcmp(usernametry, username) == 0) // return 0 if equal !=0 if not equal

Answer 4

使用strcmp比较两个字符串：

if(!strcmp(usernametry,username))

Answer 5

Your variables are not the same pointer.

Replace by:

if (!strcmp(usernametry, username))