Comparing strings in C

String library in C, <cstring> or <string.h> provides several functions to manipulate C strings and arrays. The strcmp() function compares two strings. The function returns 0 if both the strings are equal. The standard form of the strcmp() function is:

int strcmp (const char * str1, const char * str2);

The function compares the string character by character because C treats strings as characters of array.

The return values of strcmp() are either 0, >0 or <0 depending upon string comparison.

Use of strcmp() function

The following C program checks user’s password and terminates when a correct password is entered.

#include <stdio.h>
#include <string.h>

int main ()
{
  char password[] = "123xyz";

  char input[10];
  
  do {
     printf ("What is your password? ");
     fflush (stdout);
     scanf ("%s",input);
  } 
  while (strcmp (password,input)!=0);
  
  puts ("Password is correct!");
  
  return 0;
}

The output of the program can be as below:

What is your password? abcxyz

What is your password? 123456

What is your password? 123xyz

Password is correct!

Other String Comparison Functions

memcmp() – Compare two blocks of memory referenced by pointers.

strcoll() – Compare two strings using locale i.e. C localization library

strncmp() – Compare characters of two strings up to a certain number.

strxfrm() – Transform string using locale i.e. C localization library