How to check if C string is empty

Question

I\'m writing a very small program in C that needs to check if a certain string is empty. For the sake of this question, I\'ve simplified my code:

#include 

        

Answer 1

It is very simple. check for string empty condition in while condition.

1. You can use strlen function to check for the string length.

   #include<stdio.h>
   #include <string.h>   
   
   int main()
   {
       char url[63] = {'\0'};
       do
       {
           printf("Enter a URL: ");
           scanf("%s", url);
           printf("%s", url);
       } while (strlen(url)<=0);
       return(0);
   }
   

2. check first character is '\0'

   #include <stdio.h>    
   #include <string.h>
   
   int main()
   {        
       char url[63] = {'\0'};
   
       do
       {
           printf("Enter a URL: ");
           scanf("%s", url);
           printf("%s", url);
       } while (url[0]=='\0');
   
       return(0);
   }
   

For your reference:

C arrays:
https://www.javatpoint.com/c-array
https://scholarsoul.com/arrays-in-c/

C strings:
https://www.programiz.com/c-programming/c-strings
https://scholarsoul.com/string-in-c/
https://en.wikipedia.org/wiki/C_string_handling

Answer 2

With strtok(), it can be done in just one line: "if (strtok(s," \t")==NULL)". For example:

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

int is_whitespace(char *s) {
    if (strtok(s," \t")==NULL) {
        return 1;
    } else {
        return 0;
    }
}

void demo(void) {
    char s1[128];
    char s2[128];
    strcpy(s1,"   abc  \t ");
    strcpy(s2,"    \t   ");
    printf("s1 = \"%s\"\n", s1);
    printf("s2 = \"%s\"\n", s2);
    printf("is_whitespace(s1)=%d\n",is_whitespace(s1));
    printf("is_whitespace(s2)=%d\n",is_whitespace(s2));
}

int main() {
    char url[63] = {'\0'};
    do {
        printf("Enter a URL: ");
        scanf("%s", url);
        printf("url='%s'\n", url);
    } while (is_whitespace(url));
    return 0;
}

Answer 3

If you want to check if a string is empty:

if (str[0] == '\0')
{
    // your code here
}

Answer 4

You can check the return value from scanf. This code will just sit there until it receives a string.

int a;

do {
  // other code
  a = scanf("%s", url);

} while (a <= 0);

Answer 5

I've written down this macro

#define IS_EMPTY_STR(X) ( (1 / (sizeof(X[0]) == 1))/*type check*/ && !(X[0])/*content check*/)

so it would be

while (! IS_EMPTY_STR(url));

The benefit in this macro it that it's type-safe. You'll get a compilation error if put in something other than a pointer to char.

Answer 6

First replace the scanf() with fgets() ...

do {
    if (!fgets(url, sizeof url, stdin)) /* error */;
    /* ... */
} while (*url != '\n');