[c] How to check if C string is empty

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 <stdio.h>
#include <string>

int main() {
  char url[63] = {'\0'};
  do {
    printf("Enter a URL: ");
    scanf("%s", url);
    printf("%s", url);
  } while (/*what should I put in here?*/);

  return(0);
}

I want the program to stop looping if the user just presses enter without entering anything.

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

You can try like this:-

if (string[0] == '\0') {
}

In your case it can be like:-

do {
   ...
} while (url[0] != '\0')

;

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);

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

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

The shortest way to do that would be:

do {
    // Something
} while (*url);

Basically, *url will return the char at the first position in the array; since C strings are null-terminated, if the string is empty, its first position will be the character '\0', whose ASCII value is 0; since C logical statements treat every zero value as false, this loop will keep going while the first position of the string is non-null, that is, while the string is not empty.

Recommended readings if you want to understand this better:

• C strings: https://www.tutorialspoint.com/cprogramming/c_strings.htm
• C arrays: https://www.tutorialspoint.com/cprogramming/c_arrays.htm
• Relation between arrays and pointers: https://www.programiz.com/c-programming/c-pointers-arrays
• Logical operators: https://www.tutorialspoint.com/cprogramming/c_logical_operators.htm

If the first character happens to be '\0', then you have an empty string.

This is what you should do:

do {
    /* 
    *   Resetting first character before getting input.
    */
    url[0] = '\0';

    // code
} while (url[0] != '\0');

If you want to check if a string is empty:

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

strlen(url)

Returns the length of the string. It counts all characters until a null-byte is found. In your case, check it against 0.

Or just check it manually with:

*url == '\0'

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.

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;
}

Typically speaking, you're going to have a hard time getting an empty string here, considering %s ignores white space (spaces, tabs, newlines)... but regardless, scanf() actually returns the number of successful matches...

From the man page:

the number of input items successfully matched and assigned, which can be fewer than provided for, or even zero in the event of an early matching failure.

so if somehow they managed to get by with an empty string (ctrl+z for example) you can just check the return result.

int count = 0;
do {
  ...
  count = scanf("%62s", url);  // You should check return values and limit the 
                               // input length
  ...
} while (count <= 0)

Note you have to check less than because in the example I gave, you'd get back -1, again detailed in the man page:

The value EOF is returned if the end of input is reached before either the first successful conversion or a matching failure occurs. EOF is also returned if a read error occurs, in which case the error indicator for the stream (see ferror(3)) is set, and errno is set indicate the error.