Why is scanf() ignoring the part after the space in this program to reverse a string?

#include <stdio.h>

#include <string.h>

main() {
    int i;
    char given[30];
    char reversed[30];
    printf("enter your stringn");
    scanf("%s", given);
    for (i = 0; i <= (strlen(given)); i++) {
        if (i <= (strlen(given) - 1)) {
            reversed[i] = given[(strlen(given) - 1 - i)];
        } else {
            reversed[i] = given[i];
        }
    }
    printf("%s", reversed);
    return 0;
}

When I run this code, if I give input as 123 its giving back 321 but when I give 123 325 as input it is giving just 321 as output (it should be 523 321). Why is this so?

%s does not read a whole line. Take a look here. What the entry for %s says is Any number of non-whitespace characters, stopping at the first whitespace character found. A terminating null character is automatically added at the end of the stored sequence.

Maybe How do you allow spaces to be entered using scanf? or How to read one whole line from a text file using <? can point you to alternatives.

For starters the function main shall have explicitly specified return type int (or compatible)

int main( void )

As for your question then according to the C Standard (7.21.6.2 The fscanf function; the same is valid for scanf)

> s Matches a sequence of non-white-space characters.

That is as soon as a white space character is encountered filling of the corresponding character array used as an argument expression is stopped. The read sequence of non white space characters is ended with a null character '\0'. Thus only the first word 123 from user input is read and reversed by the program, the rest of the input line would be read by another call to scanf() if you had a loop.

If you want to read strings that contain embedded white space characters, then you need to use another conversion specification as for example:

scanf(" %29[^\n]", given);

Pay attention too that calling the function strlen several times is inefficient.

Also you should declare variables in minimum scopes where they are used.

Your program can look the following way

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

int main( void ) 
{
    char given[30];
    char reversed[30];

    printf( "enter your string: " );
    
    if ( scanf(" %29[^\n]", given) == 1 )
    {
        size_t n = strlen( given );
        reversed[n] = '
#include <stdio.h>
#include <string.h>
int main( void ) 
{
char given[30];
char reversed[30];
printf( "enter your string: " );
if ( scanf(" %29[^\n]", given) == 1 )
{
size_t n = strlen( given );
reversed[n] = '\0';
for ( size_t i = 0; i < n; i++ ) 
{
reversed[i] = given[n - 1 - i];
}
puts( reversed );
}
return 0;
}
';

        for ( size_t i = 0; i < n; i++ ) 
        {
            reversed[i] = given[n - 1 - i];
        }

        puts( reversed );
    }

    return 0;
}