C strings help

[folem001]

Hi all, on the C programming tutorial on this website is 'Lesson 9 - C strings:
C Strings - C++ Tutorial - Cprogramming.com

Given in this lesson is the following:

#include <stdio.h>    /* stdin, printf, and fgets */
#include <string.h>   /* for all the new-fangled string functions */

/* this function is designed to remove the newline from the end of a string
entered using fgets.  Note that since we make this into its own function, we
could easily choose a better technique for removing the newline.  Aren't
functions great? */
void strip_newline( char *str, int size )
{
    int i;

    /* remove the null terminator */
    for (  i = 0; i < size; ++i )
    {
        if ( str[i] == '\n' )
        {
            str[i] = '\0';

            /* we're done, so just exit the function by returning */
            return;   
        }
    }
    /* if we get all the way to here, there must not have been a newline! */
}

int main()
{
    char name[50];
    char lastname[50];
    char fullname[100]; /* Big enough to hold both name and lastname */

    printf( "Please enter your first name: " );
    fgets( name, 50, stdin );

    /* see definition above */
    strip_newline( name, 50 );

    /* strcmp returns zero when the two strings are equal */
    if ( strcmp ( name, "Alex" ) == 0 ) 
    {
        printf( "That's my name too.\n" );
    }
    else                                     
    {
        printf( "That's not my name.\n" );
    }
    // Find the length of your name
    printf( "Your name is %d letters long.\n", strlen ( name ) );
    printf( "Please enter your last name: " );
    fgets( lastname, 50, stdin );
    strip_newline( lastname, 50 );
    fullname[0] = '\0';            
    /* strcat will look for the \0 and add the second string starting at
       that location */
    strcat( fullname, name );     /* Copy name into full name */
    strcat( fullname, " " );      /* Separate the names by a space */
    strcat( fullname, lastname ); /* Copy lastname onto the end of fullname */
    printf( "Your full name is %s\n",fullname );
	printf("It's a pleasure to be talking with you %s\n", name);
    getchar();

    return 0;
}

I am confused as to why:

fullname[0] = '\0';

is included.
In the tutorial is states that C style strings are always terminated with a null character ('\0'). I don't understand why the beginning of the string 'fullname' would be set to this. In the code it states:

fullname[0] = '\0';            
/* strcat will look for the \0 and add the second string starting at
   that location */

Is the string 'lastname' the second string? If so why is it seemingly being put at the start of the 'fullname' string. Even though, through running this code i know this isn't what is happening.
Without 'fullname[0] = '\0';', the program crashes and I have no idea as to why this happens.

Thankyou for any help.

[MK27]

Because of the use of strcat(). Optionally (and in fact more conventionally -- I would guess the tutorial is purposefully written this way to indicate something about the functionality of strcat) that section could have been:

    strip_newline( lastname, 50 );
  /* use strcpy instead of strcat*/
    strcpy( fullname, name );     /* Copy name into full name */

in which case fullname[0] = '\0' is not necessary.

Hopefully you can connect the dots there, if not ask for further clarification!

[dwks]

The first character in fullname is set to '\0', the NULL character, to make C think it is an empty string. strcat() appends the second parameter to the end of the first parameter; it basically finds the end of the string given by the first parameter by searching for a '\0', and then copies characters over from the second parameter until it reaches a '\0' there.

If fullname is uninitialized, as it is until fullname[0] is set to '\0', there's no telling what might be in it. strcat() will try to search for the end of the string, denoted by a '\0'; it might have to go quite a way through memory until it finds a '\0' by chance, and at this point it's probably outside the memory allocated for your program and trying to overwrite some other program's memory. The operating system detects this and kills your program.

By the way, in real code it would probably be simpler to just change the first strcat() call to a strcpy() call. This copies the second parameter into the string given by the first parameter, starting at index 0, without trying to preserve any data that might have been there before; you can safely strcpy() into an uninitialized array.

[folem001]

Brilliant, thankyou!

So a code that does the same thing using strcpy:

strcpy(fullname, name);
strcat(fullname, " ");
strcat(fullname, lastname);

Thanks again!

[rags_to_riches]

Another way to accomplish that:

sprintf(fullname, "%s %s", name, lastname);

better would be to use snprintf, so you have a guarantee not to overrun your buffer:

snprintf(fullname, sizeof(fullname), "%s %s", name, lastname);

[zhangzhongke007]

You guys are so clever.I think I have understood something better.