char str[]="abc def ghi jkl";

str[3]='\0';
cout << str << endl;

str[7]='\0';
cout << str+4 << endl;

str[11]='\0';
cout << str+8 << endl;

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

int main() {
         char str[]="abc def ghi";
         char *ptr=strtok(str, " ");

        // printf would crash if we fed it NULL.
        // so we tell assert, "we are assuming ptr != NULL"
        // and if your assumption is incorrect, it dies.
        assert(ptr != NULL);
        printf("%s\n", ptr);

        ptr=strtok(NULL, " ");
        assert(ptr != NULL);
        printf("%s\n", ptr);

        ptr=strtok(NULL, " ");
        assert(ptr != NULL);
        printf("%s\n", ptr);

        ptr=strtok(NULL, " ");
        assert(ptr != NULL);
        printf("%s\n", ptr);

        ptr=strtok(NULL, " ");
        assert(ptr != NULL);
        printf("%s\n", ptr);
}

$ gcc myassert.c
$ ./a.out
abc
def
ghi
a.out: assert.c:24: main: Assertion `ptr != ((void *)0)' failed.
Aborted

$

#include <stdio.h>

char *last; /* Store the last value my_strtok used */

/**
 * A simplified strtok that only uses one char as a token.
 * The real strtok takes a string, and stops at ANY char in it.
 *
 * 'first' points to the first character in the string.
 * If we give it NULL, it assumes 'last'.
 *
 * It points to wherever it left off in the global variable 'last'.
 */
char *my_tok(char *first, char c)
{
        int pos=0;

        /* If given a string, start over here */
        if(first != NULL)       last=first;

        first=last;     /* Pick up wherever we left off */

        /* Our very first char is NULL?  Give up. */
        if(first[0] == '\0') return(NULL);

        /* Increment 'pos' until we find c or NULL */
        while(first[pos] && (first[pos] != c)) pos++;

        /**
         * If we found a separator, replace it with a NULL terminator
         * The string beginning in 'first' will now stop early, here.
         *
         * A 'while' loop is used to catch several in a row.
         */
        while(first[pos]==c)
        {
                first[pos]='\0';
                pos++;
        }

        // Remember exactly where we left off.
        last += pos;

        // Return a pointer to where we started.
        return(first);
}

int main(void) {
        char buf[128]="abcd  efgh  jklm  nop";
        char *tok=my_tok(buf, ' ');

        while(tok != NULL)
        {
                fprintf(stderr, "tok=%s\n", tok);
                tok=my_tok(NULL, ' ');
        }
}

$ gcc mystr.c
$ ./a.out

tok=abcd
tok=efgh
tok=jklm
tok=nop

$