Strtok function - sample implementation

The string token function (strtok) function splits a string into tokens.
In this example the de-limiter is ":". The input string is :

Himy*:name:is:bozo:

The function 'stok' will split the above string into individual strings. The output will be something like:

Himy*
name
is
bozo

The first time the function is called from main() , it is of the form:
finalstr = stok(mystr, ':');

The subsequent calls are of the form:
finalstr = stok(NULL, ':');

When the string runs out, the function returns null.

Here is a sample implementation:

//////////////////////////////////////////////////////////////////////
#include <stdio.h>

char* stok(char* teststr, char ch);

int main() {
char mystr[] = "Himy*:name:is:bozo:";
//char mystr[] = ":*&Fight!:the^%:^^&)Man\n";
char *finalstr;
finalstr = stok(mystr, ':');
printf("%s\n", finalstr);
while(finalstr!=NULL){
finalstr = stok(NULL, ':');
if(finalstr) printf("%s\n", finalstr);
}
return 0;
}

char* stok(char* teststr, char ch) {
char *dummystr=NULL;
char *start=NULL;
char *end=NULL;
char nullch = '\0';
char* address_of_null=&nullch;
static char* nexttok;

if(teststr!=NULL) {
dummystr = teststr;
} else {
if(*nexttok=='\0') return NULL;
dummystr = nexttok;
}

while(dummystr!=NULL) {
//empty string
if(*dummystr=='\0') break;
if(*dummystr!=ch){
if(!start) start = dummystr;
dummystr++;
//to handle the case where the delimiter is not at the end of the string.
if(*dummystr=='\0') {
nexttok = dummystr;
break;
}
}else{
if(start) {
end = dummystr;
nexttok = dummystr+1;
*end = *address_of_null;
break;
} else {
dummystr++;
}
}
}
return start;
}
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