How strtok() Works

strtok() : Split string into tokens
Syntax :

char * strtok ( char * str, const char * delimiters );

Parameters
str C string to truncate.Notice that this string is modified by being broken into smaller strings (tokens). Alternativelly, a null pointer may be specified, in which case the function continues scanning where a previous successful call to the function ended.

delimiters   C string containing the delimiter characters. These can be different from one call to another.

Example

/* strtok example */
 #include <stdio.h>
 #include <string.h>
int main (){
char str[] ="- This, a sample string.";
char * pch;
printf ("Splitting string \"%s\" into tokens:\n",str);
pch = strtok (str," ,.-");
while (pch != NULL){
printf ("%s\n",pch);
pch = strtok (NULL, " ,.-");
}
return 0;
}

code courtesy : cplusplus.com

Why Function strtok() is not Thread Safe / reentrant

The strtok() function uses a intermediate static buffer while parsing, and reuse them at each call so it’s not thread safe.

Solution :

Use  strtok_r() to use strtok as thread safe