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The first time you call strtok(), you pass the string to be parsed into tokens, the character delimiter, and the address of the variable to return the result in. The strtok() function parses the string up to the first instance of the delimiter character, replaces the character in place with a null byte ('\0'), and puts the address of the first character in the token to the passed-in variable. Subsequent calls to strtok() begin parsing immediately after the recently-placed null character.
Because strtok() modifies it's its argument, the string is subsequently unsafe and cannot be used in its original form. If you need to preserve the original string, copy it into a buffer and pass the address of the buffer to strtok() instead of the original string.
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