Summary
The ISO 9899 function strtok() is a string tokenization function which takes three arguments; an initial string to be parsed, a const-qualified character delimiter, and a pointer to a pointer to modify to return the result.
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. strtok() 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 argument the string is subsequently unsafe and cannot be used in its original form. Use of strtok() should be avoided if possible, or if necessary, copy the original string into a buffer and pass the address of the buffer to strtok() instead of the original string.
To quote the Linux Programmer's Manual (man) page on strtok(3):
Never use this function. This function modifies its first
argument. The identity of the delimiting character is
lost. This function cannot be used on constant strings.
References
Unix Man page strtok(3)