char *strtok(char *s1, const char *s2);
strtok A sequence of calls to strtok breaks the string pointed to by s1 into a sequence of tokens, each of which is delimited by a character from the string pointed to by s2. The first call in the sequence has a non-null first argument; subsequent calls in the sequence have a null first argument. The separator string pointed to by s2 may be different from call to call.
The first call in the sequence searches the string pointed to by s1 for the first character that is not contained in the current separator string pointed to by s2. If no such character is found, then there are no tokens in the string pointed to by s1 and strtok returns a null pointer. If such a character is found, it is the start of the first token.
strtok then searches from there for a character that is contained in the current separator string. If no such character is found, the current token extends to the end of the string pointed to by s1, and subsequent searches for a token will return a null pointer. If such a character is found, it is overwritten by a null character, which terminates the current token. strtok saves a pointer to the following character, from which the next search for a token will start.
Each subsequent call, with a null pointer as the value of the first argument, starts searching from the saved pointer and behaves as described above.
strtok maintains static state and is therefore not reentrant and not thread safe. See strtok_r for a thread-safe and reentrant variant.