C in 100 Seconds: String Tokenizing with strtok

String Tokenizing with strtok

strtok splits a string into tokens based on delimiters. It modifies the original string by replacing delimiters with null bytes.

Basic Usage — Split by Spaces

char sentence[] = "The quick brown fox";
char *word = strtok(sentence, " ");
while (word != NULL) {
  printf("%s\n", word);
  word = strtok(NULL, " ");
}

The first call passes the string. Every call after that passes NULL to continue tokenizing the same string.

Parsing CSV Data

char csv[] = "Alice,30,Engineer";
char *field = strtok(csv, ",");
while (field != NULL) {
  printf("Field: %s\n", field);
  field = strtok(NULL, ",");
}

Same pattern, just change the delimiter from space to comma.

Multiple Delimiters

char data[] = "red;green,blue:yellow";
char *color = strtok(data, ";,:");

Pass all delimiters in a single string — strtok splits on any of them.

Important Notes

• strtok modifies the original string (replaces delimiters with \0)
• It uses internal state, so it's not thread-safe
• For thread-safe tokenizing, use strtok_r

Student Code

Try it yourself: episode33/tokenize.c