What is strtok_r() function in C language?

The strtok_r() function is a thread-safe alternative to the strtok() function for tokenizing strings. The "_r" suffix stands for "re-entrant," meaning this function can be safely interrupted and resumed without losing its state. This makes it ideal for multi-threaded applications where multiple threads might need to tokenize strings simultaneously.

A re-entrant function can be interrupted during execution and safely resumed later. Because strtok_r() maintains its context through an external pointer, it avoids the static variable issues that make strtok() non-thread-safe.

Syntax

char *strtok_r(char *str, const char *delim, char **saveptr);

Parameters

• str − The string to be tokenized (NULL for subsequent calls)
• delim − String containing delimiter characters
• saveptr − Pointer to store the context between calls

Return Value

Returns a pointer to the next token, or NULL if no more tokens are found.

Example: Basic String Tokenization

The following example demonstrates how to use strtok_r() to split a sentence into individual words −

#include <stdio.h>
#include <string.h>

int main() {
    char input_string[] = "Hello Tutorials Point";
    char token_list[20][20];
    char *context = NULL;
    char *token = strtok_r(input_string, " ", &context);
    int num_tokens = 0;
    
    while (token != NULL) {
        strcpy(token_list[num_tokens], token);
        num_tokens++;
        token = strtok_r(NULL, " ", &context);
    }
    
    printf("Token List:<br>");
    for (int i = 0; i < num_tokens; i++) {
        printf("%s<br>", token_list[i]);
    }
    
    return 0;
}

Token List:
Hello
Tutorials
Point

Example: Multiple Delimiters

This example shows how to use multiple delimiters to parse a CSV-like string −

#include <stdio.h>
#include <string.h>

int main() {
    char data[] = "apple,banana;orange:grape";
    char *context = NULL;
    char *token = strtok_r(data, ",;:", &context);
    
    printf("Parsed fruits:<br>");
    while (token != NULL) {
        printf("- %s<br>", token);
        token = strtok_r(NULL, ",;:", &context);
    }
    
    return 0;
}

Parsed fruits:
- apple
- banana
- orange
- grape

Key Differences from strtok()

Conclusion

The strtok_r() function provides a thread-safe way to tokenize strings by using an external context pointer. It's essential for multi-threaded applications where string parsing needs to be performed safely without interference between different execution contexts.