strtok() and strtok_r() functions in C with examples

String splitting is the process of dividing a string into smaller parts based on a specified delimiter. In C, the strtok() and strtok_r() functions are commonly used to split strings into tokens.

• strtok() splits a string by a delimiter and returns one token at a time.
• strtok_r() is a reentrant version of strtok() that is safer for multithreaded programs.

strtok() Function

The strtok() method splits str[] according to given delimiters and returns the next token. It needs to be called in a loop to get all tokens. It returns NULL when there are no more tokens.

Syntax

char *strtok(char *str, const char *delims);

Parameters:

• str: It is the pointer to the string to be tokenized.
• delims: It is a string containing all delimiters.

Return Value:

• It returns the pointer to the first token encountered in the string.
• It returns NULL if there are no more tokens found.

Example: C Program to demonstrate how to split a string using strtok().

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

int main()
{
    char str[] = "Geeks-for-Geeks";

    // Returns first token
    char* token = strtok(str, " - ");

    // Keep printing tokens while one of the
    // delimiters present in str[].
    while (token != NULL) {
        printf(" % s\n", token);
        token = strtok(NULL, " - ");
    }

    return 0;
}

 Geeks
 for
 Geeks

Example: Program to demonstrates the use of the strtok() function to tokenize a string based on a delimiter.

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

// Driver function
int main()
{
    // Declaration of string
    char gfg[100] = " Geeks - for - geeks - Contribute";

    // Declaration of delimiter
    const char s[4] = "-";
    char* tok;

    // Use of strtok
    // get first token
    tok = strtok(gfg, s);

    // Checks for delimiter
    while (tok != 0) {
        printf(" %s\n", tok);

        // Use of strtok
        // go through other tokens
        tok = strtok(0, s);
    }

    return (0);
}

  Geeks 
  for 
  geeks 
  Contribute

strtok_r() Function

Just like strtok() function in C, strtok_r() does the same task of parsing a string into a sequence of tokens. strtok_r() is a reentrant version of strtok(), hence it is thread safe.

Syntax

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

Parameters:

• str: It is the pointer to the string to be tokenized.
• delims: It is a string containing all delimiters.
• saveptr: It is a pointer to a char * variable that is used internally by strtok_r() in order to maintain context between successive calls that parse the same string.

Return Value:

• It returns the pointer to the first token encountered in the string.
• It returns NULL if there are no more tokens found.

Example: a Simple C program to show the use of strtok_r().

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

int main()
{
    char str[] = "Geeks for Geeks";
    char* token;
    char* rest = str;

    while ((token = strtok_r(rest, " ", &rest)))
        printf("%s\n", token);

    return (0);
}

Output

Geeks
for
Geeks

Example: The below C program demonstrates the use of strtok_r() function for Nested Tokenization.

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

int main()
{
    char str[] = "Hello, World! Geeks for Geeks.";
    const char outer_delimiters[] = "!.";
    const char inner_delimiters[] = " ,";

    char* token;
    char* outer_saveptr = NULL;
    char* inner_saveptr = NULL;

    token = strtok_r(str, outer_delimiters, &outer_saveptr);

    while (token != NULL) {
        printf("Outer Token: %s\n", token);

        char* inner_token = strtok_r(
            token, inner_delimiters, &inner_saveptr);

        while (inner_token != NULL) {
            printf("Inner Token: %s\n", inner_token);
            inner_token = strtok_r(NULL, inner_delimiters,
                                   &inner_saveptr);
        }

        token = strtok_r(NULL, outer_delimiters,
                         &outer_saveptr);
    }

    return 0;
}

Output

Outer Token: Hello, World
Inner Token: Hello
Inner Token: World
Outer Token:  Geeks for Geeks
Inner Token: Geeks
Inner Token: for
Inner Token: Geeks

Difference Between strtok() and strtok_r()

Let us see the differences between strtok() and strtok_r() functions in a tabular form as shown below: