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

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C provides two functions strtok() and strtok_r() for splitting a string by some delimiter. Splitting a string is a very common task. For example, we have a comma-separated list of items from a file and we want individual items in an array.

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 of strtok()

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.

Examples of strtok()

Example 1: C Program to demonstrate how to split a string using strtok().

C

// C program for splitting 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;
}

                    

Output
 Geeks
 for
 Geeks

Example 2: Program to demonstrates the use of the strtok() function to tokenize a string based on a delimiter.

C

// C code to demonstrate working of
// strtok
#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);
}

                    

Output
  Geeks 
  for 
  geeks 
  Contribute

Practical Application of strtok()

strtok() can be used to split a string into multiple strings based on some separators. A simple CSV file support might be implemented using this function. CSV files have commas as delimiters.

Example 3: C Program to demonstrate the use of the strtok() function in C to implement a simple CSV file.

C

// C code to demonstrate practical application of
// strtok
#include <stdio.h>
#include <string.h>
 
// Driver function
int main()
{
    // Declaration of string
    // Information to be converted into CSV file
    char gfg[100] = " 1997 Ford E350 ac 3000.00";
 
    // 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, ", tok);
 
        // Use of strtok
        // go through other tokens
        tok = strtok(0, s);
    }
 
    return (0);
}

                    

Output
1997, Ford, E350, ac, 3000.00, 

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 of strtok_r()

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.

Examples of strtok_r()

Example 1: a Simple C program to show the use of strtok_r().

C

// C program to demonstrate working of strtok_r()
// by splitting string based on space character.
#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

Nested-Tokenization in C

Example 2: The below C program demonstrates the use of strtok_r() function for Nested Tokenization.

C

#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:

S.No.

strtok()

strtok_r()

1.

It is used to break string str into a series of tokens.It is used to decode a string into a pattern for tokens.

2.

The syntax is as follows:

char *strtok(char *str, const char *delim)

Its syntax is as follows:
char *strtok_r(char *string, const char *limiter, char **context);

3.

It uses the delimiter to proceed.It is a re-entered variant of strtok().

4.

It takes two parameters.It takes three parameters.

5.

It returns a pointer to the first token found in the string.It returns a pointer to the first token found in the string.
6.It is not thread-safe.It is thread-safe.

 

 



Last Updated : 07 Jun, 2023
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