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

Difficulty Level : Medium
Last Updated : 21 Aug, 2018

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()

// Splits str[] according to given delimiters.
// and returns next token. It needs to be called
// in a loop to get all tokens. It returns NULL
// when there are no more tokens.
char * strtok(char str[], const char *delims);

// A 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

strtok_r()
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()

There are two ways we can call strtok_r()

// The third argument saveptr 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.
char *strtok_r(char *str, const char *delim, char **saveptr);

Below is a simple C program to show the use of strtok_r() :

// 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

Another Example of strtok :

// 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 delimeter
    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
strtok can be used to split a string in multiple strings based on some separators. A simple CSV file support might be implemented using this function. CSV files have commas as delimeters.

// 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 delimeter
    const char s[4] = " ";
    char* tok;
  
    // Use of strtok
    // get first token
    tok = strtok(gfg, s);
  
    // Checks for delimeter
    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,

Reference:
1) manual page strtok_r()
2) http://stackoverflow.com/questions/15961253/c-correct-usage-of-strtok-r

This article is contributed by MAZHAR IMAM KHAN and shantanu_23. If you like GeeksforGeeks and would like to contribute, you can also write an article using contribute.geeksforgeeks.org or mail your article to contribute@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.

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