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

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() 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.

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

Example:

C

// C/C++ program for splitting a string
// using strtok()
#include& lt; stdio.h & gt;
#include& lt; string.h & gt;

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 & quot;, 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() :

CPP

// 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

Example 2

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: 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 delimiters.

Example:

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,

Let us see the differences in a tabular form as shown below as follows:

strtok()	strtok_r()
It is used to break string str into a series of tokens	It is used to decode a string into a pattern for tokens.
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);
It uses the delimiter to proceed.	It is a re-entered variant of strtok().
It takes two parameters.	It takes three parameters.
Its return value is a pointer to the first token found in the string	It is defined in header file <string.h>

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