How does strtok() split the string into tokens in C?

For those who are still having hard time understanding this strtok() function, take a look at this pythontutor example, it is a great tool to visualize your C (or C++, Python ...) code.

In case the link got broken, paste in:

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

int main()
{
    char s[] = "Hello, my name is? Matthew! Hey.";
    char* p;
    for (char *p = strtok(s," ,?!."); p != NULL; p = strtok(NULL, " ,?!.")) {
      puts(p);
    }
    return 0;
}

Credits go to Anders K.

strtok() stores the pointer in static variable where did you last time left off , so on its 2nd call , when we pass the null , strtok() gets the pointer from the static variable .

If you provide the same string name , it again starts from beginning.

Moreover strtok() is destructive i.e. it make changes to the orignal string. so make sure you always have a copy of orignal one.

One more problem of using strtok() is that as it stores the address in static variables , in multithreaded programming calling strtok() more than once will cause an error. For this use strtok_r().

you can scan the char array looking for the token if you found it just print new line else print the char.

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

int main()
{
    char *s;
    s = malloc(1024 * sizeof(char));
    scanf("%[^\n]", s);
    s = realloc(s, strlen(s) + 1);
    int len = strlen(s);
    char delim =' ';
    for(int i = 0; i < len; i++) {
        if(s[i] == delim) {
            printf("\n");
        }
        else {
            printf("%c", s[i]);
        }
    }
    free(s);
    return 0;
}

So, this is a code snippet to help better understand this topic.

Printing Tokens

Task: Given a sentence, s, print each word of the sentence in a new line.

char *s;
s = malloc(1024 * sizeof(char));
scanf("%[^\n]", s);
s = realloc(s, strlen(s) + 1);
//logic to print the tokens of the sentence.
for (char *p = strtok(s," "); p != NULL; p = strtok(NULL, " "))
{
    printf("%s\n",p);
}

Input: How is that

Result:

How
is
that

Explanation: So here, "strtok()" function is used and it's iterated using for loop to print the tokens in separate lines.

The function will take parameters as 'string' and 'break-point' and break the string at those break-points and form tokens. Now, those tokens are stored in 'p' and are used further for printing.

strtok is replacing delimiter with'\0' NULL character in given string

CODE

#include<iostream>
#include<cstring>

int main()
{
    char s[]="30/4/2021";     
    std::cout<<(void*)s<<"\n";    // 0x70fdf0
    
    char *p1=(char*)0x70fdf0;
    std::cout<<p1<<"\n";
    
    char *p2=strtok(s,"/");
    std::cout<<(void*)p2<<"\n";
    std::cout<<p2<<"\n";
    
    char *p3=(char*)0x70fdf0;
    std::cout<<p3<<"\n";
    
    for(int i=0;i<=9;i++)
    {
        std::cout<<*p1;
        p1++;
    }
    
}

OUTPUT

0x70fdf0       // 1. address of string s
30/4/2021      // 2. print string s through ptr p1 
0x70fdf0       // 3. this address is return by strtok to ptr p2
30             // 4. print string which pointed by p2
30             // 5. again assign address of string s to ptr p3 try to print string
30 4/2021      // 6. print characters of string s one by one using loop

Before tokenizing the string

I assigned address of string s to some ptr(p1) and try to print string through that ptr and whole string is printed.

after tokenized

strtok return the address of string s to ptr(p2) but when I try to print string through ptr it only print "30" it did not print whole string. so it's sure that strtok is not just returning adress but it is placing '\0' character where delimiter is present.

cross check

1.

again I assign the address of string s to some ptr (p3) and try to print string it prints "30" as while tokenizing the string is updated with '\0' at delimiter.

2.

see printing string s character by character via loop the 1st delimiter is replaced by '\0' so it is printing blank space rather than ''