Dynamically read user input strings in C [duplicate]

Question

Given a user input of unknown length (consisting of words with maximum length 100). Is there a way to read it dynamically string by string?

It is my understanding that scanf reads a string until a space is reached, so I tried using scanf but it enters an infinite loop.

char buf[101];
while (scanf("%s", buf))
{
    //something is done with the string
}

Answer 1

Given a user input of unknown length (consisting of words with maximum length 100) is there a way to read it dynamically string by string?

Building your own function, instead of scanf would help to achieve this

• Here, I've created a function scan which would dynamically take in the string by increasing it's size over each iteration

• I've use the additional header files string.h and stdlib.h for the following reasons :

  1. functions in stdlib.h (click to know more) library file are useful for dynamic allocation of memory.

  2. functions in string.h (click to know more) library file are useful to handle strings.

Note: the input from the user stops when he enters the string end.

#include <stdio.h>  //the standard library file
#include <stdlib.h> //library file useful for dynamic allocation of memory
#include <string.h> //library file with functions useful to handle strings

//the function    
char* scan(char *string)
{
    int c; //as getchar() returns `int`
    string = malloc(sizeof(char)); //allocating memory

    string[0]='\0';

    for(int i=0; i<100 && (c=getchar())!='\n' && c != EOF ; i++)
    {
        string = realloc(string, (i+2)*sizeof(char)); //reallocating memory
        string[i] = (char) c; //type casting `int` to `char`
        string[i+1] = '\0'; //inserting null character at the end
    }

    return string;
}

int main(void)
{
    char *buf; //pointer to hold base address of string

    while( strcmp((buf=scan(buf)),"end") ) //this loop will continue till you enter `end`
    {
        //do something with the string

        free(buf); //don't forget to free the buf at the end of each iteration
    }

    free(buf); //freeing `buf` for last input i.e, `end` 

}

---

let's just //do something with the string to see whether the above code works or not :)

I change the following while loop in main function :

    while( strcmp((buf=scan(buf)),"end") )
    {
        //do something with the string
    }

to

while( strcmp((buf=scan(buf)),"end") )
{
    printf("you entered : %s\n",buf);
    printf("size        : %u\n",strlen(buf));
    printf("reversing   : %s\n",strrev(buf));

    printf("\n-------------------\n");

    free(buf);
}

and now,

input :

hall
of
fame
stay in it
end

output :

you entered : hall
size        : 4
reversing   : llah

-------------------
you entered : of
size        : 2
reversing   : fo

-------------------
you entered : fame
size        : 4
reversing   : emaf

-------------------
you entered : stay in it
size        : 10
reversing   : ti ni yats

-------------------

---