C: Unknow size input with scanf()

Question

I'm trying to make a simple program where you put some text in it and it write back what you just wrote.

For example if I write "Hello World", the program should write me back "Hello World"

How I think it should work is like that :

loop to check if the current character is '\0'

if not print the current character and reallocate 1 more byte of memory

else stop the loop

So it's looks like an easy thing to do but my attempt is not working correctly, for example if you put only a few characters it is going to write you back with no problem but with longer string.. it is not working at all.

I know it is possible using fgets(), but I would like to understand why my version with scanf() isn't working.

(my code)

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

int main(void){
    int mem = 2;
    char * str = malloc(mem);

    scanf("%s", str);

    while (*str != '\0') {
        printf("%c", *str);
        realloc(str, mem++);
        str++;
    }

    free(str);

    return 0;
} 

edit : I was thinking that I only did a small mistake but, after reading the comments it looks like there is a lot of things that I did wrong in this tiny program. I'm going make sure that I better understand how C work and retry to do this program later. Thanks for the help!

Answer 1

Your program could be much more simple

#include <stdio.h>

int main() {

    char c;
    
    while( scanf("%c", &c) == 1 ) {
        
        printf("%c", c);
    }
    
    return 0;
}

Answer 2

Reading functions' manuals and searching for similar questions is a great method to get things clearer, and undestand the problem you are facing better :)

Try looking at this question: Reading input of unknown length, and particularly at this answer that uses scanf: scanf answer (I did not verify if that works, but it teaches you another method you can use).

Answer 3

If you're going to use scanf for this, you shouldn't use "%s". (You should never use "%s" without a field width, since this will potentially overflow a buffer. Using "%s" is no better that gets().) If you are going to use a variant of "%s", you need to understand that it will ignore whitespace. The following does almost what you want, except for the whitespace issue. If you want to handle whitespace with precision, you cannot use "%s".

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

void * xrealloc(void *buf, size_t num, size_t siz, void *endvp);

int
main(void)
{
    size_t cap = 2;
    char *end = NULL;
    char *str = xrealloc(NULL, cap, sizeof *str, &end);

    /*
     * Read 1 char at a time, discarding whitespace.  This should
     * be done with getchar().  If using scanf, a single char
     * should be read with "%c".  We use "%1s" here only
     * because the question is specifically about `%s`.
     * Note that %1s will allow scanf to write up to 2 characters into
     * *end, so we need to ensure there is space for the terminator.
     */
    while( 1 == scanf("%1s", end++) ){
        while( end > str + cap - 2 ){
            str = xrealloc(str, cap *= 2, sizeof *str, &end);
        }
    }
    fputs(str, stdout);
    free(str);

    return 0;
}

void *
xrealloc(void *buf, size_t num, size_t siz, void *endvp)
{
    char **endp = endvp;
    char *b = buf;
    ptrdiff_t offset = b && endp && *endp ? *endp - b : 0;
    b = realloc(b, num * siz);
    if( b == NULL ){
        perror("realloc");
        exit(EXIT_FAILURE);
    }
    if( endp != NULL ){
        *endp = b + offset;
    }
    return b;
}