How to create a substitute for scanf()

Question

After having so many problems using the scanf() function I decided to search for an alternative to read whole strings (and by whole strings I mean words mixed with white spaces and numbers etc, until a \n is found) from the keyboard. I stumbled upon an interesting bit of code here on stackoverflow. So I changed that code to be more dynamic, I wanted to allocate memmory after a char input from the keyboard. I got it to work but only with char **a as an argument and ,in main(), I would call it like this read(&w);.My idea is to create an argument-less function so that I can use it without having to declare a variable char *w each time I need to read a string. Here's the code I got:

char *read(char **a) {
int i = 0;

while (1) {
    *a = (char *)realloc(*a, (i + 1) * sizeof(char));
    scanf("%c", &(*(*a + i)));

    if ( *(*a+i) == '\n')
        break;
    else 
        i++;
}
*(*a+i) = '\0';
return *a;}

Now i'm trying to modify this function to take no arguments but I'm getting write/read access violation errors :

char *read2() {
int i = 0;
char **a = NULL;

while (1) {

    *a = (char *)realloc(*a, (i + 1) * sizeof(char)) // Error here
        scanf("%c", &(*(*a + i)));
        if (*(*a + i) == '\n') { break; }
        else { i++; }
}
    *(*a + i) = '\0';
return *a;}

Thanks in advance

POSIX has a `getline()` function that does what you want. Can you use that? — Barmar, Mar 15 '17 at 22:09
Are you trying to re-invent the 1000th version of `fgets` or `getline`? And using the name`read` is a very bad idea, because that is commonly already defined by your platforms system library. — too honest for this site, Mar 15 '17 at 22:10
Well I was trying to create an argument-less version of this `read()` function. I can´t seem to make it through all the` *` and pointers . Seemed like a good practice but I didn´t remember about the `getline()` or the `fgets()` functions. I'll try them ,thanks. — Ferro, Mar 15 '17 at 22:21

score 1 · Accepted Answer · answered Mar 15 '17 at 22:16

The problem with your function is that you've declared too many levels of indirection. You declare

char **a = NULL;

and then assign to *a. But since a == NULL, this indirects through a null pointer, which is undefined behavior.

You don't need an extra level of indirection for a local variable, that's only needed for a parameter so you can update the caller's variable.

You should also check for EOF when reading. Instead of using scanf(), use getc().

char *read2() {
    int i = 0;
    char *a = NULL;

    while (1) {
        a = (char *)realloc(a, (i + 1) * sizeof(char));
        int c = getc(stdin);
        if (c == '\n' || c == EOF) {
            break;
        } else {
            a[i] = c;
            i++;
        }
    }
    a[i] = '\0';
    return a;
}

wow , levels of indirection , not knowing this makes me feel like a complete newbie in C but thanks @Barmar it worked wonderfully. Thank you all — Ferro, Mar 15 '17 at 22:28

score 0 · Answer 2 · answered Mar 15 '17 at 22:06

0

You really don't need to roll your own, just use fgets().

Also if you MUST roll your own getline() function, do it with fgetc() instead.

answered Mar 15 '17 at 22:06

Iharob Al Asimi

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`fgets()` won't reallocate the array if it's not long enough for the whole line. – Barmar Mar 15 '17 at 22:08
Yes, that's why I suggest `getline()`. – Iharob Al Asimi Mar 15 '17 at 22:09

How to create a substitute for scanf()

2 Answers2