Allocating Memory for a whole line of input of variable length stored as a single string

Question

I'm learning C and I was reading around what is the best way to take in input line by line.

Basically I want to create strings that hold a whole line of input. The question below doesn't answer how I should handle the memory allocation for the string.

How can I scan strings with spaces in them using scanf()?

Should I first detect how much space is needed? How? Should I just realloc every time I need more space?

Please show an example

Input is from a file or from stdin(both will be possible but not at the same time)

question too general. what do you need to do? where is the input from? — Itay Sela, Oct 22 '15 at 14:15
Well ,if length will change then you would need to use `realloc` . — ameyCU, Oct 22 '15 at 14:16
I included where the input is from in the question, so remove your downvote — Noel, Oct 22 '15 at 14:18
Depending on platform, you might have a [`getline`](http://man7.org/linux/man-pages/man3/getline.3.html) function which will allocate memory for you. — Some programmer dude, Oct 22 '15 at 14:18

Paul Ogilvie · Answer 1 · 2015-10-22T17:05:24.403

0

The following repeatedly allocates buffer storage until the input has been read (malloc/realloc checks omitted):

char *myGetLine(FILE *in)
{
    char *buffer= NULL;
    int c, i, j;

    i= 0; j= 0;
    do
    {
        c= fgetc(in);
        if (i==j) {
            j += 1024;
            buffer= realloc(buffer, j);
        }
        if (c==EOF) break;
        buffer[i++]= c;
    } while (1);
    buffer[i]= '\0';
    return buffer;
}

edited Oct 22 '15 at 17:05

answered Oct 22 '15 at 14:35

Paul Ogilvie

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Why use a separate buffer `buf` and do all that copying? Just store into `buffer` if there is space, or re-allocate and then store seems to make more sense. – unwind Oct 22 '15 at 14:45
1

Better, but I think you failed to store the incoming character when reallocating. – unwind Oct 22 '15 at 14:49
2

Getting there, though. :) It might make sense to return the buffer, to make the function usuble as a reusable building block. – unwind Oct 22 '15 at 14:56
Also: you don't need the initial malloc. `buff = realloc(NULL, new_size);` works as expected – joop Oct 22 '15 at 14:59
@unwind - fixed your comment. Any reason to keep the down vote? – Paul Ogilvie Oct 22 '15 at 17:03
@BLUEPIX, think I fixed your bug report. – Paul Ogilvie Oct 22 '15 at 17:03
@PaulOgilvie We're not supposed to say, but I never voted you down for this answer. It will break now if given an empty input, since it will terminate a `NULL` buffer. – unwind Oct 23 '15 at 07:06
@Unwind, hi; it won't break on empty input; it will always allocate a buffer. It first gets `c`, then checks/allocates the buffer and only then checks for `c==EOF` and breaks if true, so there will always be a buffer to put in the `\0`. – Paul Ogilvie Oct 23 '15 at 13:19
@PaulOgilvie True! #fail. – unwind Oct 23 '15 at 13:32

Allocating Memory for a whole line of input of variable length stored as a single string

1 Answers1