How to dynamically allocate memory space for a string and get that string from user?

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Read one character at a time (using getc(stdin)) and grow the string (realloc) as you go.

Here’s a function I wrote some time ago. Note it’s intended only for text input.

char *getln()
{
    char *line = NULL, *tmp = NULL;
    size_t size = 0, index = 0;
    int ch = EOF;

    while (ch) {
        ch = getc(stdin);

        /* Check if we need to stop. */
        if (ch == EOF || ch == '\n')
            ch = 0;

        /* Check if we need to expand. */
        if (size <= index) {
            size += CHUNK;
            tmp = realloc(line, size);
            if (!tmp) {
                free(line);
                line = NULL;
                break;
            }
            line = tmp;
        }

        /* Actually store the thing. */
        line[index++] = ch;
    }

    return line;
}

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