input - How to read unlimited characters in C

Question

How to read unlimited characters into a char* variable without specifying the size?

For example, say I want to read the address of an employee that may also take multiple lines.

Answer 1

You have to start by "guessing" the size that you expect, then allocate a buffer that big using malloc. If that turns out to be too small, you use realloc to resize the buffer to be a bit bigger. Sample code:

char *buffer;
size_t num_read;
size_t buffer_size;

buffer_size = 100;
buffer = malloc(buffer_size);
num_read = 0;

while (!finished_reading()) {
    char c = getchar();
    if (num_read >= buffer_size) {
        char *new_buffer;

        buffer_size *= 2; // try a buffer that's twice as big as before
        new_buffer = realloc(buffer, buffer_size);
        if (new_buffer == NULL) {
            free(buffer);
            /* Abort - out of memory */
        }

        buffer = new_buffer;
    }
    buffer[num_read] = c;
    num_read++;
}

This is just off the top of my head, and might (read: will probably) contain errors, but should give you a good idea.