Parsing input in C and automatically allocating space for characters - c

Question

I started learning programming with Python a couple months ago, and I decided to learn C because I am interested in lower level languages that let me interact closer to the computers hardware. I'm trying to get some input from a user in C, and I am writing my own little input parser. I'm having some trouble here:
#include <stdio.h>
char prompt()
{
char resp[]; // Create a variable for the users response
for (int i = 0; i < 1000; ++i)
{
char letter = getchar(); // Get character
if (letter == '\n') // If the user hits enter break the loop
{
break;
}
else // Otherwise append the character to the response array
{
resp[i] = letter;
}
}
return resp[]; // Return the response array
}
int main() {
return 0;
}
I'm receiving errors with this code. The errors specifically say:
error: definition of variable with array type needs an explicit size or an initializer
char resp[];
I take it that I must define a set value for an array or assign it something right away. I don't understand how I can grow the character array as the user types the input if arrays in C must have a defined value. I am thinking that using pointers or memory management might work, but I haven't learned a lot about these things yet so if you do have a solution involving pointers, it would be of great help to me if you could briefly explain what the code is doing. In the meantime I will try and find a solution.

Answer 1

You probably want to allocate memory, here's how you would do it>
#include <stdio.h>
#include <stdlib.h>
char* prompt()
{
char* resp; // Create a variable for the users response
int i;
resp = malloc(sizeof(char));//allocate space for one char
for (i = 0; i < 1000; ++i)
{
resp = realloc(resp, sizeof(char)*(i+1));//allocate space for one more char
char letter = getchar(); // Get character
if (letter == '\n') // If the user hits enter break the loop
{
break;
}
else // Otherwise append the character to the response array
{
resp[i] = letter;
}
}
return resp; // Return the response array
}
int main() {
char* answer = prompt();
printf("answer is %s\n", answer);
return 0;
}
And this will work but it is highly NOT recommended to do it like this because you never free() your allocated memory.
EDIT>
How to do it? You might want to avoid creating a new function and do it all in your main(), in that case, when you no longer need your variable simply call free(resp);

Answer 2

There isn't a dynamic runtime in C. You need to explicitly allocate resources because your code essentially translates directly into machine instructions.
Here's an example.
#include <stddef.h>
#include <stdlib.h>
#include <stdio.h>
int prompt(const size_t bufsize, char response[bufsize]) {
if (fgets(response, bufsize, stdin) == NULL)
return -1; // error
return 0;
}
int main() {
const size_t bufsize = 1024;
char* response = malloc(bufsize); // Create a variable for the users response
if (!response)
return -1; // error
if (prompt(bufsize, response))
return -1; // error
printf("%s", response);
free(response);
return 0;
}