C programming: get line, 2D array and ending user input - arrays

Question

char* str = NULL;
size_t capacity = 0;
getline(&str, &capacity, stdin);
The above code is an example of using getline to dynamically allocate memory while reading a string input. But, what if I'm trying to read the input into a 2D array?
Example:
Linenumberone (enter)
Linenumbertwo (enter)
(enter) <<< enter on an empty line - stop reading user input
I do know about the function strlen, so I was thinking I could technically use that to figure out when to stop reading user input? But I'm a bit confused, is it even possible to read user input using getline into a 2D array in C as described? I've only seen people using it in C++

Answer 1

We can declare an array of pointers, then assign each line to the 2D array in a loop. See the code below:
#include <stdio.h>
#include <stdlib.h>
int main()
{
char *line[5] = {NULL}; // array of 5 pointers
size_t len = 0;
int i=0;
for(i=0;i<5;i++)
{
getline(&line[i],&len, stdin); // reading strings
}
printf("\nThe strings are \n");
for(int i=0; i<5; i++)
{
printf("%s",line[i]); // prinitng the strings.
}
return 0;
}
The output is (first 5 lines are input) :
krishna
rachita
teja
sindhu
sagarika
The strings are
krishna
rachita
teja
sindhu
sagarika

Answer 2

Each time characters = getline(&str,...) is used, new dynamic memory is allocated at the address str, with a size equal to number of characters read. It suffices to store the buffer address (value of str) into an array at each getline() call. Subsequently, the buffer address (str) in getline() is incremented by the number of characters read in the last getine(). See code and example below.
#include <stdio.h>
int main () {
char *buffer=NULL;
size_t capacity = 0;
size_t maxlines = 100;
char *lines[maxlines]; // pointers into the malloc buffer for each line
printf ("Input\n\n");
int lines_read;
int characters;
// read getline until empty line or maxlines
for (lines_read = 0; lines_read < maxlines; lines_read++) {
printf ("Enter line %d: ", lines_read + 1);
characters = getline (&buffer, &capacity, stdin);
// stop at empty line
if (characters == 1) break;
// convert end of line "\n" into zero (C null-terminated string convention)
buffer[characters - 1] = 0;
// save starting location into lines
lines[lines_read] = buffer; // save pointer to the start of this line in the buffer
buffer += characters; // set pointer to the start of a new line in the buffer
}
printf ("\nOutput\n\n");
// print lines read excluding empty line
for (int i = 0; i < lines_read; i++)
printf ("Line[%d] = %s\n", i+1, lines[i]);
return (0);
}
Example output:
Input
Enter line 1: This
Enter line 2: is
Enter line 3: an
Enter line 4: example.
Enter line 5:
Output
Line[1] = This
Line[2] = is
Line[3] = an
Line[4] = example.