Array of dynamic length - c

Question

#include <stdio.h>
#include <string.h>
#include <stdlib.h>
int main(void) {
char *names = NULL;
int capacity = 0;
int size = 0;
printf("Type 'end' if you want to stop inputting names\n");
while (1) {
char name[100];
printf("Input:\n");
fgets(name, sizeof(name), stdin);
if (strncmp(name, "end", 3) == 0) {
break;
}
if (size == capacity) {
char *temp = realloc(names, sizeof(char) * (size + 1));
if (!temp) {
if (names) {
return 1;
}
}
names = temp;
capacity++;
}
names[size] = name;
size++;
}
for (int i = 0; i < size; i++) {
printf("OUTPUT :%c\n", names[i]);
}
if (names) {
free(names);
}
}
I am trying to create an array of dynamic length in C but I don't know what is wrong with my code? I think it is cause of how I take the user input and the problem occurs when the code names[size] = name is executed.

Answer 1

You need to declare an array of pointers to strings (char**), not a pointer to a single character string (char*).
This means that when you want to add a new entry, not only do you need to make space for the new pointer in your array (type is char* not char) but you also have to separately allocate storage for the string itself and then set the pointer you just made space for to that allocated string.

Answer 2

There are multiple problems in the code:
names should be defined as a pointer to an array of char *: char **names = NULL;
you must make copies of the strings read with fgets(), otherwise all entries in names will point to the same array name.
you actually do not need to distinguish capacity and size if you reallocate one extra slot at a time.
the format "OUTPUT :%c\n" is incorrect for a string.
Here is a corrected version:
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
int main(void) {
char **names = NULL;
int capacity = 0;
int size = 0;
printf("Type 'end' if you want to stop inputting names\n");
for (;;) {
char name[100];
printf("Input:\n");
if (!fgets(name, sizeof(name), stdin)) {
break;
}
if (strncmp(name, "end", 3) == 0) {
break;
}
if (size == capacity) {
// reallocate array with geometric growth
int new_capacity = capacity + (capacity / 2) + 4;
char *temp = realloc(names, sizeof(*names) * new_capacity);
if (!temp) {
free(names);
printf("out of memory\n");
return 1;
}
names = temp;
capacity = new_capacity;
}
name[strcspn(name, "\n")] = '\0'; // strip the trailing newline if any
names[size++] = strdup(name); // allocate a copy of the string
}
for (int i = 0; i < size; i++) {
printf("OUTPUT: %s\n", names[i]);
}
free(names);
return 0;
}