How to create an array of strings of unknown size? - c

Question

I'm trying to understand how can I create a C program that declares an "array of strings" whose size is unknown at the time of declaration. This is what I've got so far:
#include <stdio.h>
#include <stdlib.h>
int main(void) {
int n, i;
char** words;
printf("How many strings you want to input? \n");
scanf("%d", &n);
words = malloc(sizeof(char*) * n);
for (i = 0; i < n; i++) {
printf("Input your %d string: ", i + 1);
scanf("%s", words[i]);
}
for (i = 0; i < n; i++) {
printf("%s\n", words[i]);
}
return 0;
}
The program compiles, but I get a Segmentation fault error.

Answer 1

You only allocated memory for the pointer to the strings, but not for the strings themselves. Attempting to store strings at non-allocated memory invokes undefined behavior.
The pointer are just pointer. You cannot store strings in them. You need to reserve memory for where the pointer should point to.
#define STR_SIZE 100 // max length of a string (incl. null terminator)
printf("How many strings you want to input? \n");
if (scanf("%d", &n) != 1)
{
fputs("Error at input", stderr);
// further error routine.
}
// allocate memory for the pointer to the strings.
words = malloc(sizeof(*words) * n);
if (!words)
{
// error routine if the memory allocation failed.
perror("malloc");
exit(EXIT_FAILURE);
}
// allocate memory for the strings themselves.
for (int i = 0; i < n; i++)
{
words[i] = malloc(sizeof(**words) * STR_SIZE);
if (!words[i])
{
// error routine if the memory allocation failed.
perror("malloc");
exit(EXIT_FAILURE);
}
}
Side notes:
Always check the return of memory-management functions if an error happened at the allocation! Same goes for input operations such as scanf().
Note that using sizeof(*words) and sizeof(**words) instead of sizeof(char*) and sizeof(char) is more safe for the case your code changes.

Answer 2

You did not allocate the memory properly for individual char pointer words[i].
This was ok - you allocated memory for the pointer to pointer to char words:
words = malloc(sizeof(char*) * n);
But then you haven't allocated for individual pointers of that words yet. To do so, you need to provision how much buffer to for individual word. As an example, if each word is 100 char, then:
for (i = 0; i < n; i++)
{
printf("Input your %d string: ", i + 1);
words[i] = malloc(sizeof(char) * 100); //<--
scanf("%s", words[i]);
}
Also remember to free the buffer once you're done.
for (int i=0; i<n; ++i) {
free(words[i]);
}
free(words);
return 0;