C program malloc with array of strings - c

Question

At the beginning of a program I need to dynamically allocate memory for an unknown number of strings with unknown number of size to later manipulate with. To get the number of strings from a user I have:
int main(int argc, char *argv[]){
int number = atoi(argv[1]);
So far so good. "number" now holds holds the number that the user inputted on the command line for executing the code. Now here comes the part I don't quite understand. I now need to dynamically store the lengths of the strings as well as the contents of the strings. For example, I want the program to function like this:
Enter the length of string 1: 5
Please enter string 1: hello
Enter the length of string 2: ...
For this I recognize that I will have to create an array of strings. However, I can't quite understand the concept of pointers to pointers and what not. What I would like is perhaps a simplification of how this gets accomplished?

Answer 1

You know from the start you will have number strings to store so you will need an array of size number to store a pointer to each string.
You can use malloc to dynamically allocate enough memory for number char pointers:
char** strings = malloc(number * sizeof(char*));
Now you can loop number times and allocate each string dynamically:
for (int i = 0; i < number; i++) {
// Get length of string
printf("Enter the length of string %d: ", i);
int length = 0;
scanf("%d", &length);
// Clear stdin for next input
int c = getchar(); while (c != '\n' && c != EOF) c = getchar();
// Allocate "length" characters and read in string
printf("Please enter string %d: ", i);
strings[i] = malloc(length * sizeof(char));
fgets(strings[i], length, stdin);
}

Answer 2

Since you want to save both the length and the string, I'll suggest that you put them together in a struct. Like
struct string
{
int length;
char* str;
};
Now you can dynamically create an array of this struct and dynamically assign memory for the individual strings.
Something like:
#include <stdio.h>
#include <stdlib.h>
struct string
{
int length;
char* str;
};
int main(void) {
int i;
char tmp[128];
int number = 3;
struct string* strings = malloc(number * sizeof *strings);
// read the input
for (i=0; i<number; ++i)
{
printf("length?\n");
if (fgets(tmp, sizeof tmp, stdin) == NULL)
{
printf("error 1");
exit(1);
}
int length;
if (sscanf(tmp, "%d", &length) != 1)
{
printf("error 2");
exit(1);
}
strings[i].length = length;
strings[i].str = calloc(length + 2, 1);
printf("string?\n");
if (fgets(strings[i].str, length + 2, stdin) == NULL)
{
printf("error 3");
exit(1);
}
if (strings[i].str[length] != '\n')
{
printf("error 4");
exit(1);
}
strings[i].str[length] = '\0';
}
// print the strings
for (i=0; i<number; ++i)
{
printf("len=%d str=%s\n", strings[i].length, strings[i].str);
}
// Clean up, i.e. free the memory allocated
for (i=0; i<number; ++i)
{
free(strings[i].str);
}
free(strings);
return 0;
}
Note: You should also check that all malloc/calloc are succesful, i.e. doesn't return NULL. For clarity I skipped that.