Print function print some strange chars - c

Question

i'm trying to create a random strings using this function:
static char *rand_string(char *str)
{
const char charset[] = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJK";
int i;
for ( i = 0; i < 10; i++)
{
int key = rand() % (int) (sizeof charset - 1);
str[i] = charset[key];
}
str[11] = '\0';
return str;
}
The problem is this: sometimes when i am going to print the pointer, it display some strange char like this:
As you can see in the first launch the chars in a string are 10, in the second and third launch the chars in a string are 11...
This is my program:
#include <stdio.h>
#include <stdlib.h>
#include <time.h>
#include <unistd.h>
static char *rand_string(char *str);
int main()
{
char *string, //str
*string_result; //str1
int dimensione= 15,
i;
for(i=0;i<dimensione;i++)
{
string_result = rand_string(string);
printf("%s\n", string_result);
}
}
static char *rand_string(char *str)
{
const char charset[] = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJK";
int i;
for ( i = 0; i < 10; i++)
{
int key = rand() % (int) (sizeof charset - 1);
str[i] = charset[key];
}
str[11] = '\0';
return str;
}
You can view and test my code here --> https://onlinegdb.com/r1yY8DWc8

Answer 1

Your whole code invoke undefined behavior, because you are passing string which is an uninitialized pointer which may "not" be guaranteed to accessible for the 11 bytes your are accessing in the rand_string() function.
You need to allocate memory from the main() and pass that buffer to be filled by the random string generator function, that way you have control of the memory you are modifying.
Also the for loop in your generator already fills indices from 0 to 9. To set the final byte to NULL, you need to use the index as 10.
Also the headers time.h and unistd.h are useless here. Here is a re-write of the same program with memory allocated through malloc(). Also modified your function rand_string() to void instead of returning a string.
#include <stdio.h>
#include <stdlib.h>
void rand_string(char *str);
int main()
{
int dimensione= 15;
char *string = malloc(11 * sizeof *string); // extra byte for NULL
if (!string) return 1;
int i;
for(i=0;i<dimensione;i++)
{
rand_string(string);
printf("%s\n", string);
}
free(string);
return 0;
}
void rand_string(char *str)
{
const char charset[] = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJK";
int i;
for ( i = 0; i < 10; i++)
{
int key = rand() % (int) (sizeof charset - 1);
str[i] = charset[key];
}
str[10] = '\0';
}
Always compile with extra flags to enable all sorts of warnings from your compiler. For e.g. with gcc always add -Wall -Wextra to see the warnings that you could fix from your side.