How to improve the code for dynamically allocated strings in C? - c

Question

So I'm practicing and learning C right now and came across a rather easy challenge from CodeWars that asked to print out a string of "Aa~", "Pa!", and "Aa!" depending on whether n was <= 6 or not. I know how we can do this with arrays but wanted to try out using dynamically allocated char arrays like with malloc for efficiency sake.
I want to make sure I get the basics down with these questions
So I know in other malloc examples with int we set up a pointer (of type int) to point to an allocated memory block. Is it still a pointer when I declare "char *ptr" to point to an allocated memory block because pardon me I thought "char *anything" meant it was a convention to represent a string. So not sure why the below is working somewhat if I'm not setting up a pointer like "char **ptr" as I thought.
why do I get a "malloc: *** error for object 0x100000fab: pointer being freed was not allocated" type of error when I try to return the answer especially when the "val" is 1 or 0? I read somewhere that changing the pointer(string?), answer, to NULL will solve the issue but not entirely sure why this works.
3 To continue in general with the above question, for freeing up space, what is the best method to do so if we dynamically allocate a memory block in a function but need to return a value from that function? As in, do we free up the space after or before?
Thank you all for your input.
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#define val 1
char *sc(int); // function declaration/prototype
int main(int argc, const char * argv[]) {
char *answer = sc(val);
printf("The answer is %s\n", answer);
answer = NULL; // why does this work
free(answer);
return 0;
}
char *sc(int n) {
// if n < 6 then will have an extra "Aa!" after "Pa!" at the nth position
char *ptr = (char*) malloc(n*4);
if (ptr == NULL){
printf("malloc failed");
}
char *first = "Aa~ ";
char *second = "Pa! Aa!";
char *third = "Pa!";
if (n <= 6 && n >1) {
for (int i = 0; i <n-1; i++){
ptr = strcat(ptr, first);
}
ptr = strcat(ptr, second);
}
else if (n> 6){
for (int i = 1; i < n; i++) {
ptr = strcat(ptr, first);
}
ptr = strcat(ptr, third);
}
else if (n <= 1){
ptr = "";
}
else {
printf("Error!");
exit(0);
}
return ptr;
}

Answer 1

The main issue you have with your code is that you're not zeroing out the buffer you get from malloc(), so the first strcat() is not necessarily going to write at the start of the string but at the end.
You can fix that with a strcpy() right after the malloc() call and check:
strcpy(ptr, "");
Or, equivalently, you can just set the first byte of the buffer to zero. Since C strings are zero-terminated strings, setting a character to zero will indicate it's at the end:
ptr[0] = 0;
You also seem to be allocating your buffer too short. If you write n-1 copies of Aa~ (4 bytes) plus one copy of Pa! Aa! (8 bytes when you include the terminating zero!) you'll actually need 4 * (n+1) as space. So either always allocate that or do so in case n < 6 which is where you need the additional bytes.
This is also a problem:
ptr = "";
Because now your ptr is no longer pointing to a buffer returned by malloc(), but to a static (empty) string in your binary. It's quite probable that this is the place where you're getting the trouble from free(), since calling it on a static string in your binary is definitely wrong.
Furthermore, after you set ptr = "" you no longer have any reference to the buffer you allocated, which means you most likely just created a memory leak!
In that case, you should simply use strcpy() or set the first byte to zero. But if you do that at the start of the program, you don't need to do it here.
Finally, free(NULL); works (as in, doesn't throw an error) because that's part of its specification, you can pass it a NULL pointer and it will do nothing. But note that it's not freeing the buffer you allocated, so you have a memory leak here too.
I'd further refactor the second part of your code so you don't have too much repetition appending the strings:
char *sc(int n) {
/* if n <= 6 then will have an extra "Aa!"
* after "Pa!" at the nth position.
*/
char *ptr;
if (n < 0) {
printf("Error!");
return NULL;
}
ptr = (char*) malloc(4 * (n+1));
if (ptr == NULL){
printf("malloc failed");
return NULL;
}
strcpy(ptr, "");
if (n > 1) {
for (int i = 1; i < n; i++){
strcat(ptr, "Aa~ ");
}
strcat(ptr, "Pa!");
if (n <= 6) {
strcat(ptr, " Ah!");
}
}
return ptr;
}
Also note you don't need to assign the result of strcat() back to ptr every time, since it always returns its first argument anyways, so assigning it there isn't changing anything really.

Answer 2

I thought "char *anything" meant it was a convention to represent a string. So not sure why the below is working somewhat if I'm not setting up a pointer like "char **ptr" as I thought.
You're allocating memory for a string, so it just needs to be char *. char ** would be used for an array of multiple strings, or for a pointer to a variable that contains a pointer to a string.
why do I get a "malloc: *** error for object 0x100000fab: pointer being freed was not allocated"
You get that when you do ptr = "";. After you do this, ptr no longer points to the memory that was allocated with malloc, it points to that string literal. If you want to set the allocated memory to an empty string, you can do
ptr[0] = '\0';
This puts a null terminator in the first element of the string.
You also need to do that before the code that uses strcat() to append to the string. Otherwise you're appending to uninitialized data. Simplest would be to do it immediately after allocating the memory (then you don't need it in the n <= 1 block.
The else block is not needed. There are no other possibilities than the 3 you test for unless the CPU is malfunctioning (in which case all bets are off). However, you should check for n < 1 before calling malloc(), as you can't allocate negative memory, and malloc(0) may return NULL.
When you allocate space for ptr, you need to add 1 byte for the terminating null of the string.
char *sc(int n) {
// if n < 6 then will have an extra "Aa!" after "Pa!" at the nth position
if (n >= 1) {
char *ptr = malloc(n*4 + 1);
} else {
char *ptr = malloc(1);
}
if (ptr == NULL){
printf("malloc failed");
exit(1);
}
ptr[0] = '\0'; // initialize empty string
char *first = "Aa~ ";
char *second = "Pa! Aa!";
char *third = "Pa!";
if (n <= 6 && n >1) {
for (int i = 0; i <n-1; i++){
ptr = strcat(ptr, first);
}
ptr = strcat(ptr, second);
}
else if (n> 6){
for (int i = 1; i < n; i++) {
ptr = strcat(ptr, first);
}
ptr = strcat(ptr, third);
}
else if (n <= 1){
// nothing to do
}
return ptr;
}