Proper Way to Free Memory of a Returned Variable

Question

I created a function designed to get user input. It requires that memory be allocated to the variable holding the user input; however, that variable is returned at the end of the function. What is the proper method to free the allocated memory/return the value of the variable?

Here is the code:

char *input = malloc(MAX_SIZE*sizeof(char*));
int i = 0;
char c;

while((c = getchar()) != '\n' && c != EOF) {
    input[i++] = c;
}

return input;

Should I return the address of input and free it after it is used?

Curious as to the most proper method to free the input variable.

Answer 1

It's quite simple, as long as you pass to free() the same pointer returned by malloc() it's fine.

For example

char *readInput(size_t size)
 {
    char *input;
    int   chr;
    input = malloc(size + 1);
    if (input == NULL)
        return NULL;
    while ((i < size) && ((chr = getchar()) != '\n') && (chr != EOF))
        input[i++] = chr;
    input[size] = '\0'; /* nul terminate the array, so it can be a string */
    return input;
 }

 int main(void)
  {
     char *input;
     input = readInput(100);
     if (input == NULL)
         return -1;
     printf("input: %s\n", input);
     /* now you can free it */
     free(input);
     return 0;
  }

What you should never do is something like

free(input + n);

because input + n is not the pointer return by malloc().

But your code, has other issues you should take care of

1. You are allocating space for MAX_SIZE chars so you should multiply by sizeof(char) which is 1, instead of sizeof(char *) which would allocate MAX_SIZE pointers, and also you could make MAX_SIZE a function parameter instead, because if you are allocating a fixed buffer, you could define an array in main() with size MAX_SIZE like char input[MAX_SIZE], and pass it to readInput() as a parameter, thus avoiding malloc() and free().

2. You are allocating that much space but you don't prevent overflow in your while loop, you should verify that i < MAX_SIZE.

Answer 2

You could write a function with return type char*, return input, and ask the user to call free once their done with the data.

You could also ask the user to pass in a properly sized buffer themselves, together with a buffer size limit, and return how many characters were written to the buffer.

Answer 3

This is a classic c case. A function mallocs memory for its result, the caller must free the returned value. You are now walking onto the thin ice of c memory leaks. 2 reasons

First ; there is no way for you to communicate the free requirement in an enforceable way (ie the compiler or runtime can't help you - contrast with specifying what the argument types are ). You just have to document it somewhere and hope that the caller has read your docs

Second: even if the caller knows to free the result he might make a mistake, some error path gets taken that doesnt free the memory. This doesnt cause an immediate error, things seem to work, but after running for 3 weeks your app crashes after running out of memory

This is why so many 'modern' languages focus on this topic, c++ smart pointers, Java, C#, etc garbage collection,...