Malloc and scanf

Question

I'm fairly competent in a few scripting languages, but I'm finally forcing myself to learn raw C. I'm just playing around with some basic stuff (I/O right now). How can I allocate heap memory, store a string in the allocated memory, and then spit it back out out? This is what I have right now, how can I make it work correctly?

#include <stdio.h>
#include <stdlib.h>

int main(int argc, char *argv[])
{
  char *toParseStr = (char*)malloc(10);
  scanf("Enter a string",&toParseStr);
  printf("%s",toParseStr);
  return 0;
}

Currently I'm getting weird output like '8'\'.

Answer 1

  char *toParseStr = (char*)malloc(10);
  printf("Enter string here: ");
  scanf("%s",toParseStr);
  printf("%s",toParseStr);
  free(toParseStr);

Firstly, the string in scanf is specifies the input it's going to receive. In order to display a string before accepting keyboard input, use printf as shown.

Secondly, you don't need to dereference toParseStr since it's pointing to a character array of size 10 as you allocated with malloc. If you were using a function which would point it to another memory location, then &toParseStr is required.

For example, suppose you wanted to write a function to allocate memory. Then you'd need &toParseStr since you're changing the contents of the pointer variable (which is an address in memory --- you can see for yourself by printing its contents).

void AllocateString(char ** ptr_string, const int n)
{
    *ptr_string = (char*)malloc(sizeof(char) * n);
}

As you can see, it accepts char ** ptr_string which reads as a pointer which stores the memory location of a pointer which will store the memory address (after the malloc operation) of the first byte of an allocated block of n bytes (right now it has some garbage memory address since it is uninitialized).

int main(int argc, char *argv[])
{
  char *toParseStr;
  const int n = 10;
  printf("Garbage: %p\n",toParseStr);
  AllocateString(&toParseStr,n);
  printf("Address of the first element of a contiguous array of %d bytes: %p\n",n,toParseStr);

  printf("Enter string here: ");
  scanf("%s",toParseStr);
  printf("%s\n",toParseStr);
  free(toParseStr);

  return 0;
}

Thirdly, it is recommended to free memory you allocate. Even though this is your whole program, and this memory will be deallocated when the program quits, it's still good practice.

Answer 2

You need to give scanf a conversion format so it knows you want to read a string -- right now, you're just displaying whatever garbage happened to be in the memory you allocated. Rather than try to describe all the problems, here's some code that should at least be close to working:

char *toParseStr = malloc(10);
printf("Enter a string: ");
scanf("%9s", toParseStr);
printf("\n%s\n", toParsestr);
/* Edit, added: */ 
free(toParseStr);
return 0;

Edit: In this case, freeing the string doesn't make any real difference, but as others have pointed out, it is a good habit to cultivate nonetheless.