Exception thrown when using malloc() to create an array of characters

Question

Essentially, I'm trying to create a program that simply takes in input from the user and then prints it, using dynamically allocated memory. Yes I know how to do this the simple way, but I'm attempting to get to grips with the intricacies of memory management in C. So what is wrong with this code? it runs without error but when I enter in a string into the command line it stops working and throws an exception at a hexadecimal address. Thank you in advance.

int main() {
  char *input;

  input = (char *)malloc(20 * sizeof(char)); 
  puts("Enter the string you wish to display"); 
  scanf_s("%s", input); 
  printf_s("%s", *input); 
  free(input);  
  return 0; 
}

Please edit your question and show your #includes and tell us what platform you use. — Jabberwocky
– Jabberwocky, Commented Feb 5, 2019 at 13:29

pmdj · Accepted Answer · 2019-02-05 13:33:48Z

2

Your compiler should be warning you about this line:

printf_s("%s", *input);

If not, you need to enable an "all warnings" setting. (On gcc and clang, add -Wextra to the command line.)

Essentially, you have a mismatch between the type of the argument (char) and the type expected by the format string (const char*). *input dereferences the character pointer, and so evaluates to the first character in the string. "%s" expects a pointer to a nul-terminated array of characters.

It should work if you remove the *:

printf_s("%s", input);

answered Feb 5, 2019 at 13:33

pmdj

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Comments

Jabberwocky · Accepted Answer · 2019-02-05 13:37:39Z

1

You probably want this:

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

int main() {
  char *input;

  input = malloc(20 * sizeof(char)); // (char *) is not needed here (but doesn't harm either)
  puts("Enter the string you wish to display");
  scanf("%s", input);
  printf("%s", input);  // *input is wrong here
  free(input);
  return 0;
}

Don't use the _s verions as they are not standard on every platform and more or less pointless, just use scanf and printf.

edited Feb 5, 2019 at 13:37

answered Feb 5, 2019 at 13:34

Jabberwocky

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3 Comments

pmdj Over a year ago

The _s versions are in C11.

pmdj Over a year ago

They defend against a few more programming errors such as NULL pointers. How useful that is in code like this is debatable I suppose.

alk Over a year ago

Although I agree with you in the broadest sense, I feel it's not really fair, to tell the OP to use differnt tools then the one s/he chose, if the latter would do the job, if used correctly.

alk · Accepted Answer · 2019-02-05 13:50:56Z

1

You are using scanf_s("%s", ... wrongly.

Verbatim from the docs:

The main difference between the more secure functions (that have the _s suffix) and the other versions is that the more secure functions require the size in characters of each c, C, s, S, and [ type field to be passed as an argument immediately following the variable.

So if input points to the 1st character of a sequence of 20 char then it should be:

  scanf_s("%s", input, 20);

Lessons learned: If in doubt, (re-)read the documentation!

edited Feb 5, 2019 at 13:50

answered Feb 5, 2019 at 13:44

alk

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