Question

For the getline() function, I tried two method to allocate memory space for string, but the first one works, the second doesn’t. Can anyone explain why the second won’t work?

The first one

#include <stdio.h>

int main()
{
  int bytes_read;
  int nbytes = 100;
  char *my_string;

  puts ("Please enter a line of text.");

  /* These 2 lines are the heart of the program. */
  my_string = (char *) malloc (nbytes + 1);
  bytes_read = getline (&my_string, &nbytes, stdin);

  if (bytes_read == -1)
    {
      puts ("ERROR!");
    }
  else
    {
      puts ("You typed:");
      puts (my_string);
    }

  return 0;
}

The second one:

#include <stdio.h>
int main()
{
  int bytes_read;
  int nbytes = 100;
  char my_string[nbytes+1];

  puts ("Please enter a line of text.");

  /* These 2 lines are the heart of the program. */
  bytes_read = getline (&my_string, &nbytes, stdin);

  if (bytes_read == -1)
    {
      puts ("ERROR!");
    }
  else
    {
      puts ("You typed:");
      puts (my_string);
    }

  return 0;
}

The second one can be compiled, but when I execute it:

bash-3.2$ ./a.out 
Please enter a line of text.
lsdfa
Bus error: 10

It says Bus error: 10

I don’t know what is the possible reason, can anyone help me?

Answer 1

The signature for getline requires a pointer to a char* so that it may be modified. This is because getline is supposed to be allowed to call realloc on the char* or allocate a char* if you pass a char* which points to 0:

getline() reads an entire line from stream, storing the address of the buffer
containing the text into *lineptr.

…

In either case, on a successful call, *lineptr and *n will be updated to
reflect the buffer address and allocated size respectively

In the first case everything is fine as the pointer you’ve passed to getline can be modified.

In the second case you’re passing a pointer to a char array, which itself cannot be modified. As you’ve found, unfortunately &my_string ends up looking like char**, so the compiler does not complain (but it may with -Wall).

Basically, since getline needs to be able to modify what lineptr points to, in your second case this cannot be done (hence the bus error).