is malloc necessary for char *s?

duryodhan · 11-14-2006, 10:52 AM

Will the following work :

char *s;
scanf("%s",s);
printf("%s",s);

IT seems to work on some comps and not on some.. what is the reason?

dmail · 11-14-2006, 10:59 AM

IT seems to work on some comps and not on some.. what is the reason?
Uninitialised pointer.
the following could cause a buffer overrun but try

Code:

#define ARRAY_SIZE 20
char s[ARRAY_SIZE];
scanf("%s",s);
printf("%s",s);

paulsm4 · 11-14-2006, 12:48 PM

duryodhan -

Yes, "allocation" is necessary.

And no, you don't necessarily need "malloc()" just to allocate space. As DMail's example shows.

Here are three equivalent examples:

Code:

#include <stdio.h>
#include <string.h>
#include <malloc.h>

#define MAXCHAR 20

char *
trim (char *s)
{
  s[strlen(s)-1] = '\0';
  return s;
}

void
get_value (char *s, int maxlen)
{
  fgets (s, maxlen, stdin);
  printf ("s= %s...\n", trim(s));
}

int
main (int argc, char *argv[])
{
  char *s1 = NULL, *s2 = NULL, s3[MAXCHAR+1];

  s1 = (char *)malloc (MAXCHAR+1);
  if (s1)
  {
    get_value (s1, MAXCHAR);
    free (s1);
    s1 = NULL;
  }

  s2 = new char[MAXCHAR+1];
  if (s2)
  {
    get_value (s2, MAXCHAR);
    delete[] s2;
    s2 = NULL;
  }

  get_value (s3, sizeof (s3));

  return 0;
}

Here's a sample compile/execute:

Quote:

g++ -g -o x x.cpp
./x
abc
s= abc...
def
s= def...
123
s= 123...

'Hope that helps .. PSM