How to print the value of a char* type data in C

The scanf() will attempt to write a sequence of chars to the 4 bytes of memory
currently occupied by the POINTER to "aaa".  There are several things wrong with this:

1) scanf("%s") expects a pointer to char.  You are passing a pointer to a pointer to char.  
You have made one of two possible common errors:
 a) You were trying to have scanf() store the input data into the bytes of memory
     currently occupied by "aaa".  See items 2 and 3 below.
 b) You were expecting scanf() to return a pointer to new or existing memory
     that holds the string, modifying the pointer (aString) to point to that new location
     (and no longer point at "aaa").  See item 4 below.

2) If the user enters more than 3 characters, the input will overflow the 4 bytes
[or 8 bytes - depending on sizeof(pointer) for your platform] overwriting adjacent data.

3) String constants may be (but are not required to be) stored in read-only memory.
Attempting to write into read-only memory will yield a segmentation fault.

4) scanf("%s") expects a pointer to an array of char large enough to hold the expected
input.  You should consider declaring aString as:   char aString[1024];
The char array will be mutable and sufficiently large to accept most input.
You can also limit the number of bytes read into aString by specifying a width
to the field read:
scanf("%u%s", &num, &aString);

I think the scanf statement should be

scanf("%u%s", &num, aString);

What if I don't know what size the maximum length of the string is
Does C has INT_MAX or something, just like C++?

> What if I don't know what size the maximum length of the string is
> Does C has INT_MAX or something, just like C++?

Yes, INT_MAX is in the ANSI C spec.  However, it sounds like you are contemplating
something like  char buffer[INT_MAX];  which would be a 2 gigabyte buffer - unwise.

Typically, users of scanf() create a "sufficiently large" buffer  (in my example, I used
1 a kilobyte buffer, however 2, 4, 8 and 16 kilobyte scratch buffers are not uncommon.
The wise programmer will also use input functions that avoid overflowing the input
buffer, like fgets() or a field width specification for scanf().

Hi nofearse,
> What if I don't know what size the maximum length of the string is
> Does C has INT_MAX or something, just like C++?
That's a classic security problem: buffer overflow. Never, ever use a function which does not limit the input size. Even if your program isn't security-sensitive, it will be much more stable when you use fgets() instead for your string.

Cheers!

Stefan

i think fgets and sscanf combination will work

char * aString, * buffer;
aString = (char *) malloc(MAX_LEN + 2);
do {
       if (!fgets(buffer, MAX_INPUT_LEN + 2, stdin))
      return 0;
       if (sscanf(buffer, "%s", aString) != 1)
      continue;
     }
while (1);

Hi  manojantony,

While are you doing a sscanf() here?  strcpy() is faster and more straight-forward.


Kent

Kent,

Its for input validation..

if (sscanf(buffer, "%s", aString) != 1)
                                                  ^
if (sscanf(buffer, "%d%d%d", aString) != 3)
                                                            ^
hope you got the point

-
MA    

Ok, but in your example I believe that you will always return 1.

Kent

Ye u r right.  I agree.. its fine .. for that particular example. :-)