C Char pointers

Question

Let us say we have a array of pointers:

char *ptr[30];

Now this works fine, and doesn\'t seems to do anything unexpected! I can input names easi

Answer 1

You've allocated space for 30 pointers, but you haven't initialized them to point anywhere meaningful. Unless you declared the array outside of a function, each element in the array will contain some random bit string that may or may not correspond to a writable memory location. If we drew a picture, it would look something like this (all addresses are pulled out of thin air; don't assume this corresponds to any real architecture):

Item       Address       0x00  0x01  0x02  0x03
----       -------       ----  ----  ----  ----
ptr        0xbbc81230    0x??  0x??  0x??  0x??
           0xbbc81234    0x??  0x??  0x??  0x??
           0xbbc81238    0x??  0x??  0x??  0x??
           ...
           0xbbc812a8    0x??  0x??  0x??  0x??           

where 0x?? represents a random byte value. For the behavior you've described, each of the random values just happens to point to writable memory, and writing over whatever's stored there just happens to not have any immediate ill effects.

Bad juju: it looks like your code is working properly when in reality it's behaving very badly, and can lead to some nasty runtime problems elsewhere in your program that's a pain to debug.

You will need to explicitly set each element of the ptr array to point to a valid memory location before attempting to write through it.

Suppose we add the following code:

ptr[0] = malloc(strlen("foo") + 1);
strcpy(ptr[0], "foo");
ptr[1] = malloc(strlen("bar") + 1);
strcpy(ptr[1], "bar");

We've dynamically allocated some extra memory to hold a couple of strings, and stored the pointers to those new buffers to ptr[0] and ptr[1].

Our picture would now look something like this:

Item       Address       0x00  0x01  0x02  0x03
----       -------       ----  ----  ----  ----
           0x80ff0000     'f'   'o'   'o'  0x00
           ...
           0x80ffcdc0     'b'   'a'   'r'  0x00
           ...
ptr        0xbbc81230    0x80  0xff  0x00  0x00
           0xbbc81234    0x80  0xff  0xcd  0xc0
           0xbbc81238    0x??  0x??  0x??  0x??
           ...
           0xbbc812a8    0x??  0x??  0x??  0x??           

ptr[0] now contains the address of a buffer sized to hold 4 char values, and we copy the string "foo" to that buffer. Similarly, ptr[1] contains the address of another 4-byte buffer that now contains the string "bar".