Sometimes you're not sure at first how much memory you'll need. For example, if you need to store a series of items you read from the user, and if the only way to know how many there are is to read them until the user types some ``end'' signal, you'll have no way of knowing, as you begin reading and storing the first few, how many you'll have seen by the time you do see that ``end'' marker. You might want to allocate room for, say, 100 items, and if the user enters a 101st item before entering the ``end'' marker, you might wish for a way to say ``uh, malloc, remember those 100 items I asked for? Could I change my mind and have 200 instead?''
In fact, you can do exactly this, with the realloc function. You hand realloc an old pointer (such as you received from an initial call to malloc) and a new size, and realloc does what it can to give you a chunk of memory big enough to hold the new size. For example, if we wanted the ip variable from an earlier example to point at 200 ints instead of 100, we could try calling
ip = realloc(ip, 200 * sizeof(int));Since you always want each block of dynamically-allocated memory to be contiguous (so that you can treat it as if it were an array), you and realloc have to worry about the case where realloc can't make the old block of memory bigger ``in place,'' but rather has to relocate it elsewhere in order to find enough contiguous space for the new requested size. realloc does this by returning a new pointer. If realloc was able to make the old block of memory bigger, it returns the same pointer. If realloc has to go elsewhere to get enough contiguous memory, it returns a pointer to the new memory, after copying your old data there. (In this case, after it makes the copy, it frees the old block.) Finally, if realloc can't find enough memory to satisfy the new request at all, it returns a null pointer. Therefore, you usually don't want to overwrite your old pointer with realloc's return value until you've tested it to make sure it's not a null pointer. You might use code like this:
int *newp;
newp = realloc(ip, 200 * sizeof(int));
if(newp != NULL)
ip = newp;
else {
printf("out of memory\n");
/* exit or return */
/* but ip still points at 100 ints */
}
If realloc returns something other than a null pointer, it succeeded, and we set ip to what it returned. (We've either set ip to what it used to be or to a new pointer, but in either case, it points to where our data is now.) If realloc returns a null pointer, however, we hang on to our old pointer in ip which still points at our original 100 values.Putting this all together, here is a piece of code which reads lines of text from the user, treats each line as an integer by calling atoi, and stores each integer in a dynamically-allocated ``array'':
#define MAXLINE 100
char line[MAXLINE];
int *ip;
int nalloc, nitems;
nalloc = 100;
ip = malloc(nalloc * sizeof(int)); /* initial allocation */
if(ip == NULL)
{
printf("out of memory\n");
exit(1);
}
nitems = 0;
while(getline(line, MAXLINE) != EOF)
{
if(nitems >= nalloc)
{ /* increase allocation */
int *newp;
nalloc += 100;
newp = realloc(ip, nalloc * sizeof(int));
if(newp == NULL)
{
printf("out of memory\n");
exit(1);
}
ip = newp;
}
ip[nitems++] = atoi(line);
}
We use two different variables to keep track of the ``array'' pointed to by ip. nalloc is how many elements we've allocated, and nitems is how many of them are in use. Whenever we're about to store another item in the ``array,'' if nitems >= nalloc, the old ``array'' is full, and it's time to call realloc to make it bigger.Finally, we might ask what the return type of malloc and realloc is, if they are able to return pointers to char or pointers to int or (though we haven't seen it yet) pointers to any other type. The answer is that both of these functions are declared (in <stdlib.h>) as returning a type we haven't seen, void * (that is, pointer tovoid). We haven't really seen type void, either, but what's going on here is that void * is specially defined as a ``generic'' pointer type, which may be used (strictly speaking, assigned to or from) any pointer type.
