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3.2.2.4 Changing the Size of a Block

Often you do not know for certain how big a block you will ultimately need at the time you must begin to use the block. For example, the block might be a buffer that you use to hold a line being read from a file; no matter how long you make the buffer initially, you may encounter a line that is longer.

You can make the block longer by calling realloc. This function is declared in stdlib.h.

— Function: void * realloc (void *ptr, size_t newsize)

The realloc function changes the size of the block whose address is ptr to be newsize.

Since the space after the end of the block may be in use, realloc may find it necessary to copy the block to a new address where more free space is available. The value of realloc is the new address of the block. If the block needs to be moved, realloc copies the old contents.

If you pass a null pointer for ptr, realloc behaves just like ‘malloc (newsize)’. This can be convenient, but beware that older implementations (before ISO C) may not support this behavior, and will probably crash when realloc is passed a null pointer.

Like malloc, realloc may return a null pointer if no memory space is available to make the block bigger. When this happens, the original block is untouched; it has not been modified or relocated.

In most cases it makes no difference what happens to the original block when realloc fails, because the application program cannot continue when it is out of memory, and the only thing to do is to give a fatal error message. Often it is convenient to write and use a subroutine, conventionally called xrealloc, that takes care of the error message as xmalloc does for malloc: 

     void *
     xrealloc (void *ptr, size_t size)
     {
       register void *value = realloc (ptr, size);
       if (value == 0)
         fatal ("Virtual memory exhausted");
       return value;
     }

You can also use realloc to make a block smaller. The reason you would do this is to avoid tying up a lot of memory space when only a little is needed. In several allocation implementations, making a block smaller sometimes necessitates copying it, so it can fail if no other space is available.

If the new size you specify is the same as the old size, realloc is guaranteed to change nothing and return the same address that you gave.