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If no more space is available, malloc returns a null pointer. You should check the value of every call to malloc. It is useful to write a subroutine that calls malloc and reports an error if the value is a null pointer, returning only if the value is nonzero. This function is conventionally called xmalloc. Here it is:

     void *
     xmalloc (size_t size)
       register void *value = malloc (size);
       if (value == 0)
         fatal ("virtual memory exhausted");
       return value;

Here is a real example of using malloc (by way of xmalloc). The function savestring will copy a sequence of characters into a newly allocated null-terminated string:

     char *
     savestring (const char *ptr, size_t len)
       register char *value = (char *) xmalloc (len + 1);
       value[len] = '\0';
       return (char *) memcpy (value, ptr, len);

The block that malloc gives you is guaranteed to be aligned so that it can hold any type of data. In the GNU system, the address is always a multiple of eight on most systems, and a multiple of 16 on 64-bit systems. Only rarely is any higher boundary (such as a page boundary) necessary; for those cases, use memalign, posix_memalign or valloc (see Aligned Memory Blocks).

Note that the memory located after the end of the block is likely to be in use for something else; perhaps a block already allocated by another call to malloc. If you attempt to treat the block as longer than you asked for it to be, you are liable to destroy the data that malloc uses to keep track of its blocks, or you may destroy the contents of another block. If you have already allocated a block and discover you want it to be bigger, use realloc (see Changing Block Size).