malloc
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 section Allocating 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 section Changing the Size of a Block).
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