@node Safe Allocation Macros
@section Safe Allocation Macros

@mindex safe-alloc
The standard C library malloc/realloc/calloc/free APIs are prone to a
number of common coding errors.  The @code{safe-alloc} module provides
macros that make it easier to avoid many of them.  It still uses the
standard C allocation functions behind the scenes.

Some of the memory allocation mistakes that are commonly made are

@itemize @bullet
@item
passing the incorrect number of bytes to @code{malloc}, especially
when allocating an array,
@item
unchecked integer overflow when calculating array sizes,
@item
fail to check the return value of @code{malloc} and @code{realloc} for
errors,
@item
forget to fully initialize memory just allocated with @code{malloc},
@item
duplicate calls to @code{free} by forgetting to set the pointer
variable to @code{NULL},
@item
leaking memory in calls to @code{realloc} when that call fails.
@end itemize

The @code{safe-alloc} module addresses these problems in the following way:

@itemize @bullet
@item
It defines macros that wrap around the standard C allocation
functions.  That makes it possible to use the compiler's knowledge of
the size of objects for allocation; it also allows setting pointers
passed in as arguments when appropriate.
@item
It uses return values only for a success/failure error condition flag,
and annotates them with GCC's @code{__warn_unused_result__} attribute.
@item
When allocating a fresh array, it uses @code{calloc} instead of
@code{malloc} so that the array's contents are zeroed.
However, memory added to an already-existing array is uninitialized.
@end itemize

@defmac {int} ALLOC (ptr)
@findex ALLOC
Allocate @code{sizeof *ptr} bytes of memory and store the address of
allocated memory in @code{ptr}.  Fill the newly allocated memory with
zeros.

Returns @minus{}1 on failure, 0 on success.
@end defmac

@defmac {int} ALLOC_N (ptr, count)
@findex ALLOC_N
Allocate an array of @code{count} elements, each @code{sizeof *ptr}
bytes long, and store the address of allocated memory in
@code{ptr}.  Fill the newly allocated memory with zeros.

Returns @minus{}1 on failure, 0 on success.
@end defmac

@defmac {int} ALLOC_N_UNINITIALIZED (ptr, count)
@findex ALLOC_N_UNINITIALIZED
Allocate an array of @code{count} elements, each @code{sizeof *ptr}
bytes long, and store the address of allocated memory in
@code{ptr}.  The allocated memory is not initialized.

Returns @minus{}1 on failure, 0 on success.
@end defmac

@defmac {int} REALLOC_N (ptr, count)
@findex REALLOC_N
Reallocate the memory pointed to by @code{ptr} to be big enough to hold
at least @code{count} elements, each @code{sizeof *ptr} bytes long,
and store the address of allocated memory in @code{ptr}.  If
reallocation fails, the @code{ptr} variable is not modified.
If the new array is smaller than the old one, discard excess contents;
if larger, the newly added storage is not initialized.

Returns @minus{}1 on failure, 0 on success.
@end defmac

@defmac {void} FREE (ptr)
@findex FREE
Free the memory stored in @code{ptr} and set @code{ptr} to
@code{NULL}.
@end defmac
