Subject: Re: Many thanks and a new question...
To: Joshua Dinerstein <email@example.com>
From: Jeremy Cooper <firstname.lastname@example.org>
Date: 07/21/1997 16:06:57
On Mon, 21 Jul 1997, Joshua Dinerstein wrote:
> > > [ Use malloc() or stack for large variables/buffers? ]
> > [ Use malloc(). ]
> 1- There appear to be 2 malloc functions in the kernel: MALLOC and
> malloc. These 2 functions are taking quite a different set of arguments.
> 2- What is the MALLOC function for? (Specifically) And when
> should it be used over the malloc function.
> 3- Vice versa for the malloc function. When should it be used instead
> of MALLOC. The MALLOC allows you to set the "type" of memory to be allocated.
The "type" and "flag" arguments to malloc() serve two purposes. The
"type" argument is really more like a "purpose" argument. It helps
the VM statistics gathering functions keep track of the amount of memory
currently allocated for a specific use. Check sys/malloc.h for a type
that is appropriate to your usage.
The "flags" argument is a way to state the urgency of the operation.
The only two flags that I know of are M_NOWAIT and M_WAITOK. The M_NOWAIT
flag informs the kernel that you are not willing to block or sleep for the
memory - M_WAITOK implies the opposite. M_NOWAIT is typically used in
code that has no associated process context, such as inside an interrupt
handler, while M_WAITOK is used everywhere else.
Finally, MALLOC() is a macro that calls malloc(). You use it to reduce
the amount of typing needed to make a proper call to malloc(), and to help
improve memory diagnostics when you compile a kernel with the DIAGNOSTIC
option. The following two code fragments are the same.
MALLOC(myints, int *, sizeof(int) * 1024, M_DEVBUF, M_WAITOK);
myints = (int *) malloc((u_long) sizeof(int) * 1024, M_DEVBUF, M_WAITOK);